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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 pigment— be 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 three 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.

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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.

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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 compounds 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 CO2 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 Braatz 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 anaerobic cavities much more of the matrix has

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disappeared than is the case in those cavities to which oxygen has free access.

Temperature. Bacteria are the most widely distributed of all living things when we consider them from the point of view of temperature.

Foster1 and Fischer 2 have demonstrated that a number of bacteria, amongst them the species producing phosphorescence, thrive and multiply at 0° C. Per contra Miguel has described an organism that flourishes and forms spores at 70° C. (158° F.), a temperature at which ordinary albumen is coagulated! These two extremes however only give the limits of actual growth, the limits of passive resistance being much wider. For instance, Pictet found spores to resist exposure to the temperature of frozen oxygen (-213° C.) for a short time, and that they easily withstand. a temperature of 120° C. for twenty hours, rapidly developing when thawed. Rapid freezing and thawing was found to be more injurious than a long exposure to a low degree of cold.

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The ordinary bacterial plasm of most organisms enters into heat rigor at 42° to 45° C., and a prolonged temperature of 55° C. will destroy almost all bacterial bodies; but the fact does not apply to the spores, those of B. subtilis requiring three hours' continuous boiling in water or steam to destroy them. The point at which death occurs is termed "thermal death point," and varies considerably for various species. The property of resistance to temperature as high as boiling was one of the experiments by which bio-genesis was sought to be proved among others by van Helmont, who devised therefrom a process of producing artificial mice!

The spores present in the boiled fluid develop into adult forms as soon as the temperature has fallen sufficiently. From this, and the fact that the bacterial bodies themselves were easily destroyed by boiling, Tyndell devised what is known as intermittent sterilization. The medium, which would be spoiled by a high temperature, is boiled for twenty minutes on three successive days. In the interval between the operations the spores germinate to adult forms which are killed at the next boiling.

Saprophytic bacteria of soil and water grow best at about 20° C.,

Centralbl. f. Bakt., ii., 1887; xii., 1892.

2 Ibid., iv., 1888.

3 Arch. des Sci. Phys. et Nat., xxx., 1893, p. 293.

growth ceasing at about 5° C. The pathogenic organisms have their optimum temperature at the body heat of about 37° C., although each of them generally exhibits a preference of some definite degree of heat which is termed "optimum temperature."

The relative resistance of different bacteria is often made practical use of to isolate the more hardy species. It is also used in the determination of the presence or absence of endospores. The cultivation or material to be tested is maintained at a temperature of 80° C. for half an hour. At the end of the time the tube is replaced in the incubator for twenty-four hours. If spores are present they resist the action of the heat and develop rapidly when incubated; when no spores are present no development

occurs.

Heat is applied to "attenuate" pathogenic cultivations for inoculation purposes. Pasteur found that by incubating the anthrax bacillus at a temperature of 40° to 42° C. no spores are formed, and the pathogenic power of the bacilli is greatly reduced. Even spores themselves if maintained for considerable periods at 80° lose their pathogenic power. The digestive and bacterial enzymes are mostly destroyed by temperatures above 70°, being more resistant than the vegetative forms but less so than the endospores.

The foregoing facts have a very practical bearing upon sterilization and will be again referred to under that heading: and in passing it may perhaps be as well to point out that freezing or cold-storage does not destroy the bacteria, but merely restrains their activity for the time being.

Reaction of Medium.-Most bacteria grow best when the reaction of the substratum is neutral or faintly alkaline, the majority of the putrefactive bacteria and most of the pathogenic bacteria are favoured by an alkaline reaction; some organisms, however, are able to grow on an acid medium, while a few are directly favoured by the presence of acid, as for instance the acetic acid bacilli, which ferment acetic acid to CO, and water. B. butyricus is another of these acid-loving organisms. The mouth bacteria all prefer a somewhat alkaline medium, most of them refusing to develop in the presence of acid; a few—particularly the mouth streptococcus-will grow in acid media. This ability of bacteria to develop in an acid medium must not be confounded with the production of an acid reaction by the vital activity of the organism.

The reaction of the culture medium in which bacteria are

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