Chapter 9: Bacteria and Its Economic Importance
In the 1670s, in the Dutch town of Delft, a cloth merchant named Antonie van Leeuwenhoek spent his evenings grinding small glass lenses. He had begun making them to check the quality of cloth, but curiosity took him further. He set a drop of rainwater in front of a lens, held the little instrument close to his eye, and found the drop crowded with moving specks.
He wrote to the Royal Society in London about these ‘animalcules’ — little animals, as he called them. Water from a pond, water in which pepper had been soaked, water from a rooftop gutter: every sample had its own inhabitants. In 1683 he scraped the soft white matter from between his own teeth, mixed it with clean rainwater, and saw more of them, some straight like tiny rods, some bent, some drifting in loose groups. He drew what he saw.
The learned men in London did not believe him at first. Nobody had suspected that a clear drop of water could hold living things too small to see, and it took several careful demonstrations before the reports were accepted.
Van Leeuwenhoek had found bacteria. He never learned what they do — that some rot our food, some enrich our fields, some make us ill, and some are turned into medicines. That is what this chapter is about.
Diagram
Fig. 9.0-1 — Van Leeuwenhoek’s Single-Lens Microscope Line drawing of a hand-held single-lens microscope of the seventeenth century, shown from the front and edge-on; label the small glass lens, the metal plate holding it, the specimen pin, the focusing screws and the position of the eye. Beside it, a small inset panel showing simple sketches of rod-shaped, curved and rounded bodies as an early observer would have drawn them.
9.1 Bacteria — A General Study
Bacteria are the most primitive living organisms we know. Each bacterium is a single cell, and that one cell carries out every activity the organism needs — feeding, respiring, growing and dividing.
The genetic material of a bacterium lies loose in the middle of the cell. It is not shut inside a nuclear membrane, as it is in your cells or in the cells of a leaf. An organism whose cell has no true nucleus is called a prokaryote.
This single fact decides where bacteria are placed in the living world. In the five-kingdom classification, bacteria are not counted as plants. Plants are made of many cells, and each of those cells has a true nucleus; a bacterium is one cell without one.
9.1.1 Where Bacteria Are Found
There are several thousand known species of bacteria, and they occur in air, water, soil, food, and on or inside the bodies of other living things. In short, they are practically everywhere.
Your own body is no exception. Bacteria live in your mouth, on your skin, inside your intestines and in the genital tracts. By one estimate, the bacteria living on the skin of a single person outnumber all the human beings on Earth.
Most of these are harmless, and many are useful. Bacteria in the intestine make certain vitamins of the B group, which the body then absorbs. The ones on the skin are useful in a different way: by occupying the space, they keep harmful kinds from settling there.
A small number of species do cause disease, and those are the ones people usually think of first. They are a minority. The rest of this chapter will show how much of what we eat, grow and use depends on bacteria.
9.1.2 Size And Shape Of Bacteria
Bacteria are the smallest living organisms on Earth. An average bacterium is about 2 micrometres long and about 0.5 micrometre thick.
A micrometre is one-thousandth of a millimetre. Look at the millimetre marks on your ruler; imagine one of those tiny gaps divided into a thousand parts, and you have the width of a bacterium. This is why van Leeuwenhoek needed a lens before anyone could see them at all.
9.1.2.1 The Four Shapes
Whatever their size, bacteria are usually of four shapes, shown in Fig. 9.1-1.
- Cocci — spherical bacteria.
- Bacilli — rod-shaped bacteria.
- Spirilla — spiral or twisted bacteria.
- Vibrio — comma-shaped bacteria.
Shape is one of the first things a scientist records about an unfamiliar bacterium, because it can be seen directly under the microscope without any test.
9.1.2.2 Patterns Of Grouping
Many bacteria live singly. Others stay attached to one another after dividing, and the pattern they form is a second useful clue to identity.
Bacteria that occur in pairs are called diplococci. Those that stay joined in long chains are streptococci, and those that collect in irregular clusters, like a bunch of grapes, are staphylococci. All three names end in -coccus, so each of these patterns is a pattern of spherical cells; the first half of each name — double, chain-like, grape-like — describes the arrangement.
Diagram
Fig. 9.1-1 — Kinds Of Bacteria Line drawing showing the four shapes of bacteria, much magnified, in four labelled groups: coccus (spherical), bacillus (rod-shaped, some with flagella), spirillum (spiral or curved) and vibrio (comma-shaped). Alongside the coccus group, show and label the three patterns of grouping — diplococcus (in pairs), streptococcus (in chains) and staphylococcus (in clusters).
9.1.3 Structure Of A Bacterial Cell
A bacterium is a simple cell, but it is not a bag of jelly. Each part of it does a definite job, and Fig. 9.1-2 shows how those parts are arranged.
The living substance of the cell is its protoplasm, sometimes called the protoplast. It is made up of the cytoplasm together with the bacterial chromosome, and it lies within the cell membrane.
Around the outside is a stiff, non-living cell wall. The wall gives the cell its shape and keeps it from bursting. It is made of peptidoglycan — not of cellulose, which is what the wall of a plant cell is made of.
Immediately beneath the wall lies a thin cell membrane, which surrounds the cytoplasm. The membrane is the layer that decides what enters and leaves the cell; the wall outside it is only a rigid casing.
The Genetic Material
There is no well-defined nucleus. Instead, the chromatin or DNA material lies in the central region of the cell as a single closed circle, and this is called the bacterial chromosome. It has no nuclear membrane around it, but it is attached to the cell membrane at one point.
Many bacteria also carry a plasmid — a small, separate circle of DNA, quite distinct from the bacterial chromosome. It is small, but it will matter twice in this chapter: once when bacteria pass genes to one another, and again when we ask how a bacterium can be made to produce a human hormone.
The cytoplasm contains vacuoles and granules, often granules of stored glycogen. In some kinds of bacteria it contains chlorophyll as well.
Outside the cell wall there is sometimes a slimy protective layer called the capsule. It is not present in every bacterium, and where it is present it shields the cell against drying and against attack.
Diagram
Fig. 9.1-2 — Structure Of A Bacterial Cell Diagrammatic drawing of a single rod-shaped bacterial cell cut open to show the interior; label capsule, cell wall, cell membrane, cytoplasm, bacterial chromosome, plasmid, glycogen granules and flagellum. Mark the cytoplasm and bacterial chromosome together with a bracket labelled protoplasm. Straight, non-crossing lead lines.
9.1.4 Movement
Most bacteria cannot move on their own at all. They are carried from place to place by wind, by water, or by contact with something that touches them.
Some bacteria, however, possess one or more whip-like flagella. A flagellum pierces through the cell wall and the capsule, so it is anchored inside the cell and projects into the surroundings.
