BACTERIA AND PLANT GROWTH

DECAY is one of the first necessities of life – without it life would soon cease. The world’s surface would become covered with a vast collection of dead bodies and the amount of food available for the plants and animals that still lived would become less and less until finally none would remain. This is one explanation of the great importance of Bacteria to all other forms of life.

When the milk turns sour – when the joint gets high – when there is a bulge in the tin of canned fruit – when the patient suffers from pneumonia or influenza – the cause is to be found in the growth and activities of these minute organisms.

No sooner are animals and plants dead than Bacteria set to work to bring about their destruction. Proteids, carbohydrates, and fats are all broken up. The final products of the destruction are nitrates, carbon dioxide, water, sulphuretted hydrogen, marsh gas, and hydrogen.

To understand Bacteria, however, they must be looked at from their own point of view, rather than entirely from the point of view of other living things whose lives they affect. Bacteria themselves are not concerned in being either public benefactors or public scourges – they are simply minute living organisms that require food and energy for their own growth and for the reproduction of their species.

An abundant growth of Bacteria can be obtained-in a few days by chopping up some hay, putting it in water, and then leaving it to stand in a warm place. Soon a gelatinous film forms on the surface. This is due to the growth of innumerable Bacteria, whose jelly-like outer coverings cohere to form a slimy mass. On examination of a drop of the water under the microscope tiny rod-like microorganisms are seen .

Without microscopes of higher power than those usually available in schools it is impossible to investigate the actual structure of Bacteria.

They are very minute, colourless, unicellular plants, and are among the smallest of known organisms.

They are classed as plants because they take in food in the liquid or gaseous form, generally the former. They have no chlorophyll, but some of them are able to live on inorganic substances, as the green plants do, manufacturing glycogen, a substance that is allied to starch and which is always present in their protoplasm. Usually, however, they feed on highly organised plants and animals.

There is a very simple group of water plants termed the Cyanophycew, or Blue-green Algae, with which the Bacteria appear to have affinities. The two groups are alike in their simplicity of structure, in their general life-history, and in their method of division by fission. It may be that life first mani- fested itself in some such simple forms as these. The primi- tive forms of life must have lived in water and probably they obtained energy in different ways, some directly from light, others from oxidation or other similar chemical process.

Bacteria vary in form, being spherical, rod-like, filamentous, or spiral.

The spherical forms are dis- tinguished as cocci or micrococci ; rod-like forms are termed bacilli ; a slightly undulate form is named vibrio – spiral forms are termed spirilli .

All the forms may break up into spherical units or micro- cocci. Micrococci generally multiply very rapidly by fission.

The rod-like individuals may be joined end to end to form long filaments – the cocci may form chains or masses .

Notwithstanding their affinities with plants, bacterial cells are not bounded by a cellulose wall. The limiting layer appears to be of a pro- tein substance.

This layer produces a jelly-like film round each individual. It is by this film that in- dividuals are held to- gether, thus producing the filamentous and chain-like forms.

Sometimes the jelly-like films swell greatly, so that a large mass of jelly-like substance is produced as a result of the fusion of the layers of the individual units. When in this state described as being in the zooglwa phase.

The protoplasm within the cell is not clearly differentiated into nucleus and cytoplasm. The whole contents pick up nuclear stains. It is therefore probable that the material which is usually included within the nucleus in cells of higher plants ,is here diffused throughout the whole protoplasm. At the same time it must be remembered that the cells are so extremely minute that detailed investigation is most difficult.

Bacteria are frequently motile. Movement is brought about by the lashing of cilia . These may cover the whole surface, or be restricted to the ends, or to one end only, or there may be only a single cilium .

It is not possible to see the cilia in the living specimen, as they are so transparent, but the move- the Bacteria are ment of the Bacterium, which is due to their action, can be watched. They are made visible in the dead organism by deep staining.

