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cultivated, has considerable bearing on their development, different species showing a somewhat marked preference for certain percentages of alkalinity-in fact, different races even of a given species of organism will show differences when grown on media containing slight differences in reaction, so much so that it is often possible to pick out a given race by these means. It follows, therefore, that in all practical bacteriological work definite and careful methods of standardisation should be adopted (see chap. 4).

Food Supply. The rôle of bacteria in nature is the breaking up of the complex chemical compounds of the bodies of plants and animals with the release of the chemical constituents so that they may be again recombined and utilised in building up fresh living things. A small section of the vast number of existing bacterial species have become so modified in their mode of life that they are able only to exist in the bodies of animals or plants, and developing in such situations initiate pathological changes with various symptoms peculiar to special diseases. It follows, therefore, that the food material required by one species is not always adapted to the development of another; in some cases-such for instance as the diplococcus of pneumonia, or the gonococcus-the organism can only be grown at first upon a medium smeared with fresh blood; but even these refractory organisms in time adapt themselves to their new environment, and may be cultivated upon the ordinary laboratory media. The greatest number of bacteria, however, are remarkably adaptable, and may be cultivated upon what are termed “artificial media.”

An attempt is always made to reproduce, as far as possible, the natural food condition enjoyed by bacteria; but to do so exactly is generally impossible owing to the complex nature of the natural food stuffs, and moreover to the frequent presence of organisms other than the special one it is sought to isolate. In a certain number of cases, however, the artificial cultivation may present better opportunities for the growth of a given organism than is possible in its natural habitat, many organisms excreting bodies harmful to other species and also to their own development. Two organisms therefore which, when growing together, exhibit mutual antagonism, may individually grow more easily when separated in pure culture in artificial media.

Bacteria do not always antagonise one another, and many cases are known where the presence of one species of bacteria actually

assists the development of another. Thus in the well-known fermentation that takes place when the juice of grapes is expressed furnishes an excellent example. The crude wine-must when it comes first from the press contains a large and varied flora, amongst which are yeasts. These yeast forms finding the surroundings especially fitted for their development ferment the sugar present to alcohol and CO, until the alcohol reaches a certain percentage when they are unable to develop further. Another series of organisms now comes into play, contained like the yeasts in the original winemust. These organisms attack the alcohol and change it to acetic acid, and as the alcohol becomes used up, cease their activity and give place to a third series, which having a special taste for acid solutions were unable to develop before their particular food was obtainable. As a result of their growth the acetic acid is fermented to CO, and water, and the reaction of the medium becomes again neutral or faintly alkaline. The way is thus prepared for the putrefactive organisms which have gained access from the air, or from the original grape skins; these bacteria change the remaining proteid matters into CO, water, nitrogen and various evil-smelling gases that generally accompany putrefaction. Such a cycle is the common phenomenon in most spontaneous decompositions. The alcohol stage may be and often is omitted, direct change of carbohydrate into acid taking place.

The whole process, one class of organisms clearing the way for the activity of another, is termed a "metabiotic cycle," or "metabiosis." It often happens that two or more bacteria grow side by side and each assists the other, as for instance the B. tetanus, which is an anäerobic organism under ordinary circumstances, may be grown äerobically if the culture is also inoculated with B. pyocyaneus; such a phenomenon is termed symbiosis, and is of great importance in many pathological conditions of mixed infection.

Bacteria, as will be easily understood from the foregoing paragraphs, produce various chemical substances as the result of their growth; some of these are due simply to the splitting up of the various molecules of food stuff into simpler parts; the one is absorbed by the organism and built up into its protoplasm, the other remains in the solution. Other compounds are probably excreted by the organisms, and others again are only obtainable in any quantity from the bodies of the bacteria themselves. Some of these bacterial products are brightly coloured pigments, and it is easy to note in

examining a cultivation of such a chromogen that the pigment produced is not always confined to the actual area of growth of the organism but diffuses widely into the nutrient medium, demonstrating clearly the method in which the products of an organism taint the medium in which it grows, and in this way may check its own development (cf. B. pyocyaneus).

The products of vital activity of bacteria are of many kinds, the whole of which are generally now included under the term "fermentation-products."

These fermentations require considering in detail.

Production of Acid and Alkali.-The majority of bacteria produce an alkaline reaction when grown in a medium free from carbohydrate; some, e.g., the diphtheria bacillus, when grown in ordinary broth containing traces of carbohydrate, for the first few days give an acid reaction; later the reaction changes to alkaline, often due to the presence of ammonia.