The flagella lash from side to side. In a liquid, this pushes the cell along, giving it active locomotion instead of a passive ride.
9.1.5 Nutrition
Most bacteria have no chlorophyll, so they cannot make their own food. They depend on ready-made food taken from other sources, and this way of feeding is called heterotrophic nutrition.
Heterotrophic bacteria are of two kinds:
- Saprotrophic bacteria draw nourishment from decaying dead organisms.
- Parasitic bacteria draw nourishment from the body of a living host.
The food outside the cell is usually solid, and a solid cannot cross the cell membrane. The bacteria get round this by secreting powerful enzymes out of their cells into the food-containing material around them.
These enzymes break the food down and make it soluble. The dissolved food is then absorbed into the bacterial cell as a solution. Digestion, in other words, happens outside the body of the bacterium and only the products come in.
9.1.6 Respiration
Every living cell must release energy from its food, and bacteria differ in whether they need oxygen to do it.
Some bacteria absorb oxygen from the air around them and respire with it. These are aerobic bacteria. Others need no free oxygen at all and are called anaerobic bacteria.
For many anaerobic species, oxygen is not merely unnecessary but harmful, and they are killed if they are exposed to air. This is why such bacteria are found in airless places — deep in the soil, inside sealed cans, in the mud at the bottom of a pond. Table 9.1 sets the two kinds side by side.
9.1.7 Reproduction
Reproduction in bacteria is only asexual. It takes place by fission, that is, by simple cell division.
9.1.7.1 Binary Fission
The steps are set out in FC 9.1 and drawn in Fig. 9.1-3. First the circular DNA duplicates, so that the cell now holds two identical copies instead of one. The cell then expands, and the two copies of the DNA are pulled apart towards opposite ends.
The cell now constricts at the middle. The constriction deepens until the parent cell has been separated into two cells, each with one copy of the DNA.
What happens next depends on the kind of bacterium. In some, the two daughter cells stay attached — which is how the pairs, chains and clusters of section 9.1.2.2 arise. In others they separate completely and live as independent cells.
Flowchart
FC 9.1 — Binary Fission In A Bacterium Circular DNA duplicates → cell expands → the two DNA circles are pulled to opposite ends → cell constricts at the middle → two daughter cells formed → daughter cells either remain attached or separate → each daughter cell grows and the cycle begins again
Diagram
Fig. 9.1-3 — Reproduction In Bacteria By Fission Four-stage vertical sequence of a rod-shaped bacterium dividing; label original DNA, DNA duplicates, copy DNA, constriction at the middle, and the two daughter cells. Straight, non-crossing lead lines.
Under ideal conditions some bacteria divide once every half hour. Starting from a single bacterium and dividing at that rate, there would be 281,474,976,710,656 bacteria at the end of twenty-four hours. Nothing like that number is ever reached in practice, because food runs short and wastes build up — but it explains why milk left out on a warm afternoon turns in a few hours.
9.1.7.2 Conjugation
A highly primitive kind of sexual reproduction has been found in a very few bacteria. It is extremely simple, and it is called conjugation.
Two bacteria of different strains, but of the same species, come together. One of them, the donor, grows a hollow tube-like extension that reaches the other, the recipient.
The plasmid of the donor is transferred through this tube into the recipient. Notice that it is the plasmid that travels and not the bacterial chromosome, so what the recipient gains is a small extra set of genes rather than a whole new set.
9.1.8 Spore Formation
Conditions do not stay favourable for ever. Vegetation dries up, water disappears, temperatures rise. A bacterium that could do nothing about this would simply die.
When such unfavourable conditions set in, the bacterial cell draws its contents into a spherical mass. This mass becomes surrounded by a thick, hard protective wall.
This little rounded body is a spore, and it lies at first within the original cell wall, as Fig. 9.1-4 shows. Later the old cell wall ruptures and the spore is set free. Such spores are resting stages, and they are extraordinarily tough:
- they can tolerate extreme dryness;
- some cannot be killed even at the temperature of boiling water or of frozen ice;
- some can tolerate poisonous chemicals.
Once free, spores are carried far and wide by wind, by water or by contact. On meeting favourable conditions again, the spore germinates: its dormant protoplasm becomes active and streams out of the ruptured wall as a new bacterium. FC 9.2 traces the whole cycle.
Diagram
Fig. 9.1-4 — A Bacterium Forming A Spore Inside Itself Drawing of a rod-shaped bacterial cell containing a single rounded spore; label spore wall, spore contents and bacterial wall. Straight, non-crossing lead lines.
Flowchart
FC 9.2 — Spore Formation And Germination Unfavourable conditions set in → cell contents drawn into a spherical mass → thick hard protective wall formed around it → spore lies within the old cell wall → old cell wall ruptures and the spore is released → spore carried by wind, water or contact → favourable conditions met → protoplasm becomes active and streams out → new bacterium
One point must be kept straight. A single bacterium forms a single spore, and that spore later gives one bacterium. The number of individuals does not increase. Spore formation is therefore not a method of reproduction at all — it is a method of escaping unfavourable conditions.
9.2 Useful Role Of Bacteria In Medicine
Medicine is the field in which the usefulness of bacteria is easiest to see. Bacteria are used in the production of antibiotics, serums and vaccines — three quite different kinds of protection against disease.
9.2.1 Antibiotics
Living things compete. A microorganism growing in soil is surrounded by others feeding on the same material, and some of them defend their patch chemically, by releasing substances that stop their neighbours from growing. Medicine has borrowed those substances, and we call them antibiotics.
Today dozens of antibiotics are in wide use against numerous infections. Among the best known are streptomycin, chloromycetin, tetracycline and penicillin.
9.2.1.1 The Discovery Of Penicillin
The first antibiotic to reach medical use came not from a bacterium but from a mould — one of the fungi you will meet in the next chapter.
The clear ring around the mould is worth picturing, because it is the whole discovery in one image. It is drawn in Fig. 9.2-1.
Diagram
Fig. 9.2-1 — Fleming’s Contaminated Culture Plate Surface view of a circular culture plate; show a mould colony growing at one edge, a clear zone of no bacterial growth immediately around it, and dense bacterial colonies further away. Label mould colony, clear zone with no bacterial growth, and bacterial colonies. Straight, non-crossing lead lines.
One thing about penicillin is easily got wrong. Penicillin is the product of a mould, which is a fungus, and not of a bacterium. Bacteria give us many antibiotics, but not this one.
The antibiotic streptomycin was discovered next, by Selman Waksman, working in a similar way. Its source organism was the bacterium Streptomyces griseus, and streptomycin was the first drug that worked against tuberculosis.
9.2.1.2 Some Well-Known Antibiotics And Their Sources
Table 9.2 lists four antibiotics with the organism each is obtained from and a disease it is used against.
