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certain reagents, among them being the ordinary physiological salt solution (0.75 gm. NaCl in 100 gms. of water). Various staining reagents bring about this phenomenon. The plasmolytic effect may be produced and removed without any apparent injury to the living cell.

CHAPTER II.
The Biology of Bacteria.

BACTERIA are greatly influenced by their surroundings, and on the other hand often profoundly modify the substratum in which they are growing, be it living tissue or nutrient solution, whilst the very products resulting from their activity are among the chief inhibitory influences restraining their indefinite development.

Cohn estimated that a single bacillus 2 u long and 1 u broad, weighing 0.000000001571 mgrm., and which reproduced itself by binary fission once in half an hour, will in two days' time have a progeny of 281 billions, occupying a volume of half a litre, while in another three days the mass would be sufficient to fill the beds of all the oceans of the globe, the number of the progeny being represented by 37 places of figures ! That such enormous development does not take place is due partly to the antagonism displayed by one species towards another, partly by the insufficiency of nutriment obtainable, but chiefly to the products of the organisms' own activity. The more important conditions related to the development of bacteria are-light, temperature, gaseous environment, moisture, food supply. The first four factors are related more particularly to the development of the bacteria, whilst the question of food supply is largely complicated by the various chemical changes induced by bacterial action.

Effect of Light.—The antagonism of light to disease was a fact established by empirical observation long before bacteria were known to exist, and the old Italian proverb, “ Where the sun does not enter, the doctor does,” is illustrative of this popular knowledge gained by observation. Since the discovery of bacteria many experiments have proved the scientific basis of the empirical deduction; the majority of bacteria, and certainly all the known pathogenic forms, are particularly sensitive to the action of light.

Direct sunlight is the most powerful agent; exposure to the direct rays of the sun will kill tubercle bacilli in half an hour, anthrax spores in an hour. Diffused daylight has a similar, but less energetic effect, the time of exposure being about three times as long. The light from an electric arc has a similar effect to diffused daylight; recently advantage has been taken of the fact in the treatment of lupus. Tuberculous sputum and typhoid dejecta may thus be deprived of their respective organisms under natural conditions. Even when the organisms are not entirely destroyed by the process of insolation their pathogenic powers are greatly attenuated, the subsequent cultivations developing less luxuriantly than before the experiment, and the power of producing poisonous products greatly diminished.

Among the bacteria producing pigment, light has marked effect in altering the power of chromogenesis. Thus if a cultivation of the B. rouge d'Kiel—an organism producing a fine red pigmentbe exposed to sunlight for a time insufficient to entirely destroy the organisms, subsequent cultures may be obtained which have lost the power of colour-formation and remain colourless indefinitely. A few non-pathogenic bacteria, such as B. violaceous, which forms a purple pigment, apparently thrive best in the light. The red chromogens are generally more resistant to the action of light than other colour-forming species.

Downes and others have attributed the destructive action of sunlight to a disengagement of nascent oxygen which attacks the bacterial plasma, such a process depending largely on the composition of the medium containing the bacteria during exposure. The blue and violet portion of the spectrum, i.e., the most chemically active rays, were found most energetic in action, the red and yellow rays the least. The destruction was apparently independent of the temperature, and took place when the heat rays were excluded. The more translucent the medium the greater the action.

Buchner found in a series of experiments conducted in the clear water of Lake Starnberg, that the bactericidal effect of light was apparent at the depth of two metres below the surface. Sunlight must therefore exercise a powerful influence in cleansing watercourses polluted with excremental matter; the marked diminution in the number of bacteria present, say, a mile below a sewage

British Medical Journal, January, 1902.

outfall, undoubtedly depends upon such action, besides sedimentation and other processes.

Moisture.-Water is necessary for bacterial development as for other forms of life; the optimum percentage of water is about 80 per cent. Some bacteria will withstand desiccation for long periods, others rapidly succumb; the spore-bearing organisms resist drying to a much greater degree than the non-sporulating varieties, the arthrosporous forms holding an intermediate position (Hueppe). Anthrax spores will survive drying in dust for two years or more. A cultivation of B. typhi abdominalis kept in my laboratory for nine months, and which had become so dry that the medium could be broken with ease, gave subcultures in twenty-four hours.

B. diphtheria will resist drying for a week or two. Tubercle bacilli remain alive for long periods in rooms occupied by tubercular persons, especially in dark situations, as behind pictures, &c. On the other hand the cholera spirillum is destroyed by th ree to four hours' drying.

Most Schizomycetes exhibit their maximum development upon fluid media, bacilli often growing out into long chains of filaments : “ dry rot” and “mould” are always associated with dampness.

Relation to Gaseous Environment.—Bacteria, in common with other living things, require oxygen for their existence, but not in all cases is it necessary that the gas should be present in the free state, as certain organisms can obtain the necessary oxygen from chemical compounds in which it is present in a loosely combined form.

Such organisms are termed anäerobic, that is, they will live even though free air is excluded; others however require oxygen in a free state, and are termed äerobic. Intermediate between these two extremes come those organisms which, although they develop best in the presence of air, are yet capable of existence when air is excluded; such are termed facultative-anäerobic, and comprise the largest number of the known Schizomycetes.

Anäerobiosis is probably an ancestral trait going back to the first appearance of life upon this planet, when the atmosphere contained but little oxygen in a free state. It is possible experimentally to change the character of an organism that, though at first it is äerobic, subcultures will ultimately become anäerobic in habit, and vice-versa. Such experiments have been performed by Hueppe with the cholera spirillum.

100.

Hydrogen and nitrogen are indifferent gases for anäerobes, while sulphuretted hydrogen and carbon dioxide are poisons. On the other hand the majority of mouth bacteria are able to develop in an atmosphere of carbon dioxide-in fact certain species are favoured by its presence.

The Beggiatoa group, as observed by Winogradsky, are able to exist in sulphuretted hydrogen, and change that gas into its component parts, storing up the sulphur in solid particles.

The changes induced in the substratum by anäerobic bacteria differ from the changes taking place in the presence of free oxygen. The maintenance of life without free oxygen depends solely upon the availability of compor nds from which oxygen may be split off.

“ The amount of chemical change therefore is relatively much less intense than in äerobic conditions ; thus if 1,000 grammes of sugar be completely oxidised to CO, and water in the presence of free oxygen, 3,939 calories or heat units are produced ; if however it is split into butyric acid, hydrogen and CO2, only 414 calories are evolved. It follows therefore that anäerobic bacteria must superficially disintegrate a far larger quantity of material to obtain this necessary oxygen than äerobic organisms—a circumstance that has considerable significance in the large production of toxines by organisms growing in the living body. Again, during anäerobic fermentation the secondary products are not oxidised to other and simpler compounds, and they therefore accumulate, a good example being afforded by the so-called “bottom fermentation."

It has also been shown by Fajans that cholera cultures retain their virulence much longer under anäerobic than under äerobic conditions; and Braatzi has called attention to the fact that bacteria in suppuration foci are living without atmospheric oxygen.

A great pressure of carbon dioxide is said to deprive B. anthracis of the power of sporulation.

It is probable that facultative anäerobic organisms are largely concerned in dental caries after the granular layer has been passed, and the rapid progress and undermined character of the cavities generally formed is due to the phenomena connected with anäerobiotic growth; gelatin, the end-product of the tooth cartilage or collogen, is a substance from which anäerobes are able to obtain their oxygen. A relatively large amount is therefore attacked, and in these anäerobic cavities much more of the matrix has

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