Very many bacteria-and of these a considerable number are mouth bacteria are capable of fermenting carbohydrate with the production of acid. The fermentability of various carbohydrates differs widely, glucose being the most easily fermentable. Lactose is also fermented to acid by mouth bacteria. The carbohydrates of the mono-saccharide group are those most easily acted upon, the general equation of fermentation being :

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The carbohydrates of the di-saccharide group, C12H22O11, are first inverted to the mono-saccharide form and then fermented. Thus:

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+

=

12

Dextrose.

Lævulose.

2 (C3H6O3).

The carbohydrates of the poly-saccharide group are more complex than the other two groups, and require preliminary inversion before fermentation to acid occurs. Thus:

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According to Brown1 and Morris soluble starch has the formula (C&H10O3)30.

6

These equations do not express the whole of the reaction as some of the sugar is used by the bacteria themselves, and quantities of gas are often evolved during the process.

2

In the formation of alcohol from sugar large amounts of CO2 are evolved, thus:

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It has been shown by Maly2 that proteid is not attacked to any appreciable extent as long as any carbo-hydrate remains in the solution, the organisms first attacking the carbo-hydrate, and only when this is used up is the proteid acted upon.

Gas Formation.-Many bacteria produce gas, and a well known example is found in B. coli. com., which will produce large quantities of gas even in gelatin cultures. The chief gases formed by bacteria are CO2, N, H, CH, SH2. Unless the organism be anäerobic the gas formed generally escapes undetected into the air. When anäerobic a well-marked bubble may be seen around the colonies, as in stab gelatin cultures of Bacillus tetanus. In ordinary äerobic fluid cultures the formation of gas is best demonstrated by Durham's tube, a small test tube placed in the fluid, which becomes filled with the gas and floats in the liquid.

Heat. Some bacteria evolve a considerable amount of heat in their growth, and at times have been held responsible for spontaneous combustion of vegetable matter (hay). Some of these organisms do not thrive below 40° C., whilst they exhibit their greatest activity at 50° to 60° C. Several species have been isolated and studied. Practically they are utilised in the formation of "ensilage," the heat evolved in the anäerobic silos being due to the fermentative activity of these "thermophilic bacteria."

Nitrification. The conversion of ammonia to nitrite and nitrate is accomplished by a number of bacteria, which are able to grow in purely inorganic material. These bacteria, which are present for the most part in the top layers of surface soil, are of great importance to the agriculturist. In artificial media a number of bacteria produce nitrates, the presence of which may be demon

1 J. Chem. Soc., Lond., 1888, 610.

2 Hermann's Handbuch, Bd. v. (2) S. 239.

strated by appropriate chemical means. Certain others of these nitrogen-loving organisms assist in fixing the nitrogen of the air.

Phosphorescence.-A number of bacteria, many of them developing at 0° C., produce well-marked phosphorescence. Pflüger in 1875 first demonstrated this relationship of bacteria to the silvery phosphorescence seen on unsound fish and meat. The amount of light produced has been shown to be sufficiently great to photograph small objects placed near the cultivation. The class, as a whole, show a well marked preference to certain food-stuffs; in all cases a good supply of oxygen and about 3 per cent. of salt (NaCl) are required for the production of phosphorescence.

Chromogenesis.—A considerable number of organisms, many of them belonging to the pathogenic varieties, produce various colouring matters, or pigments. The colouring matter may be confined. to the bacteria themselves, or become diffused through the medium in which they grow. The pigments are often composed of several distinct chemical bodies which may be separated by chemical means. For instance B. pyocyaneus, the bacillus found in green pus, has been found to produce two if not three varieties of pigment. One may be easily obtained by extracting a cultivation with chloroform and crystallising out, when long delicate needles of a bluish-green tint are obtained, which changes to red on the addition of weak acid. The other pigment of B. pyocyaneus is a fluorescent green.

B. prodigiosus, B. rouge de Kiel, and several bacilli found in the mouth, produce a fine red pigment. Two of the pyogenic cocci, Staphylococcus pyogenes aureus, and S. pyogenes citreus, produce well-marked pigmentation; the colouring is not diffused into the culture medium. I have met with all the above bacteria in the mouth. Among other chromogens from time to time met with in the mouth Sarcina aurantiaca and Sarcina lutea are common. B. liquefasciens fluorescens, producing a fluorescent blue-green pigment, is also frequently met with. I have found this organism present in several cases of " green stain."

The chromogenic function of bacteria is considerably modified by environment. Most chromogens only produce pigment when grown at a low temperature, 20° C., and it is often possible to artificially produce a variety of non-colour-producing organisms by simply growing them at 37° C. for several generations. For some little time after the organism has been subjected to the higher

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