Read down the middle column and a pattern appears. Three of these four source organisms are bacteria; only the first is a mould. Bacteria are the main producers of the antibiotics we use.
9.2.1.3 Criteria For A Good Antibiotic
Not every substance that kills bacteria can be given to a patient. A useful antibiotic has to meet three conditions.
- It should be able to kill a variety of disease-producing microorganisms. Such an antibiotic is called a broad-spectrum antibiotic.
- It should not produce undesirable side-effects.
- It should not kill the normal bacteria of the host.
The third condition follows from what you learnt in section 9.1.1. The body’s own bacteria make vitamins and keep harmful kinds out; a drug that wipes them out solves one problem and creates another.
9.2.1.4 Other Uses Of Antibiotics
Antibiotics are not used only on patients. They are also used:
- as food preservatives, especially for fresh meat and fish;
- for treating animal feed;
- for controlling plant pathogens, that is, the organisms that cause disease in crops.
9.2.2 Serums
An antibiotic attacks the bacterium. A serum works on the other side of the problem: it attacks the poison the bacterium leaves behind.
Serum means blood plasma from which fibrinogen has been removed. Plasma is the pale liquid part of blood, and fibrinogen is the protein in it that makes blood clot; take the fibrinogen out and what remains cannot clot. Serum may contain numerous chemical substances, including antibodies and antitoxins.
9.2.2.1 Toxins And Antitoxins
Toxins are poisonous proteins released by disease-causing bacteria, either while they are growing or when they break down after dying. Much of the harm in a bacterial disease is done by the toxin rather than by the bacterium itself.
An antitoxin is a substance produced in animal bodies which reacts with the toxin produced by the invading germs and neutralises it. Serums contain antitoxins for one particular pathogen, so a serum is specific: the one prepared against a given disease is of no use against another.
Because a serum acts on a poison already present, it is used as a preventive measure against bacterial invasion rather than as a cure long afterwards.
9.2.2.2 How A Serum Is Prepared
The preparation begins with the toxin itself, and the steps are set out in FC 9.3.
A small dose of bacterial toxin is introduced into the blood of a healthy animal such as a horse or a cow. The dose is small enough not to harm the animal, but large enough for its body to respond by producing antitoxins that neutralise the effect of the toxin.
The injection is repeated several times, so that the animal’s blood builds up a large store of antitoxins. Blood is then taken from the animal and left to chill. As it chills, it clots, and a straw-coloured liquid separates out from the clot. That liquid is the serum.
Flowchart
FC 9.3 — Preparation Of A Serum Small dose of bacterial toxin injected into a healthy horse or cow → animal’s body produces antitoxins → injections repeated several times → antitoxin level in the blood rises → blood drawn from the animal → blood chilled and allowed to clot → straw-coloured liquid separates from the clot → serum collected
9.2.2.3 Products From Genetically Modified Bacteria
Some substances the body needs cannot be collected from animals in any useful quantity. Genetic engineering has made it possible to introduce human genes into certain bacteria, such as Escherichia coli, usually written E. coli.
These bacterial cultures grow very rapidly, as section 9.1.7.1 showed. A bacterium carrying a human gene follows that gene’s instructions, so the product of the gene can be extracted from the culture in bulk. The hormone insulin was the first such substance produced by E. coli.
A few serum compounds produced by genetically modified bacteria are:
- blood clotting factor VIII, for the treatment of haemophilia A;
- factor IX, for the treatment of haemophilia B.
Haemophilia is a condition in which the blood clots very slowly because one clotting factor is missing. Supplying the missing factor allows a person with haemophilia to control bleeding.
9.2.3 Vaccines
A serum brings in ready-made antitoxins from another animal. A vaccine does something better: it makes your own body produce them, before the disease ever arrives.
Vaccines are available for a variety of diseases such as cholera, typhoid, measles and chickenpox. On injecting a vaccine into the body, the person gets the disease in a very mild form, and the body is stimulated to produce antitoxins.
Those antitoxins remain, and they provide immunity against any future attack by that particular disease germ. This is why a vaccine is given to a healthy person and not to a sick one. FC 9.4 shows the sequence.
Flowchart
FC 9.4 — How A Vaccine Protects Vaccine of weakened or dead germs introduced into the body → disease produced in a very mild form → body stimulated to produce antitoxins → antitoxins remain in the body → germ of that disease enters later → antitoxins act on it → person does not fall ill
Two Vaccines Grown From Bacteria
Killed bacteria — the TAB vaccine, given against typhoid.
Living weakened bacteria — the BCG vaccine, given against tuberculosis.
Both are prepared by growing the bacteria in bulk and then either killing them or weakening them, so the bacteria themselves are the raw material of the vaccine.
9.2.4 Toxoids
There is a third possibility. Instead of using the whole germ, weakened or killed, the toxin itself can be treated so that it is no longer poisonous.
Toxoids are the inactivated toxins of a particular bacterium, which can still stimulate the production of the respective antibodies. An antibody is a protein the body makes in response to a foreign substance, and it acts against that substance alone.
Toxoids are used for producing immunity against diphtheria and tetanus — two diseases in which the damage is done chiefly by the toxin.
You will study a fuller account of vaccination and immunisation in Class X.
9.3 Bacteria — Role In Agriculture
Nearly four-fifths of the air is nitrogen gas, \(N_2\). Every plant and animal needs nitrogen, because proteins cannot be built without it — and yet a plant standing in all that nitrogen cannot use a molecule of it.
The reason is that the two nitrogen atoms in \(N_2\) are held together very tightly, and a plant has no way of pulling them apart. Bacteria can. Almost all the nitrogen in your food passed through a bacterium first.
9.3.1 Nitrogen-Fixing Bacteria Of Root Nodules
A special category of soil bacteria, Rhizobium, is found living in small swellings called nodules on the roots of leguminous plants such as beans. Fig. 9.3-1 shows the nodules, a nodule cut open, and the bacteria taken out of one.
These bacteria take up free nitrogen from the soil and the atmosphere and combine it into compounds the plant can use. The gas is first converted into ammonia, \(NH_3\), and ammonium compounds; these are then converted in the soil into nitrates, which are the form the roots absorb.
The arrangement suits both partners. The plant supplies the bacteria with food and a protected place to live; the bacteria supply the plant with usable nitrogen. This is why the nitrogen compounds are used by the host plant and also by other plants sown later in the same soil — the nodules stay behind in the field.
The conversion of free atmospheric nitrogen into nitrogen compounds is called nitrogen fixation.
Diagram
Fig. 9.3-1 — Root Nodules Of A Leguminous Plant Three-part figure. A: the root system of a leguminous plant pulled from the soil, showing rounded nodules along the roots; label nodules. B: a single nodule in section, showing the mass of bacteria inside the plant tissue; label bacteria and nodule tissue. C: a few rod-shaped bacteria as seen when taken out of a nodule, much magnified. Straight, non-crossing lead lines.
