These are minute unicellular organisms found practically everywhere, in the air, water, soil, and in and upon other living organisms. Some bacteria are motile in water, swimming by means of flagella or cilia. The spirilla worm their way by a corkscrew-like motion.


Each bacterium is a small mass of protoplasm covered by a firm membrane. There is no visible nucleus.


Bacteria have no chlorophyll and cannot carry out photosynthesis. They therefore feed on organic food either dead or alive. Those which feed on dead matter are called saprophytes ; those which live on other living organisms, termed hosts, are called parasites. They secrete enzymes into the surrounding food, where extracellular digestion occurs, the products of digestion diffusing into the cell to be used for growth, respiration and reproduction. Some bacteria can make part of their food from simpler substances, using as energy not light but that obtained through certain chemical reactions.


Some bacteria use oxygen for respiration and are called aerobic bacteria ; others do not, but release energy by a chemical reaction, e.g. the lactic acid bacterium which sours milk, converts milk sugar into lactic acid. These latter are called anaerobic bacteria.


In suitably warm conditions bacteria grow and multiply rapidly by binary fission, often as frequently as once every half-hour. A single bacterium can in this way give rise to 2 bacteria in twelve hours, over 17^ million, in fact. In adverse conditions bacteria become dormant, and in some species form highly resistant spore-like bodies.

Historical Note

Louis Pasteur was the first man to demonstrate that living matter did not arise spontaneously from dead matter. He showed that if beef broth was sterilized by heating and then protected from the dust of the air, it did not decay as it normally did when left exposed to the air. This he did as follows :—

After placing the broth in a flask he softened the glass neck by heating it and bent it downwards. The broth was then boiled for some time to kill all living things in it and then left. By this arrangement air could reach the broth, but dust could not setde into it. Since the broth did not decay, Pasteur concluded that the bacteria and other forms of life, e.g. fly maggots, which appear in decaying broth, arose from previously existing living forms in the dust in the air and not spontaneously from the broth itself.

Preservation of Food from Bacterial Decay

From Pasteur ’s work originated the practice of preserving food by heating it to kill any bacteria present and then sealing it while still hot in airtight tins. ’Bottling ’of fruit is a similar practice. Previous to this time meat could only be preserved by salting it. Milk and fruit juices, which would be spoilt by boiling, are pasteurized, I.e. heated to a temperature of 70 to 75 C. for about twenty minutes. This is sufficient to kill most active bacteria, though not their spores. Food can also be preserved by refrigeration, since bacteria cannot carry on active life at low temperatures, or by desiccation.

Bacteria are usually killed by exposure to strong sunlight and also by acids—hence vinegar is sometimes used as a good preservative, as in pickles. Silage is preserved by the acids produced by certain bacteria during the fermentation produced in the storage silos. Sulphur dioxide is often used for preserving acid fruits. About the middle of last century, when the connection between disease and bacteria was recognized, mainly owing to the advocacy of Lord Lister, began the practice of using antiseptics and disinfectants to kill bacteria in medical work such as the dressing of wounds and in the performing of operations. These, by killing pathogenic bacteria in the neighbourhood of wounds, prevented to a large extent the mortality due to blood poisoning, which once so frequently followed so-called sucoessful operations. Nowadays surgeons endeavour to exclude bacteria as much as possible by employing ’aseptic ’conditions in the operating room, sterilizing all instruments, dressings, etc., before use.

Bacteria which are Useful to Man

The great work of bacteria in the world is to clear up all dead matter, converting the combined elements in it into simple compounds, e.g. C02, H20 and NH3. These elements thus become available for conversion into food by green plants. By decay of organic matter in the soil the fertility of the soil is maintained, and since Man and his domestic animals are dependent for food on crops, bacteria are thus essential for the well-being of Man. Bacteria which produce decay are called putrefying bacteria. Included in the work of maintaining soil fertility is that of the nitrifying and nitrogen-fixing bacteria.

Special Cases

Bacteria responsible for producing flavours in butter and in cheese.

Retting of flax by bacteria to obtain linen.

Bacteria employed in making silage.

Production of vinegar by bacteria from weak alcoholic solutions.

Bacteria which are Harmful to Man

Putrefying bacteria can be harmful when they act on food, especially since some of them produce poisonous substances which cause food poisoning.

Bacteria which are parasitic on man and other animals produce disease. Generally the symptoms, e.g. high temperature, muscular weakness, headache, are not produced by the bacteria directly but by their poisonous waste substances known as toxins. These circulate in the blood and often produce effects which are far more dangerous and widespread than the actual damage to the tissues caused by the feeding of the bacteria. In tetanus the bacteria are found infecting the tissues in the neighbourhood of a cut or a scratch only, but their toxins produce widespread rigidity of the muscles, including those of the face.

Bacteria gain entrance to the body through the delicate linings of the nose, throat, lungs and alimentary canal, and also through cuts and wounds and dead hair roots.

The body has means of defence against invading bacteria. The white corpuscles of the blood and lymph feed on bacteria, while antibodies are produced in the blood which either kill the bacteria or neutralize their toxins. After a sucoessful resistance to the attack of a particular disease, a reserve of antibodies remains and confers an acquired immunity against further attacks for a longer or shorter period, varying with the disease and the individual. Immunity acquired towards one disease does not mean, however, immunity against others.

Means of Combating Disease

Prevention of infection. This may be done by isolation of infected persons, and by killing off or reducing the numbers of bacteria, e.g. blood poisoning can be prevented by the use of antiseptics or by the use of aseptic methods in surgery.

Sterilization of food and water likely to carry disease-producing bacteria. Milk is pasteurized to kill off disease-producing bacteria in it, particularly those of tuberculosis. Drinking-water is sterilized by means of chlorine to prevent infection by waterbornc diseases such as typhoid and dysentery. Exposure to direct sunlight also kills germs in open reservoirs.

By attention to sanitation and general cleanliness.

Maintenance of bodily health by good food, exercise and sufficient rest and sleep to stimulate natural resistance to disease. Sunbathing in moderation and with suitable precautions against sunburn is a valuable aid.

Artificial immunity produced by vaccination and inoculations, e.g. diphtheria immunization.

Food inspection by the Health Authorities and measures to exterminate carriers of disease, e.g. flies and rats. Flies carry disease germs, including those causing infantile diarrhoea. Black rats carry plague germs.

The use of antibiotics. These are substances excreted by certain fungi which are capable of destroying bacteria within the human body without materially causing harm to the body itself. The two best-known examples are Penicillin and Streptomycin. Injections of Penicillin have proved to be very efficacious in cases of ’blood poisoning. ’

S. The use of drugs popularly known as M & B which inhibit the multiplication of bacteria in the body and so assist the natural defences of the body to overcome them.

Practical Work on Bacteria

Examine specimens of water in which beans, hay and horse dung have been decaying, under the high power of the microscope, and observe motile and non-motile bacteria and their shapes.

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