act on the right-handed lævulose, which is a mere laboratory product, and has never been encountered in nature. 2. Oxygen. While many organisms grow only in the presence of air (obligatory aerobes), others do so only when oxygen is completely excluded (obligatory anærobes). But the great majority of bacteria, although thriving best in the presence of oxygen, can also grow when its supply is more or less cut off (facultative anaerobes). This is true of almost all pathogenic forms, which are thus uninjured in the body cavity, where the oxygen is scanty or absent. But it is probable that in this situation their vegetative capacity is diminished, and the formation of toxic metabolic products increased. The exclusion of oxygen suspends chromogenesis in nearly all colourforming organisms, and destroys the phosphorescence of all photogenic bacteria. 3. Temperature.-In their relation to temperature, bacteria present wide differences. Thus, while most water bacteria can grow at the freezing temperature, many of the saprophytic forms flourish at the boiling temperature. Between these cold-loving and heat-loving bacteria lie almost all parasites which grow best at the blood-heat. Although most organisms grow within a definite range of temperature, yet the maximum development takes place at a certain point called the "optimum". The range of temperature necessarily varies with each class of bacterium, and is well brought out in the following table :— It will be observed that while B. subtilis ("hay bacillus ") can grow within very wide limits, the B. tuberculosis, like all strict parasites, has the narrowest range of all. It is also apparent that a necessary condition for pathogenesis is the approximation of the optimum temperature to blood-heat. Where such an approximation does not obtain the animal does not "take" the disease, i.e., it is "immune". It is partly for this reason that man is insusceptible to the hay bacillus, and fowls are immune to anthrax. When the latter are immersed in cold water their temperature is brought down, and made to correspond with the optimum of B. anthracis, with the result that they speedily succumb to the disease. Their Thermophilic bacteria are commonly met with in the fermentation of hay, cotton waste, tobacco, etc. optimum reaches the temperature of coagulation of most proteids. It is probable that their protoplasm is different from that of ordinary cells. Cold arrests the growth of organisms, but does not destroy their vitality. Most bacteria withstand a temperature of several degrees below 0°; and yet at this temperature the ordinary life processes must cease. Under such conditions bacteria are neither living nor dead-they pass into a "third state," or a condition of suspended animation. Heat, on the other hand, has a decidedly injurious action on the vitality of bacteria. If the optimum temperature be raised by 5° to 10° their power of growth and virulence are reduced, but not altogether lost. But if the maximum temperature is exceeded death follows owing to the coagulation of protoplasm. A temperature of 60° C. for ten minutes, or 70° C. for five minutes, is sufficient to kill most sporeless bacteria. To destroy spores the medium is exposed to live steam for fifteen minutes on three successive days, and during the intervals kept at 25° C. to 30° C. to allow spores to vegetate (intermittent or fractional sterilisation). 4. Light. Direct sunlight checks the growth of all bacterial forms, and under favourable conditions may be bactericidal in its effects. Even when the light is not very intense, or when the exposure is too short, attenuation of the virus may result. If the action of heat be excluded by first carrying the light through a layer of water, no alterations in these results follow, showing that the germicidal action is due to light rays alone. The rays that are most effective are those of the highest refrangibility, i.e., the blue and violet rays of the spectrum. In considering these results we must remember that they are obtained in the laboratory, and under conditions which would rarely occur in nature. For practical disinfection, therefore, it would be most unsafe to rely on sunlight alone. But by introducing plenty of light and air into our dwellings we may attenuate the vitality of the germs, and thereby, to a certain extent, safeguard ourselves against infection. 5. Association of Bacteria.-Although for purposes of study pure cultures of bacteria are essential, we must not forget that in nature they often occur in combination. Their precise relations to each other can hardly be imitated in the laboratory, but recent investigations have thrown an interesting light on this question. When several species are associated in the same culture one may take precedence and the others may grow later (metabiosis); or two or more species may develop simultaneously, and co-operate for their mutual advantage (symbiosis); or the growth of one species may prevent the development of another (antibiosis). Metabiosis is well seen in the fermentation of " grape must," where, although a number of organisms exist side by side, yeasts develop first, then the vinegar organisms, and lastly the putrefying bacteria. Symbiosis, however, is more important, and may be illustrated by numerous examples. Thus, some bacteria cannot play their ordinary rôle without the aid of others, e.g., the organisms of tetanus and of suppuration. Again, the virulence of certain organisms may be heightened by means of association, e.g., diphtheria bacillus and streptococci. And the symbiotic relationship of legumes with the bacteria of root-nodules, is an even more beautiful illustration, and will be discussed in a subsequent chapter. As regards the antagonism of bacteria, it has been found that bacillus pyogenes fœtidus prevents the growth of spirillum cholera; and Emmerich has pointed out that animals injected with anthrax bacilli may be saved by subsequent infection with streptococcus pyogenes. According to Hankin, micrococcus Ghadialli destroys typhoid and colon bacilli; and he has therefore suggested the use of this coccus to purify water polluted with typhoid stools. It is thus apparent that bacterial growth is modified not only by environment, but also by association with other species. Therefore, when a pure culture of a species is obtained, and doomed to a separate existence, certain alterations are necessarily induced in the biological characters of the species concerned. A closer study of this question is most desirable, not only for the sake of confirming our notions about the life-history of bacteria, but also to show how far a species can be associated with another so as to give rise to the development of powers which would otherwise remain unobserved and unutilised. B. THE VITAL PHENOMENA OF BACTERIA. As is the case with all living cells, the amount of energy derived from the combustion of food-stuffs is not wholly expended in the building up of new protoplasm, but is, in part, applied to the production of diverse phenomena. For the sake of convenience they may be classified as follows: 1. Optical. Several species of bacteria have been isolated from sea-water which, when cultivated in suitable media, are markedly phosphorescent. The faint glow on decaying fish is also caused by the presence of these "photogenic bacteria". The phosphorescence is a vital phenomenon of the organism, and is therefore lessened by anything that interferes with its vitality. The death of the organism, or the exclusion of oxygen, quenches the light at once, while a more abundant supply of oxygen increases it. Phosphorescence is absent in perfectly smooth water, and is best seen at the crest of the waves, where the oxygen is most plentiful. 2. Thermic. The rise in temperature so frequently observed in the decomposition of organic materials, e.g., hay, manure, tobacco, is no doubt due to bacterial agency. Sometimes the temperature may be so high as to cause "spontaneous combustion". It appears that these results are brought about by the growth of "thermophilic bacteria". 3. Mechanical.-If a drop of stagnant water be examined under the microscope it will be observed that bacteria tend to crowd round particles of organic matter to which they are mysteriously attracted (chemiotaxis). This is not the result of an instinct for food, but of the chemical nature of the substance employed as stimulant. The |