9.3.2 Free-Living Nitrogen-Fixing Bacteria
Not every nitrogen-fixing bacterium needs a plant. Certain free-living bacteria in the soil, such as Azotobacter and Clostridium, do the same work on their own.
They use the free nitrogen of the soil and combine it with hydrogen to form ammonia. The ammonia is then converted into amino acids and nitrates, and the soil is enriched.
So a field gains nitrogen in two ways: through the bacteria housed inside root nodules, and through these bacteria living loose in the soil.
9.3.3 Nitrifying Bacteria
Plants need nitrogen for synthesising proteins, but they cannot make use of free atmospheric nitrogen. They can obtain it only in the form of nitrates, which they absorb from the soil through their roots.
Crops remove those nitrates as they grow. The nitrates are replenished in the soil by bacteria, through a process called nitrification, and the sequence is set out in FC 9.5.
For this, some bacteria first convert the nitrogenous wastes of animals and the dead remains of plants and animals into ammonia. The ammonia is converted into ammonium compounds.
The ammonium compounds are then converted into nitrites by bacteria such as Nitrosomonas, and the nitrites into nitrates by bacteria such as Nitrobacter. Each of these steps is brought about by its own specific bacteria — no single organism carries the material the whole way.
Flowchart
FC 9.5 — Nitrification Nitrogenous wastes of animals and dead remains of plants and animals → ammonia (\(NH_3\)) → ammonium compounds → nitrites (\(NO_2^-\)), by Nitrosomonas → nitrates (\(NO_3^-\)), by Nitrobacter → absorbed by roots of plants
9.3.4 Denitrifying Bacteria
If every step ran one way, the atmosphere would slowly be emptied of its nitrogen. It is not, because one group of bacteria works in the opposite direction.
Denitrifying bacteria are present in the soil and break down soil nitrates to release nitrogen gas, \(N_2\), which enters the atmosphere. Pseudomonas is an example.
From a farmer’s point of view this is a loss, since the nitrates removed are exactly what the crop wanted. From the point of view of the whole living world it is a return, and it completes a cycle.
9.3.5 The Nitrogen Cycle
Put the four groups of bacteria together and the movement of nitrogen becomes a closed circle, drawn in Fig. 9.3-2.
Nitrogen leaves the atmosphere by biological fixation by Rhizobium, and also by industrial fixation, the manufacture of nitrogen fertilisers in factories. In the soil, ammonia is converted through nitrites into nitrates.
Plants take up the nitrates and build their proteins. Animals eat the plants and build their own proteins from them. When plants and animals die, and in the wastes they give out while alive, this nitrogen returns to the soil as decaying matter and is converted back into ammonia.
Denitrifying bacteria complete the circle by breaking nitrates down and releasing nitrogen gas to the atmosphere, where the cycle began.
Diagram
Fig. 9.3-2 — The Nitrogen Cycle Circular flow diagram. At the top, a box labelled atmospheric nitrogen (\(N_2\)). Arrows leading down from it labelled biological fixation by Rhizobium and industrial fixation, both entering a central box labelled in soil, containing ammonia → nitrites → nitrates. An arrow from that box to a box labelled plants; from plants an arrow labelled death, and another arrow to a box labelled animals; from animals an arrow labelled death. Both death arrows lead to a box labelled decay of living matter, which returns by an arrow to the in-soil box. A further arrow from the soil box back up to atmospheric nitrogen, labelled denitrification by bacteria.
9.3.6 Decay And Putrefaction
Thousands of plants and animals die every moment, and every living organism gives out organic wastes. None of it piles up. Bacteria act on dead bodies and on excreta and break them down into simpler chemical compounds — nitrates, sulphates and carbon dioxide — which plants then use for their growth.
This breaking down happens in two ways, compared in Table 9.4.
Decay is the complete breakdown of organic matter by bacteria without giving out a foul smell. Cow dung exposed to air is reduced to soil in a few days, and no bad smell is produced.
Putrefaction is the incomplete breakdown of organic matter by bacteria, and it emits a foul smell. The smell comes from the half-broken compounds left behind when the breakdown does not go all the way.
Decay and putrefaction are among the greatest uses of bacteria in nature. Without them, the nitrogen locked in dead bodies would stay locked, and the cycle of section 9.3.5 would stop at the point of death.
9.3.6.1 Sewage Treatment
Big cities have sewage treatment plants, where the collected human excreta is decomposed by sewage bacteria. Delhi has one such unit.
The process produces a gas, which is collected and used for cooking. The liquid and solid products that remain are used as manure in agricultural fields, so material that would have been a health hazard returns to the soil as a resource.
9.3.6.2 Biogas Plants
The gobar gas plants, also called biogas plants, are designed on the same principle. FC 9.6 traces what happens inside one.
Cow dung is fed into a closed tank, where there is no free oxygen. Fermenting bacteria degrade the cellulose of the dung and release inflammable gases, mainly consisting of methane, which collect above the slurry and are drawn off by a pipe.
Installing a biogas plant serves a dual purpose: it supplies fuel, and it supplies rich manure — the spent slurry left in the tank.
Flowchart
FC 9.6 — Working Of A Biogas Plant Cow dung mixed with water fed into the closed digester tank → no free oxygen inside → fermenting bacteria degrade the cellulose of the dung → inflammable gases, mainly methane, released → gas collects above the slurry and is drawn off by pipe for cooking → spent slurry removed and used as manure
Diagram
Fig. 9.3-3 — A Biogas Plant Vertical section of a fixed-dome biogas plant; label inlet tank for the dung and water mixture, digester tank, gas holder or dome, gas outlet pipe leading to a burner, and outlet tank for the spent slurry used as manure. Straight, non-crossing lead lines.
9.3.7 Synthesis Of Vitamins And Digestion
Several bacteria habitually live in our intestines, where, as section 9.1.1 noted, they synthesise certain vitamins — especially those of the B complex group, and vitamin K.
In herbivorous animals the intestinal bacteria do something more. Grass and leaves are largely cellulose, and a cow has no enzyme of its own that can digest it. The bacteria in its gut supply that enzyme, and the cattle live on the products.
This is a partnership of the same shape as the one in the root nodule: the animal provides shelter and a steady supply of food, and the bacteria provide a chemical skill the animal lacks.
9.4 Bacteria — Role In Industry
Bacteria have a very extensive use in industry. You have already read about their part in producing certain kinds of medicine on a large scale; here are some of their other uses.
In every case the industry is doing the same thing: it is letting bacteria break down a material that would be difficult, slow or expensive to break down by any other means, and then stopping the process at the right moment.
