36 BIOLOGY AND TECHNIQUE 



also take place in cultures in which complex food products are split 

 up by one species, furnishing substances for ingestion by species with 

 a lesser digestive ability. 



RELATIONS OF BACTERIA TO PHYSICAL ENVIRONMENT 



Relation of Temperature. Like all other living beings, bacteria 

 develop and multiply by virtue of a series of chemical and physical 

 processes, by means of which growth energy is obtained by destruc- 

 tion or catabolism, and the lost tissues resupplied by absorption of 

 nutritive materials. It is natural, therefore, that the conditions of 

 external temperature should intimately affect the metabolic proc- 

 esses. The range of temperature at which bacteria may grow is sub- 

 ject to wide variations among different species. Each species, on the 

 other hand, may thrive within a more or less elastic range of tem- 

 perature, each one having an optimum, a minimum, and a definite 

 maximum temperature. When the optimum temperature is present 

 in the environment, the functions of absorption and excretion keep 

 pace with each other, and the chemical balance is well preserved. 

 When the temperature is lower than the optimum, all metabolic 

 processes take place more slowly, and the bacterium gradually enters 

 into a resting or latent stage, at which actual growth may be ex- 

 ceedingly slow or entirely inhibited. When the temperature is higher 

 than the optimum, the destructive processes are carried on more 

 rapidly than the substitution of waste products by absorption, and a 

 gradual weakening of vital energy, or even a gradual death of the 

 bacterium, may take place. When certain bacteria form spores, they 

 become very much more resistant against both high and low temper- 

 atures, probably because a true resting stage has been reached, dur- 

 ing which metabolism has been reduced to a minimum, there being 

 practically no nutritive material taken in and correspondingly little 

 destruction taking place within the body of the microorganism. 



The optimum temperature for various bacteria depends upon the 

 habitual environment, in which the particular species is accustomed 

 to exist. Thus, for the large majority of bacteria pathogenic for 

 human beings, the optimum temperature is at or about 37.5 C. 

 There are a large number of bacteria common in water, however, 

 which grow hardly at all at the body temperature, but thrive most 

 luxuriantly at temperatures of about 20 C. F. Forster, 24 moreover, 



24 F. Forster, Cent, f . Bakt., ii, 1887. 



