Читать книгу The Fundamentals of Bacteriology - Charles Bradfield Morrey - Страница 22

The temperature conditions for bacterial existence and growth have been determined more accurately than any of the other general conditions. The maximum for existence must be placed at or near 100° since it is known that all bacteria including spores may be killed by boiling in time. Nevertheless, certain forms have been reported as thriving in hot springs where the water temperature was 93°. This is the highest known temperature for development. The minimum for existence lies at or near the absolute zero (-273°) since certain organisms have been subjected to the temperature produced by the sudden evaporation of liquid hydrogen (-256° to −265°) and have remained alive. Whether they could withstand such temperatures indefinitely is not known. The minimum for development is near the freezing-point of water, since reproduction by division has been observed in the water from melting sea-ice at a temperature of −1.5°. Thus bacteria as a class have a range for existence of about 373° (-273° to +100°) and for development of 94.5° (-1.5° to +93°) certainly much wider ranges than any other group of organisms.5

The optimum temperature for development varies within rather wide limits for different organisms. In general it may be stated that the optimum temperature is approximately that of the natural habitat of the organism, though there are exceptions. The optimum of the “hot spring” bacteria just mentioned is apparently that of the springs (93° in this case). Many soil organisms are known whose optimum is near 70° (a temperature rarely, if ever, attained in the soil), but only when grown in air or oxygen; but is very much lower when grown in the absence of oxygen. Many other soil organisms exhibit very little difference in rate or amount of growth when grown at temperatures which may vary as much as 10° or 15°, apparently an adaptation to their normal environment. The disease-producing organisms show much narrower limits for growth, especially those which are difficult to cultivate outside the body. For example, the bacterium of tuberculosis in man scarcely develops beyond the limits of 2° or 3° from the normal body temperature of man (37°), while the bacterium of tuberculosis in birds grows best at 41° to 45°, the normal for birds, and the bacterium of so-called tuberculosis of cold-blooded animals at 14° to 18°.

Those bacteria whose optimum temperature is above 40° are sometimes spoken of as the “thermophil” bacteria. The fixing of the “thermal death-point” that is, the minimum temperature at which the bacteria are killed is a matter of great practical importance in many ways and numerous determinations of this have been made with a great many organisms and by different observers. The factors which enter into such determinations are so many and so varied that unless all the conditions of the experiment are given together with the time of application, the mere statements are worthless. It may be stated that all young, actively growing (non-spore-containing) disease-producing bacteria, when exposed in watery liquids and in small quantities are killed at a temperature of 60° within half an hour. It is evident, that this fact has very little practical application, since the conditions stated are rarely, if ever, fulfilled except in laboratory experiments. (See Sterilization and Pasteurization, Chapter XIII.)