9.4.1 Curing Of Tea
Freshly plucked tea leaves have very little of the taste or colour of the tea in a cup. Both develop after the leaves are picked, during a stage the industry calls curing.
The plucked leaves are spread out and kept under controlled warmth and moisture. Certain bacteria act on them during this period, and different flavours of tea are produced according to how long the treatment runs and under what conditions.
The leaves are then dried, which stops the process. A cured leaf that is not dried in time goes on changing, and the flavour is spoilt.
9.4.2 Tanning Of Leather
A hide taken from an animal is not yet leather. It carries hair, fat and soft flesh, all of which would rot, and these must be removed before the hide can be tanned into a lasting material.
Certain bacteria do this removal. They break down the soft perishable parts of the skin, loosening the hair and the tissue beneath it, while leaving the tough fibrous layer that becomes leather. The hide is then treated with tanning materials, which make it durable and resistant to decay.
The purpose of tanning is to produce a skin that bacteria can no longer feed on. Bacteria are used to prepare the hide, and then shut out of it for good.
9.4.3 Other Industrial Uses
Three further uses are worth naming.
- Bacteria are used in the production of vinegar, and in the processing of coffee and tobacco.
- The fibres for making rope, linen and similar goods are obtained by loosening the fibres of hemp and flax by retting, which is a kind of rotting brought about by bacteria. The steps are given in FC 9.7.
- Curing of hides and skins is also done with the help of bacteria.
In retting, the cut stems are kept in water for days. Bacteria attack the soft tissue that holds the long fibres together, but not the fibres themselves, so the fibres can then be separated out clean and whole. Fig. 9.4-1 shows where those fibres lie in the stem.
Flowchart
FC 9.7 — Retting Of Flax Or Hemp Cut stems bundled and kept submerged in water → bacteria act on the soft tissue holding the fibres together → soft tissue rots away while the long fibres remain unharmed → stems removed and dried → fibres separated by beating and combing → fibres spun into rope or linen
Diagram
Fig. 9.4-1 — Transverse Section Of A Flax Stem Transverse section of a flax stem as a labelled line drawing; label epidermis, the ring of long fibre bundles, the soft tissue around the fibres that bacteria break down during retting, vascular tissue and central pith. Beside it, a small inset of a retted stem teased apart to show the long fibres separating from the softened tissue. Straight, non-crossing lead lines.
9.5 Spoilage Of Food By Bacteria
Bacteria that break down dead organic matter cannot tell the difference between a fallen leaf and the food on your plate. The same activity that clears the forest floor also ruins a bowl of cooked rice.
9.5.1 How Bacteria Spoil Food
Many bacteria spoil cooked food, milk, fruits and vegetables by the process of decay, also called fermentation. Spoilage is fastest during summer, because warmth lets the bacteria divide at the rate you met in section 9.1.7.1.
The change follows the pattern of saprotrophic nutrition. Bacteria settle on the food and multiply, secrete enzymes into it, and break its substances down into simpler compounds. Those compounds are what you notice — the sour taste of turned milk, the smell of stale food, the softening of a fruit. FC 9.8 sets out the sequence.
Flowchart
FC 9.8 — How Food Spoils Bacteria settle on the food from air, water, hands or utensils → warmth and moisture allow rapid multiplication → enzymes secreted into the food → food substances broken down → changes in taste, smell, colour and texture → in some cases toxins accumulate → food poisoning if eaten
Spoilage may sometimes go far enough to cause food poisoning, an illness caused by eating food in which harmful bacteria or their toxins have built up.
Botulism is a very serious food poisoning, due to a special bacterium sometimes found in tinned and sealed foods. The bacterium grows where there is no free oxygen, which is exactly the condition inside a sealed tin.
Because of all this, preservation of food, and making it free of disease-causing germs, is very necessary.
9.5.2 Methods Of Food Preservation
Every method below works in one of three ways: it kills the bacteria, it removes something they need, or it makes conditions they cannot grow in. Keep that in mind and the seven methods stop being a list to memorise.
Killing The Bacteria
Boiling or heating at high temperatures (sterilisation): boiling water kills all bacteria except their spores. Higher temperatures — about 110 °C at increased pressure, as in an autoclave or a pressure cooker — kill even the spores. The canning industry heats food to kill bacteria before packing and sealing it.
Pasteurisation: this is a technique of partial sterilisation, applied usually to milk. The milk is heated to about 63 °C for 30 minutes and then chilled quickly. Pasteurisation kills a majority of the bacteria present, including the disease-causing ones. Remember that pasteurisation does not completely sterilise milk: souring bacteria may still be present in it, but by keeping the milk sufficiently cool they are prevented from multiplying.
Irradiation: radioactive radiation has been tried for sterilising certain foods without the food itself becoming radioactive, but the practice is not yet in much use. Ultra-violet light is found useful in a related way — not only for sterilising the air in schools, hospitals and factories, but also for killing mould spores in bakeries.
Removing What Bacteria Need
Dehydration (drying): most microbes cannot grow without water. Drying foods such as grains, meat, fish and vegetables is therefore an effective method of preservation. Dried milk powder is another excellent example.
Salting: salting is a common method of preserving foods such as fish, pickles and cheese for long periods. Mixing these foods with salt prevents their bio-degradation under normal conditions. Treatment with salt for preservation purposes is called curing.
Strong concentration and chemicals: jams and pickles do not get spoiled, because they have a strong concentration of sugar and of salt respectively. The increased concentration of the solutes causes plasmolysis and death of any bacteria or mould that may creep in. Preservatives such as sodium benzoate are sometimes added to tinned foods and squashes as well.
Making Conditions Unsuitable
- Refrigeration (cold temperature): microbes, both bacteria and moulds, do not multiply at or below the freezing point of water, and multiply only very slowly a little above it. Chilling milk and keeping vegetables and fruits in cold storage are popular methods of preserving them, and frozen fish or frozen vegetables keep fresh for a long time. A domestic refrigerator cools to 0–5 °C for short-term storage; deep freezing cools to −20 °C to −30 °C for long-term storage.
Notice that refrigeration does not kill the bacteria — it only holds them still. This is why food taken out of a refrigerator and left standing spoils quickly.
9.6 Bacterial Diseases In Plants And Animals
A parasitic bacterium draws its nourishment from a living host, and the host suffers for it. A small minority of bacterial species live this way, and it is this minority that causes disease.
9.6.1 Diseases In Plants
Two very common diseases of plants caused by bacteria are black rot of mustard and cauliflower, and bacterial blight of cowpea, the plant known as ‘lobia’.
In black rot the bacteria enter the leaf and spread along its veins, which darken and die, so the damage spreads inward from the leaf margin. Fig. 9.6-1 shows an affected leaf.
A diseased crop is a loss twice over: the plants that die are lost, and the ones that survive yield less. Diseases of this kind are one reason antibiotics are used for controlling plant pathogens, as section 9.2.1.4 mentioned.
