RELATION TO ENVIRONMENT CLASSIFICATION 31 



develops luxuriantly, and the bacillus of plague is completely inhibited 

 when streptococci are present in the culture. l 



Mutual inhibition may also be due to the monopolizing of the nutri- 

 tion in the medium by the predominating species or to the change in re- 

 action produced by its growth. This last consideration is probably the 

 secret of the inhibitory effect exerted by acid-producers upon bacteria of 

 putrefaction, and has received practical therapeutic application in 

 Metchnikoff's lactic-acid bacillus therapy, which see. 



When simultaneous presence of two species in the same environment 

 favors the development of both, the condition is spoken of as symbiosis. 

 Such dependence is not so frequent as antagonism, but it does occur. 

 Examples of such a condition have been observed in cultures containing 

 diphtheria bacilli and streptococci 2 and have been frequently observed 

 in cultures containing both aerobic and anaerobic bacteria, where the 

 former favor the development of the latter by monopolizing the supply 

 of free oxygen. Symbiosis may also take place in cultures in which com- 

 plex food products are split up by one species, furnishing substances for 

 ingestion by species with a lesser digestive ability. 



RELATIONS OP 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 destruction 

 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 processes. The 

 range of temperature at which bacteria may grow is subject to wide 

 variations among different species. Each species, on the other hand, 

 may thrive within a more or less elastic range of temperature, each one 

 having an optimum, a minimum, and a definite maximum tempera- 

 ture. 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 ac- 

 tual growth may be exceedingly slow or entirely inhibited. When the 

 temperature is higher than the optimum, the destructive processes are 



1 Bitter, Rep. Egypt Plague Com., Cairo, 1897. 

 * Hilbert, Zeit. f. Hyg., xxix, 1895. 

 4 



