THE ENVIRONMENT OP BACTERIA. 71 



Temperature. 



In addition to a suitable nutrient soil and a proper 

 supply of oxygen, bacteria require a certain temperature 

 for their development. As with other forms of life, each 

 bacterial species has a minimum, optimum and maximum 

 temperature at which it will develop. In some species the 

 range of temperature at which they can grow is very 

 limited. Thus, in the case of the tubercle bacillus and the 

 gonococcus the maximum and minimum temperatures may 

 not be more than iive degrees apart. On the other hand, 

 a few species are known which can grow anywhere from 

 15 to 68°. 



The above instances, a narrow limit of five degrees 

 and a very wide limit of fifty degrees, may be considered as 

 exceptional. In general it may be said that bacteria do 

 not grow, or but very poorly, below 10° and above 40°. 



The optimum temperature should especially be sought 

 for vhen cultivating bacteria. It may be said to vary with 

 each individual species. In the case of the parasitic bac- 

 teria, those which grow in the living body, the temperature 

 of the host may be considered as best adapted for their 

 development. Consequently the optimum temperature for 

 the parasitic group of bacteria may be placed at 35 to 40°. 

 On the other hand, the saprophytic bacteria, those which 

 grow in nature on dead matter, find the warm summer tem- 

 perature to be most favorable for their growth. As is well 

 known, fermentative and putrefactive changes are favored 

 by a warm, and retarded by a low temperature. Hence the 

 optimum temperature for saprophytic bacteria may be said 

 to range from 25 to 30°. From what has been said, it is 

 evident that, as a rule, pathogenic bacteria should be cul- 

 tivated in an incubator at, or near, the temperature of the 

 body (37.5°); where3,s, non-pathogenic bacteria are grown 

 best at- the ordinary room temperature. 