Diagram
Fig. 9.6-1 — A Cauliflower Leaf Affected By Black Rot Surface view of a single cauliflower leaf; show yellowed wedge-shaped patches spreading inward from the leaf margin, with the veins inside these patches darkened. Label leaf margin, yellowed patch and blackened veins. Straight, non-crossing lead lines.
9.6.2 Diseases In Animals
Two common bacterial diseases of cattle are anthrax and bovine tuberculosis. Table 9.6 sets them side by side.
Anthrax produces swellings on the body of the affected animal, and the milk yield falls. Bovine tuberculosis affects the lungs, and an affected animal has a dry, husky cough.
9.6.3 Diseases In Humans
Some common bacterial diseases of humans are whooping cough, cholera, tuberculosis, diphtheria, typhoid, pneumonia and tetanus.
Look back through this chapter and you will find most of these names already. Penicillin is used against tetanus and diphtheria; streptomycin against tuberculosis; the TAB vaccine protects against typhoid and the BCG vaccine against tuberculosis; toxoids give immunity against diphtheria and tetanus.
That is the honest summary of the chapter so far. The same group of organisms causes these diseases and supplies most of the means of preventing and treating them. A fuller account of bacterial diseases in humans comes in a later chapter.
9.7 Bioweapons
Every property of bacteria that this chapter has described as useful can also be turned the other way. A bioweapon is a weapon that uses living organisms, or substances made by them, to cause disease deliberately.
It has come to knowledge that some countries may be preparing ‘germ bombs’, which on being exploded would simply release disease germs such as anthrax bacteria. An infection spread in this way may take epidemic shape — that is, it may spread through a large population in a short time — and cause widespread death, generation after generation.
Section 9.1.8 explains why an organism like the anthrax bacterium is spoken of in this connection. It forms spores, and spores survive drying, heat and chemicals for very long periods, so the danger does not end when the immediate event does.
The subject belongs in a Biology textbook for one reason only: the biology is the same biology. The same knowledge of how bacteria grow, spread and survive is what allows a physician to prevent an epidemic and what would allow a bioweapon to be made. What separates the two is not the science but the use to which people put it.
9.8 Summary
- Bacteria are the most primitive living organisms; each is a single cell that carries out all its own life activities.
- The genetic material of a bacterium is not enclosed in a nuclear membrane, so a bacterium is a prokaryote and is not classified as a plant.
- Bacteria occur in air, water, soil, food and on or inside the bodies of living things, including the human mouth, skin and intestines.
- Most bacteria are harmless or useful; only a small number cause disease.
- An average bacterium is about 2 micrometres long and 0.5 micrometre thick; a micrometre is one-thousandth of a millimetre.
- Bacteria are of four shapes: cocci (spherical), bacilli (rod-shaped), spirilla (spiral or twisted) and vibrio (comma-shaped).
- Cells that stay attached after dividing form pairs (diplococci), chains (streptococci) or clusters (staphylococci).
- A bacterial cell has a stiff cell wall of peptidoglycan, a thin cell membrane beneath it, and cytoplasm containing vacuoles and granules; the cytoplasm together with the bacterial chromosome forms the protoplasm.
- The genetic material lies in the central region as a single circle of DNA attached to the cell membrane; many bacteria also carry a small separate circle called a plasmid. A slimy capsule is sometimes present outside the cell wall.
- Most bacteria are moved passively by wind, water or contact; some possess whip-like flagella that pierce the wall and capsule and give active locomotion in a liquid.
- Most bacteria lack chlorophyll and are heterotrophic, either saprotrophic (feeding on decaying dead organisms) or parasitic (feeding on a living host). They secrete enzymes onto the food, which is digested outside the cell and absorbed as a solution.
- Aerobic bacteria respire using atmospheric oxygen; anaerobic bacteria need no free oxygen, and many of them are killed on exposure to air.
- Reproduction is asexual, by binary fission: the circular DNA duplicates, the cell expands, the copies separate, and the cell constricts into two daughter cells that may stay attached or separate.
- A very primitive sexual process, conjugation, occurs in a few bacteria: a plasmid passes from a donor to a recipient of a different strain of the same species through a hollow tube-like extension.
- In unfavourable conditions a bacterial cell forms a spore — its contents drawn into a rounded mass inside a thick, hard wall. Spores resist dryness, extreme temperatures and poisonous chemicals, and germinate when conditions improve. Spore formation is a means of survival, not of reproduction.
- An antibiotic is a chemical substance produced by a living microorganism that stops the growth of, or kills, some disease-producing bacteria and fungi.
- Penicillin, discovered by Alexander Fleming in 1929, was the first antibiotic used in medicine; it is the product of the mould Penicillium chrysogenum, not of a bacterium. Streptomycin was discovered next, by Selman Waksman, from Streptomyces griseus.
- A good antibiotic should act against a variety of disease-producing microorganisms, should not produce undesirable side-effects, and should not kill the normal bacteria of the host. Antibiotics are also used as food preservatives, in animal feed and against plant pathogens.
- Serum is blood plasma from which fibrinogen has been removed. Toxins are poisonous proteins released by disease-causing bacteria; antitoxins, produced in animal bodies, neutralise them.
- A serum is prepared by injecting small doses of a bacterial toxin repeatedly into a healthy horse or cow, then drawing its blood and separating the straw-coloured liquid that clears from the clot on chilling.
- Human genes introduced into bacteria such as Escherichia coli allow useful substances to be extracted from rapidly grown cultures; insulin was the first such substance, and clotting factor VIII and factor IX are used for haemophilia A and haemophilia B.
- A vaccine is a preparation of weakened germs or dead germ substances; vaccination introduces it into the body to develop resistance to a particular disease by stimulating the body’s own antitoxins. The TAB vaccine of killed bacteria protects against typhoid and the BCG vaccine of living weakened bacteria against tuberculosis.
- Toxoids are inactivated bacterial toxins that still stimulate antibody production; they give immunity against diphtheria and tetanus.
- Rhizobium in the root nodules of leguminous plants fixes atmospheric nitrogen into compounds the plant can use, enriching the soil for later crops. Free-living soil bacteria such as Azotobacter and Clostridium fix nitrogen independently.
- In nitrification, bacteria convert nitrogenous wastes and dead remains into ammonia and ammonium compounds, then into nitrites by Nitrosomonas and into nitrates by Nitrobacter; plants absorb nitrogen only as nitrates.
- Denitrifying bacteria such as Pseudomonas break down soil nitrates and release nitrogen gas to the atmosphere, completing the nitrogen cycle.
- Decay is the complete breakdown of organic matter by bacteria without a foul smell; putrefaction is incomplete breakdown with a foul smell. Both return simple compounds such as nitrates, sulphates and carbon dioxide to the soil.