Bacteria reproduce by fission with amazing rapidity. Fission may occur as frequently as once every twenty or thirty minutes ; consequently one individual, by dividing once every half-hour, will produce over a million descendants in less than ten hours ! Rate of multiplication depends on amount of food and suitability of temperature. When conditions are adverse another method of reproduction is resorted to in some species. This is the method of spore formation. The contents of the bacterial cells become rounded off and a thick resistant wall is formed within each mother-cell wall. In time the wall of the parent cell dissolves and the spore is free. Only one spore is formed from each cell, therefore this is not a method of multiplication of individuals. It is really a resting stage during unfavourable conditions. The spores are very resistant. They can withstand a nigh temperature and a considerable degree of desiccation. As a matter of fact, a bacterial spore is the most resistant form of living protoplasm known. In this form Bacteria are dispersed far and wide. They are carried on air currents, and under suitable conditions may germinate far from their place of origin.

No method of conjugation or of sexual reproduction has been observed in any species of Bacteria.

Bacteria are almost ubiquitous. They occur in every con- ceivable situation. Both air and water are alive with them – they do not, however, float long in the atmosphere but tend to accumulate on all surfaces. They gather round the bases of cut flowers in water, and thus prevent the water from rising up the stems ; hence the advisability of cutting off the ends of the stems each day. They do not like high altitudes, and mountain air is almost devoid of them. In a comparison of the air at the top of St. Paul’s Cathedral with that of the churchyard below, Bacteria were found to be present in a ratio of I to 10.

The discovery of Bacteria in hailstones, however, proves that they do float upwards to a considerable height. Their growth is affected by light, heat, oxygen, water, food, and various chemical substances.

Most Bacteria grow well in feeble light. Direct exposure to sunlight is harmful to them. For this reason water that is to be used for drinking is exposed to sunlight in open reservoirs. The blue, violet, and ultra-violet rays are the most destructive to them.

The amount of heat required by Bacteria is very varied.

For common species the minimum temperature is ° – Io° C., the optimum 25°-35° C., and the maximum 42°500 C. Many species thrive at much higher temperatures than these, but the majority are killed if the temperature is raised to about 55° C. All, when in the active condition, are killed by boiling water.

On the other hand, Bacteria can resist very low temperatures.

At o° C., the freezing point of water, activity is suspended, because the activity of protoplasm itself is checked at this temperature. But the Bacteria are not killed, and they resume activity when the temperature rises.

In the spore stage Bacteria are much more resistant. Bacterial spores can even withstand prolonged boiling.

The chief interest of Bacteria is physiological and is bound up with their methods of obtaining food and energy.

In most living things energy is liberated by the oxidation of substances that have been absorbed as food. Oxidation is a form of combustion. In the combustion of coal, carbon is oxidised into carbon monoxide and carbon dioxide, with liberation of energy in the form of heat and light. Oxidation in living organisms takes place at ordinary temperatures and is termed respiration. Respiration, then, is a process by which energy is made available and which requires a constant supply of oxygen.

A large number of Bacteria must have free oxygen for respiratory purposes – some can do without oxygen for a time – others, again, cannot live in its presence. The latter are anaerobic. Those that need oxygen are aerobic and get available energy by respiration, that is, by oxidation either of their own tissues, or of the tissues of the host on which they live.

All Bacteria must have some supply of energy in order to carry on their activities. When this supply results from the destruction of a host without oxidation the process is termed fermentation, and has been described as lire without oxygen. Recently this term has been extended to certain manufacturing processes in which oxygen plays some part.

When fermentation of proteid substances takes place it is accompanied by the evolution of offensive gases. Such fermentation is putrefaction.

Putrefaction of proteid substances is brought about by a series of different Bacteria, each being responsible for one step of the process. The bad smell which results when organic remains are putrefying, as, for example, in a heap of rotting leaves, can be removed by letting in air. The evil-smelling compounds are oxidised into non-smelling substances.

The vast majority of Bacteria feed, as animals do, on highly organised substances, such as proteids and carbohydrates. They live either as parasites or saprophytes . They act on the food substances very much as do the enzymes in the alimentary tract of animals. They digest, then absorb, the food.

While all animal life is infested with parasitic Bacteria, living plants are almost entirely free from them. Those that occur in the root-tubercles of many members of the Leguminosm are symbiotic rather than parasitic .

Some few Bacteria are able to build up their food as green plants do, using simple inorganic substances. Some of these use the carbon dioxide of the atmosphere in the manufacture of carbohydrates.

