[Chap. XLII THE BIOLOGY OF BACTERIA 519 



solution equivalent to dissolving 1 cc. of free oxygen in 100 liters of 

 w^ater. If the oxygen content of the medium is increased to 30 times this 

 amount, most bacteria die. In other words, oxygen at such concentrations 

 has the same effect on bacteria as a solution of formaldehyde, or bi- 

 chloride of mercury, two of our commonly used antiseptics. Even pro- 

 nouncedly aerobic bacteria cannot withstand very high concentrations 

 of oxygen. If the oxygen content of the air above the nutrient medium is 

 increased to 4 times the usual amount, the growth of some strains of 

 Streptococcus is stopped. Even the same strain of bacteria differs in its 

 susceptibility to oxygen according to the medium in which it is growing. 

 Anaerobic bacteria grow better within liquid media or below the surface 

 of agar media because of the decreased oxygen content. Among the bac- 

 teria there is a marked diversity in their endurance of molecular oxygen. 



Hydrogen peroxide (H2O2) is toxic to bacteria because it changes 

 readily to water and atomic oxygen, which is very active chemically. 

 In the presence of free oxygen, hydrogen peroxide may be formed in the 

 medium in which the bacteria are growing. The hydrogen peroxide may 

 in turn be toxic to the bacteria. 



Temperature. When compared with other plants, bacteria as a group 

 can live under a very wide range of temperature conditions. In this 

 respect they are perhaps equaled only by the blue-green algae. The 

 temperature of the bacteria is of course the same as that of the media in 

 which they grow. Within a limited range, high temperatures accelerate 

 life processes and low temperatures retard them. Food is used less 

 rapidly at lower temperatures, and bacteria with a limited food supply 

 live longer under such conditions. 



Some of the bacteria that thrive in hay infusions multiply at tempera- 

 tures as low as 40° F. or as high as 110° F. The minimum temperature 

 at which some species can grow may be higher than the maximum 

 temperature at which others can sur\'ive. The optimum temperature for 

 most bacteria lies between 70° and 100° F. Few species continue vegeta- 

 tive development at temperatures higher than 115° F. Some bacteria, 

 however, that bring about the rapid decay of organic matter, such as 

 silage, may live at temperatures as high as 175° F. Living bacteria have 

 also been found in hot springs at temperatures but little below this 

 figure. This endurance of high temperature is remarkable in view of the 

 fact that man\' proteins begin to coagulate and some fats begin to liquefy 

 and separate at 145° F. 



Although bacteria are often distinguished as aerobes and anaerobes as 