- Sewage treatment plants and biogas plants use bacteria to decompose human excreta and cow dung, yielding methane-rich gas for fuel and rich manure for fields.
- Bacteria in the human intestine synthesise vitamins of the B complex group and vitamin K; in herbivores, gut bacteria digest cellulose.
- Industrial uses of bacteria include curing of tea, tanning of leather, production of vinegar, processing of coffee and tobacco, retting of hemp and flax, and curing of hides and skins.
- Bacteria spoil cooked food, milk, fruits and vegetables by decay, especially in warm weather, and may cause food poisoning; botulism results from a bacterium growing in tinned and sealed foods, which distends the can with gas.
- Food is preserved by sterilisation, pasteurisation, irradiation, drying, salting, strong concentrations of sugar or salt with chemical preservatives, and refrigeration.
- Bacterial diseases include black rot of mustard and cauliflower and bacterial blight of cowpea in plants; anthrax and bovine tuberculosis in cattle; and whooping cough, cholera, tuberculosis, diphtheria, typhoid, pneumonia and tetanus in humans.
- Bioweapons use disease-causing organisms such as anthrax bacteria deliberately, and could spread infection on an epidemic scale.
Flowchart
FC 9.9 — The Economic Importance Of Bacteria At A Glance Bacteria → useful roles → medicine (antibiotics, serums, vaccines, toxoids); agriculture (nitrogen fixation, nitrification, decay and putrefaction, biogas, vitamins and cellulose digestion); industry (tea curing, leather tanning, vinegar, retting) Bacteria → harmful roles → spoilage of food and food poisoning; diseases of plants, animals and humans; use as bioweapons
9.9 Key Terms
- Aerobic bacteria — bacteria that respire using oxygen absorbed from the air around them.
- Ammonium compounds — nitrogen compounds formed from ammonia in the soil, and the stage from which nitrites are made.
- Anaerobic bacteria — bacteria that respire without free oxygen; many kinds are killed on exposure to air.
- Anthrax — a bacterial disease of cattle marked by swellings on the body and a fall in milk yield.
- Antibiotic — a chemical substance made by a living microorganism that stops the growth of, or kills, some disease-producing bacteria and fungi.
- Antibody — a protein made by the body in response to a foreign substance, and acting against that substance alone.
- Antitoxin — a substance produced in an animal body that neutralises a particular bacterial toxin.
- Bacilli — rod-shaped bacteria.
- Bacterial chromosome — the single closed circle of DNA lying in the central region of a bacterial cell.
- Binary fission — the asexual division of one parent cell into two daughter cells, each with a copy of the genetic material.
- Biogas plant — a closed tank in which bacteria break down cow dung, releasing methane for fuel and leaving manure behind.
- Bioweapon — a weapon that uses living organisms, or substances made by them, to cause disease deliberately.
- Botulism — a serious food poisoning caused by a bacterium that grows in tinned and sealed foods.
- Bovine tuberculosis — a bacterial disease of cattle affecting the lungs, with a dry, husky cough.
- Capsule — a slimy protective layer present outside the cell wall of some bacteria.
- Cell membrane — the thin layer beneath the cell wall that controls what enters and leaves the cell.
- Cell wall — the stiff non-living outer covering of a bacterial cell, made of peptidoglycan.
- Cocci — spherical bacteria.
- Conjugation — a primitive sexual process in which a plasmid passes from a donor bacterium to a recipient of a different strain of the same species.
- Cytoplasm — the living material of the cell surrounded by the cell membrane, containing vacuoles and granules.
- Decay — the complete breakdown of organic matter by bacteria, without a foul smell.
- Denitrifying bacteria — soil bacteria that break down nitrates and release nitrogen gas into the atmosphere.
- Diplococci — spherical bacteria that occur in pairs.
- Flagella — whip-like threads that pierce the cell wall and capsule and lash to move the bacterium through a liquid.
- Food poisoning — illness caused by eating food in which harmful bacteria or their toxins have built up.
- Heterotrophic nutrition — feeding on ready-made food made by other organisms.
- Micrometre — a unit of length equal to one-thousandth of a millimetre.
- Nitrates — the form of nitrogen that plant roots absorb from the soil.
- Nitrification — the conversion by bacteria of nitrogenous wastes and dead remains into ammonia, then nitrites, then nitrates.
- Nitrites — the nitrogen compounds formed from ammonium compounds by Nitrosomonas, and converted to nitrates by Nitrobacter.
- Nitrogen fixation — the conversion of free atmospheric nitrogen into nitrogen compounds that living things can use.
- Parasitic — drawing nourishment from the body of a living host.
- Pasteurisation — partial sterilisation of milk by heating it to about 63 °C for 30 minutes and chilling it quickly.
- Penicillin — the first antibiotic used in medicine, obtained from the mould Penicillium chrysogenum.
- Peptidoglycan — the material of which a bacterial cell wall is made.
- Plasmid — a small circle of DNA in a bacterium, separate from the bacterial chromosome.
- Prokaryote — an organism whose cells have genetic material lying free in the cytoplasm, without a nuclear membrane.
- Protoplasm — the living substance of a bacterial cell, the cytoplasm together with the bacterial chromosome.
- Putrefaction — the incomplete breakdown of organic matter by bacteria, giving off a foul smell.
- Retting — the loosening of the fibres of hemp and flax by bacterial rotting of the soft tissue holding them.
- Saprotrophic — drawing nourishment from decaying dead organisms.
- Serum — blood plasma from which fibrinogen has been removed, containing antibodies and antitoxins.
- Spirilla — spiral or twisted bacteria.
- Spore — a rounded resting body with a thick, hard wall, formed by a bacterium to survive unfavourable conditions.
- Staphylococci — spherical bacteria occurring in irregular clusters.
- Sterilisation — the killing of all bacteria and their spores by heating, usually at about 110 °C under increased pressure.
- Streptococci — spherical bacteria occurring in long chains.
- Streptomycin — an antibiotic obtained from the bacterium Streptomyces griseus, used against tuberculosis.
- Tanning — the treatment that turns a raw hide into leather that does not decay.
- Toxin — a poisonous protein released by disease-causing bacteria while growing or on breaking down.
- Toxoid — an inactivated bacterial toxin that still stimulates the body to produce antibodies.
- Vaccination — the introduction of a vaccine into the body to develop resistance to a particular disease.
- Vaccine — a preparation consisting of weakened germs or dead germ substances.
- Vibrio — comma-shaped bacteria.
9.10 Points To Ponder
Spore formation does not increase the number of bacteria — one cell gives one spore, and that spore gives one cell again. What, then, does a species gain by having this ability at all, and why might it matter more to bacteria living in soil than to bacteria living inside an animal’s intestine?