The micro-organisms that are able to build up their own food from inorganic materials are the Sulphur, Iron, and Nitrifying Bacteria. As they contain no chlorophyll they cannot obtain energy directly from sunlight. They obtain the necessary energy, therefore, by other means : (1) The Sulphur Bacterium gets the energy for food-building by oxidation of sulphuretted hydrogen (H2S), that is present in ponds and marshes : 2H2S + 02 2H20 4- S2.

The sulphur that is produced in this reaction is stored in the protoplasm of the Bacterium in the form of granules. The vast deposits of sulphur that occur in Sicily are due to the activities of Sulphur Bacteria. (2) The Iron Bacteria are found in stagnant pools that contain soluble bicarbonate of iron. In such pools sulphuretted hydrogen is always present – when this interacts with the iron salt, iron sulphide is produced. The Bacteria get the energy they need by oxidising this iron sulphide, the process resulting in a brown deposit of ferric hydroxide. (3) The Nitrifying Bacteria have already been described . Of these, one obtains energy by oxidising ammonium salts to nitrites, and the other by oxidising the nitrites to nitrates.

There is a small group of Bacteria that is able to make use of the free nitrogen of the atmosphere. The way in which they get their energy is obscure. Those that live symbiotically in the root-tubercles of many Leguminosw have a similar power, but they, of course, may derive energy from the breaking down of the carbohydrates of the plant with which they are associated.

All these forms are little understood. Their interest lies in their power of building up food substances from simple inorganic materials. This is a power that makes them independent of all other living things and suggests their possible relationship to the first formed organisms of the world. Those among them that are able to fix free nitrogen play a most important part in adding to the available supply of nitrates in the soil.

One small group of Bacteria undoes, to some extent, the useful work of Nitrobacter, which oxidises nitrites to nitrates. This is the group of the de-nitrifying Bacteria. Their effect seems to be entirely harmful. When they are in a state of oxygen-poverty they get the gas by reducing nitrates to nitrites, with evolution of free oxygen.

Water s absolutely essential for the growth of Bacteria, as it is for all living things. Spores, however, survive great desiccation.

Bacteria are killed by the presence of a number of chemical substances. Among these are boracic acid, carbolic acid, and iodine. For this reason these substances are used as antiseptics .

The metabolic processes of the Bacteria are often associated with great heat and light. Spontaneous combustion may occur in a hayrick that has been made while the hay was damp, thus giving a suitable environment for bacterial growth.

Many marine Bacteria are phosphorescent. Bones and scraps of meat lying on dust-heaps near the sea get infected and shine at night with a phosphorescent light. Even the meat in the butchers’ shops may glow in the same way.

So far Bacteria have been studied from their own standpoint as living organisms needing food and energy for their growth and multiplication. But because of their saprophytic, or parasitic, mode of life, they are responsible for many changes which affect the world in general.

It was the great French chemist, Pasteur, who discovered that fermentation and putrefaction were brought about by living organisms. Many manufacturing processes depend upon fermentation. This is true of the . manufacture of cheese from milk and of vinegar from alcohol.

The decomposition of cellulose which is such an important constituent of plants is brought about by Bacteria. The process is accompanied by evolution of marsh gas (CH4) and sulphuretted hydrogen (H2S). In a pond where there is a large amount of vegetable refuse, a smell of sulphuretted hydrogen can always be detected, and bubbles of marsh gas are liberated by stirring up the decaying matter with a stick.

The higher animals do not produce a special enzyme for the digestion of cellulose, although so much is eaten by herbiVorous creatures. They rely on the co-operation of Bacteria. These cellulose-digesting Bacteria occur in large numbers in the complicated stomach of the ruminants and in the intestine of the horse. Here there is very little available oxygen, and much of the cellulose remains undigested. The rest is only partially oxidised and a good deal of marsh gas (CH4) is formed.

In man cellulose is not digested, but it plays a useful part in preventing constipation.

It is thus evident that Bacteria are in part responsible for the circulation of carbon in nature. The green plant absorbs carbon dioxide and water and builds up carbohydrates, including cellulose. The cellulose-destroying Bacteria act on the cellulose, breaking it up into carbon dioxide, marsh gas, and water.

A large number of the diseases of man and the lower animals are due to parasitic Bacteria. Some of these live naturally in soil containing organic material – it is only occasionally that they enter the bodies of animals. These are partial parasites. Others live entirely in animals and are termed complete parasites.

