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So.

chemiotaxis is said to be positive when the living cell approaches, and negative when it recedes from the chemical substance. Asparagin, peptone, and oxygen are strongly chemiotactic, but free acids and alkalies negatively

Alcohol is despised by bacteria. Chemiotaxis is not confined to the bacterial cell, but is also exhibited by leucocytes. It is in virtue of this property that the latter move towards the focus of production of toxins, and try to envelop the invading bacteria. If, however, the leucocytes are repelled, the infective agents grow unchecked, and death may follow as a consequence.

4. Chemical.-In order to render nutrient media more suitable for their absorption, bacteria resort to a peculiar device. They secrete ferments of various kinds, e.g., proteolytic (transforming albumin into soluble substances), diastatic (converting starch into sugar), emulsifying, coagulating or fat-splitting. The action of these ferments, and the metabolism of the bacterial cells, give rise to various phenomena, the more important of which may be arranged in four great groups.

(i.) Fermentation.

(ii.) Putrefaction, or putrid fermentation," is similar to fermentation, except that instead of occurring in carbohydrates, it takes place in dead nitrogenous material. The albumin is first peptonised, and then further broken up into such substances as carbon dioxide, sulphuretted hydrogen, hydrogen, nitrogen, methane, butyric acid, indol, skatol, etc.

Sometimes the innocuous albumin is changed into distinct animal alkaloids called ptomaines. These may be formed outside the body, and become a source of danger when ingested with food. Poisoning with ice-cream or meat is frequently due to the presence of these substances.

The malodorous, products of putrefaction are chiefly produced by anærobic bacteria, and are consequently most pronounced in situations where the supply of oxygen is limited or cut off. When a dead body is exposed to the air the anærobes migrate from the intestine, which is their normal habitat, and invade the tissues in all directions. They are, no doubt, favoured in their growth by the simultaneous presence (symbiosis) of the ærobes, which, while carrying on the surface decomposition, consume the oxygen and create an (anærobic) atmosphere so favourable to the continued development of the anærobes.

The organisms concerned in putrefaction are mostly facultative anærobes, of which the following are best known: Proteus vulgaris, proteus Zenkeri, bacillus fluorescens liquefaciens, bacillus coli communis, bacillus saprogenes.

(iii.) Chromogenesis.---An historical interest is attached to this property, as it was observed so long ago as the Middle Ages. The moist consecrated wafers left on the church altar were found the next day to be covered with little blood-red drops. This was supposed to foreshadow some dire calamity, and much capital was generally made out of this “ miracle”. It is now known, however, that this colour is produced by the growth of a common organism—the bacillus prodigiosus. It grows best at 25° C. on boiled rice or potato, and produces a red pigment which is insoluble in water, but soluble in alcohol. Owing to the ready production of pigment, the organism is frequently employed in various bacteriological investigations. It is also suitable for the study of the mutability of species. By repeated cultivations on faintly acid media the cells lose their globular shape, and become actively motile rods or threads. Again, by growing the bacillus at 38° C. a variety may be obtained which no longer produces pigment.

aureus.

In addition to red, various other colours may be formed by bacterial activity. Thus, violet colouring matter is formed by B. violaceus ; green or blue by B. pyocyaneus or B. fluorescens ; and yellow by staphylococcus pyogenes

A yellow coloration is also produced by sulphurbacteria, which decompose H,S, and store up the sulphur granules in their substance. Similarly, iron-bacteria oxidise the iron carbonates, and the oxide thus formed is deposited in the bacterial cell, causing it to assume a reddish-brown colour.

The pigment formation is best seen on solid media grown in the presence of air, away from sunlight, and at a low temperature. Chromogenic bacteria are frequently associated with putrefactive processes, and give rise to beautiful rainbow-like tints in putrescent substances.

(iv.) Toxin production.—Like many higher plants (e.g., abrus precatorius, ricinus communis) bacteria secrete poisonous substances, which represent the specific action of the corresponding organism. They will, however, be more conveniently dealt with at a later stage.

THE PHENOMENA OF PUTREFACTION.

It may generally be said that the function of bacteria is to return all organised beings to earth. Every form of dead organic matter is immediately brought under their influence, and broken up into its elementary constituents. It is for this reason that fallen trees and dead bodies are cleared away from the ground, and make room for the succeeding generation. Thus far the microbes are our best friends.

But unfortunately their activities proceed further, for in the search for food they often prey upon living beings. When microbes attack living tissues they produce the manifestations of disease; but when the dead organic matter is attacked we have the phenomena of putrefaction and decay. At present, however, we are only concerned with the latter.

Bacteria as Scavengers.-If we examine a fallen tree in a forest it will be observed that it remains apparently unaltered for some time. Then it gets softened into a friable substance, which finally crumbles into a brownish powder and is incorporated with the soil. How are these changes brought about? The hard woody substance is first softened by various fungi which grow into it; next woodeating insects appear; and finally bacteria complete the decomposition. A similar dissolution is observed in the case of a dead

animal, but with this difference, that the agency here is chiefly bacterial.

The importance of these cleansing processes can hardly be over-estimated. Without bacteria the surface of the earth would be encumbered with dead organic matter, and there would be no room for vegetable and animal life. Microbes are the important agents of universal hygiene. They clear away the remains of all that has had life. Without them life would be impossible, for death would be incomplete.

The Nitrogen Cycle. But the microbes do not merely scavenge and cleanse the surface of the earth. They do much more—they elaborate this material for further use. It is well known that plants are constantly being eaten by animals, and the latter in turn again by plants. But dead animals cannot be assimilated by plants unless they are brought into a proper condition by bacterial agency. Were it not so, owing to the constant loss of food-stuffs from the soil, vegetation would cease, and even animal life would become impossible.

It must not, however, be supposed that any new material is created by the micro-organisms concerned. It is the same food which is used over and over again, first by plants, then by animals, and then again by plants. This circulation of matter lies at the root of the continuation of our food supply, and, as we shall see presently, is largely brought about by the activity of microbes. A closer study of this question is pertinent to our present inquiry, not only on account of the profound interest of the subject itself, but also because it throws interesting sidelights on the behaviour and general biology of bacteria.

Plants obtain their food partly from the atmosphere and partly from the soil. Of the food-stuffs obtained from

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