The same bacteria decay a fallen leaf on the forest floor and spoil the food in your kitchen. Is being useful or harmful a property of the bacterium itself, or of the place where it happens to be working? Try to defend your answer using two examples from this chapter.
Denitrifying bacteria undo the work that nitrogen-fixing bacteria do. Imagine that every denitrifying bacterium disappeared tomorrow. Trace what would happen to the nitrogen in the soil, in living bodies and in the atmosphere over a very long period.
A serum acts at once but its protection is short; a vaccine takes time to act but its protection is long. Think of a situation in which only a serum would help, and one in which only a vaccine would, and explain what makes each situation different.
One condition for a good antibiotic is that it should not kill the normal bacteria of the host. Using what section 9.1.1 and section 9.3.7 say about the bacteria that live in us, work out what could go wrong for a person if that condition were ignored.
Refrigeration does not kill bacteria; it only stops them multiplying. What does this tell you about the difference between keeping food fresh and making food safe to eat? Which of the seven preservation methods do both?
9.11 Objective Questions
- Choose the correct option. (1 mark each)
- The cell wall of a bacterium is made of:
- cellulose
- peptidoglycan
- chitin
- glycogen
- Spore formation in bacteria is best described as:
- a method of reproduction
- a method of nutrition
- a way of surviving unfavourable conditions
- a way of moving from place to place
- Nitrites in the soil are converted into nitrates by:
- Nitrosomonas
- Nitrobacter
- Rhizobium
- Pseudomonas
- The BCG vaccine is prepared from:
- killed bacteria
- living weakened bacteria
- inactivated toxins
- antitoxins obtained from a horse
- Pasteurisation of milk:
- kills all bacteria together with their spores
- kills a majority of bacteria, including the disease-causing ones
- kills only the spores
- removes fibrinogen from the milk
- The cell wall of a bacterium is made of:
- For each pair below, select the correct option from (a) to (d). (1 mark each) Options:
- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
- Assertion (A): A serum protects a person almost immediately after it is given. Reason (R): A serum supplies ready-made antitoxins produced in the body of another animal.
- Assertion (A): Jam kept in a closed jar does not spoil easily. Reason (R): The high concentration of sugar causes plasmolysis of any bacterium that enters it.
- Assertion (A): Bacteria are not classified as plants. Reason (R): Bacteria have no chlorophyll of any kind.
- Match the items in Column A with those in Column B. (5 marks)
| Column A | Column B |
|---|---|
| a. Rhizobium | i. source of streptomycin |
| b. Streptomyces griseus | ii. denitrification |
| c. Penicillium chrysogenum | iii. root nodules of leguminous plants |
| d. Pseudomonas | iv. protection against typhoid |
| e. TAB vaccine | v. a mould that yields an antibiotic |
- Fill in the blanks. (1 mark each)
- Spherical bacteria that remain joined in long chains are called ____.
- The small circle of DNA that is separate from the bacterial chromosome is the ____.
- The incomplete breakdown of organic matter by bacteria, giving off a foul smell, is called ____.
- Inactivated bacterial toxins used to give immunity against diphtheria and tetanus are called ____.
- State whether the following are true or false. (1 mark each)
- A bacterial cell has its genetic material enclosed in a nuclear membrane.
- Penicillin is obtained from a bacterium.
- Plants absorb nitrogen from the soil mainly in the form of nitrates.
- Refrigeration kills the bacteria present in food.
- Name the following. (1 mark each)
- The shape of a bacterium that is described as comma-shaped.
- The whip-like structure that gives a bacterium active locomotion.
- The process by which a plasmid passes from one bacterium to another.
- The blood plasma left after fibrinogen has been removed from it.
- The gas, mainly methane, produced when bacteria act on cow dung in a closed tank.
- Pick the odd one out and give a reason for your choice. (2 marks each)
- Rhizobium, Azotobacter, Clostridium, Nitrobacter
- Salting, drying, refrigeration, sterilisation
- Cholera, typhoid, black rot, pneumonia
9.12 Check Your Concepts
Define nitrogen fixation, and name one bacterium that carries it out. (2 marks)
Give a reason: penicillin is described as an antibiotic, yet it is not a product of a bacterium. (2 marks)
State two ways in which bacteria living in the human body are useful to us. (2 marks)
Why is spore formation in bacteria not regarded as a method of reproduction? (2 marks)
Differentiate between decay and putrefaction, stating the basis of difference. (3 marks)
Explain, with an example, what is meant by saprotrophic nutrition, and state how such a bacterium takes in food that is solid. (3 marks)
State the three criteria that a good antibiotic should satisfy, and explain why the third of them matters to the patient. (3 marks)
Differentiate between a serum and a vaccine, stating the basis of difference in each case. (4 marks)
Describe binary fission in a bacterium, and explain why bacteria such as streptococci occur in chains. (4 marks)
Why does cooked food kept outside spoil much faster in summer than in winter? Explain in terms of what bacteria do to the food. (4 marks)
Explain how nitrogen from the air reaches the proteins of an animal’s body, naming the bacteria responsible at each step. (5 marks)
Describe any five methods of food preservation, stating in each case how the method prevents bacteria from acting on the food. (5 marks)
What would happen to a forest over many years if all the bacteria of decay and putrefaction in its soil were destroyed? Give your answer as a chain of consequences. (5 marks)
9.13 Figure-Based Questions
Study Fig. 9.1-2, which shows the structure of a bacterial cell. (4 marks)
- Name the layer that lies immediately outside the cell membrane, and state what it is made of.
- Name the part in which the genetic material of the cell lies, and state one way in which it differs from the nucleus of a plant cell.
- Name the small separate circle of DNA shown in the figure, and state one use humans have made of it.
- State one function of the outermost slimy layer where it is present.
Redraw Fig. 9.1-4, which shows a bacterium forming a spore inside itself, and label the spore wall and the bacterial wall. State one condition under which such a structure is formed. (3 marks)
Fig. 9.2-1 shows a culture plate on which a mould has grown by accident. (3 marks)
- Name the clear region seen around the mould colony, and state what has happened there.
- Name the scientist who first reported this observation and the substance he obtained from the mould.
- State what this observation proved about the mould.
Study Fig. 9.3-2, which represents the nitrogen cycle. (4 marks)
- Name the process by which nitrogen gas from the atmosphere enters the soil through the roots of a leguminous plant.
- Name the process by which nitrogen returns from the soil to the atmosphere, and name one bacterium that brings it about.
- Name the form in which plants take up nitrogen from the soil.
- State what would happen to the cycle if the step marked as decay of living matter did not occur.
Fig. 9.3-1 shows the roots of a leguminous plant. Identify the structures seen along the roots, name the bacterium found inside them, and state one benefit to the crop that is sown in that field afterwards. (3 marks)