Since parasitic Bacteria carry out all their metabolic processes within the host it is to be expected that the host will be greatly influenced as a result. The Bacteria may be perfectly harmless to their hosts. They are harmful when they produce poisonous secretions, or toxins .

Such toxins would invariably kill the hosts in time were it not for the production of antitoxins in the blood and the activity of phagocytes . Contagious diseases are due to complete parasites.

Bacteria that cause infectious diseases are able to live for a time outside the body. They are carried in the air, and it is for this reason that the disease spreads, or is caught. Sometimes the germs are carried by an individual who is merely a carrier. Such a person does not suffer himself from the dis,ease, but may cause its outbreak in any place he visits.

The soil is the normal habitat of some Bacteria, and they only enter the human body occasionally. The tetanus bacillus (Lockjaw) is of this type. It occurs in all soils that are manured with horse or cow-dung. The Bacterium lives in the intestines of these animals, generally without doing them any harm, and the spores can pass long resting periods in the soil. It is for this reason that it is so important to treat with iodine any scratch received while gardening.

Bacteria of this class can only get into the body by means of a scratch or wound. They cannot penetrate the skin, but, having once gained an entry, they get into the blood-stream and there produce the toxin. The Bacterium which causes hydrophobia is introduced with the saliva on the teeth of a mad dog. Insect bites of various kinds are responsible for the entry of Bacteria that produce plague and other diseases .

The understanding of the part played by Bacteria in entering the body through a wound is due to the work of Joseph Lister, afterwards Lord Lister. He was the first to treat wounds with substances that kill Bacteria. Such substances are termed antiseptics. In these days preventive or aseptic methods are adopted in all surgical operations – all instruments that are used are carefully sterilised to guard against bacterial infections.

An understanding of the universality, the persistence, and the activities of Bacteria, makes it clear that the greatest care must be taken to prevent their access to food. There are various ways of accomplishing this end. 41 A dry temperature of 16o° C. for half an hour, or of 180° C. for ten minutes, is enough to kill all Bacteria, including the spores.

To sterilise a liquid it should be gently boiled in a flask for several minutes with a plug of cotton-wool in the neck. All active Bacteria are killed and the plug prevents the entry of others as the flask cools. If, however, there were Bacterial spores in the liquid these may have survived. It is well, therefore, to ensure complete sterilisation by boiling two or three times in succession. Between each boiling the flask should be left in a warm place so that the spores may germinate and thus lose their resistant qualities.

Milk can, of course, be sterilised by boiling. This process, however, rather spoils its flavour. Milk supplied to our large towns is now frequently pasteurised. To this end the milk is kept at a temperature of 68° C. for twenty minutes, as it has been determined that this is sufficient to kill all the Bacteria.

To rid water that is to be used for drinking purposes of Bacteria, it is passed through several feet of sand or a mixture of sand and charcoal. A film forms on the surface, of the sand and this catches the Bacteria. Alum or some other uninjurious substance is sometimes used to produce the film artificially.

There are many ways in which food may be preserved from infection. The following are the chief : (1) By cold. Food may be kept in a refrigerator. Bacteria are not thereby killed, but their action is checked. (2) By preventing access of air. This is the principle underlying the preservation of eggs in water-glass. The method is perfectly satisfactory if the eggs are free from infection at the start. (3) By drying. As a rule Bacteria do not grow if the water content falls below about 25 per cent. Dried peas and lentils can be kept indefinitely. So, too, can dried fruits. (4) By heat. This is the method used in the canning of fruit and vegetables. The cans are heated and then hermetically sealed so that no Bacteria can enter after the heating. Although this method is generally satisfactory, there is always the danger that some resistant spores may have survived the heating process. (5) By the action of preservatives. A common preservative is salt, which is used for fish, pork, etc. Salt does not kill ‘Bacteria, but their growth is checked by it. Another preservative in general use is sugar, which is used to preserve fruit in the making of jam. It is used, too, in the preparation of condensed milks. Many chemical substances are used as preservatives, such as formalin, boracic acid, and salicylic acid. As these are often extremely harmful, especially to young children, the law requires their notification when they are used commercially.

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