The Fundamentals of Bacteriology. Charles Bradfield Morrey
Some bacteria and especially most spores may live when dried in the air or by artificial means for months and even years, while some are destroyed in a few hours or days when dried (typhoid, cholera, etc.). The optimum amount of moisture has not been determined with any great accuracy and certainly a rather wide range in percentage of water is permissible with many, though a liquid medium is usually most favorable for artificial growth. The “water bacteria” have been mentioned. In the soil a water content of 5 to 15 per cent. seems to be most suitable for many of the organisms which aid in plant growth. In animals and man the organisms infecting the intestinal tract prefer a high percentage of moisture as a rule, especially those causing disease here. Those found on the surface of the body (pus cocci) need a less amount of water, while those invading the tissues (tuberculosis, black-leg, etc.) seem to be intermediate in this respect. In artificial culture media a water content of less than 30 per cent. inhibits the growth of most bacteria.
As a general rule those bacteria which require the largest percentage of water are most susceptible to its loss and are most readily killed by drying. The typhoid and cholera organisms die in a few hours when dried, while pus cocci and tubercle bacilli live much longer.
TEMPERATURE.
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.)
LIGHT.
Speaking generally, it can be said that light is destructive to bacteria. Many growing forms are killed in a few hours when properly exposed to direct sunlight and die out in several days in the diffuse daylight of a well-lighted room. Even spores are destroyed in a similar manner, though the exposure must be considerably longer. Certain bacteria which produce colors may grow in the light, since the pigments protect them. Some few kinds, like the sulphur bacteria, which contain a purplish-red pigment that serves them to break up H2S, need light for their growth. Since disease-producing bacteria are all injuriously affected by light, the advantage of well-lighted habitations both for men and animals is obvious.
OXYGEN SUPPLY.
Oxygen is one of the constituents of protoplasm and is therefore necessary for all organisms. This does not mean that all organisms must obtain their supply from free oxygen, however, as animals and plants generally do. This fact is well illustrated by the differences among bacteria in this respect. Some bacteria require free oxygen for their growth and are therefore called aërobic bacteria or aërobes (sometimes strict aërobes, though the adjective is unnecessary). Others cannot grow in the presence of free oxygen and are therefore named anaërobic bacteria or anaërobes (strict is unnecessary). There are still other kinds which may grow either in the presence of free oxygen or in its absence, hence the term facultative anaërobes (usually) is applied to them. The distinction between facultative aërobe and facultative anaërobe might be made. The former means those which grow best in the absence of free oxygen, though capable of growing in its presence, while the latter term means those which grow best in the presence of free oxygen, but are capable of growing in its absence. The amount of oxygen in the atmosphere in which an organism grows may be conveniently expressed in terms of the oxygen pressure, i.e., in millimeters of mercury. It is evident that the maximum, minimum and optimum oxygen pressures for anaërobic bacteria are the same, namely, 0 mm. Hg. This is true only for natural conditions, since a number of anaërobic organisms have been gradually accustomed to increasing amounts of O, so that by this process of training they finally grew in ordinary air, that is, at an oxygen pressure of about 150 mm. Hg. (Normal air pressure is 760 mm. Hg. and oxygen makes up one-fifth of the air.) The minimum O pressure for facultative anaërobes is also 0 mm. Hg. Some experiments have been made to determine the limits for aërobes, but on a few organisms only, so that no general conclusions can be drawn from them. To illustrate: Bacillus subtilis (a common “hay bacillus”) will grow at 10 mm. Hg. pressure but not at 5 mm. Hg. It will also grow in compressed oxygen at a pressure of three atmospheres (2280 mm. Hg.), but not at four atmospheres (3040 mm. Hg.), though it is not destroyed.
Parodko has determined the oxygen limits for five common organisms as follows:
| Maximum. | Minimum | |||
| In atmospheres. | Mm. Hg. | Vol. per cent. | Mm. Hg. | |
| Bacterium fluorescens | 1.94 to 2.51 | 1474 to 1908 | 0.00016 = 0.0012 | |
| Sarcina lutea | 2.51 to 3.18 | 1908 to 2417 | 0.00015 = 0.0011 | |
| Proteus vulgaris | 3.63 to 4.35 | 2749 to 3306 | 0 | |