FACTORS INFLUENCING BACTERIAL GROWTH 17 



typhoid bacilli exposed to -175 C. for thirty minutes 10 per cent 

 remained alive. 



Antibiosis and Symbiosis. In nature many species of bacteria 

 exist side by side ; pure cultures are seldom found outside of the 

 laboratory. In some instances the products of one species are 

 antagonistic to the well-being of another and the weaker is able 

 to multiply very slowly or not at all. Saprophytic forms soon 

 overpower the comparatively less resistant disease-producing forms. 

 Thus infected carcasses eventually become purified by the same 

 processes that destroyed them. 



Sometimes, on the contrary, there is a certain amount of 

 cooperation between two species and the presence of one induces 

 the more luxuriant growth of the other. This latter condition is 

 spoken of as symbiosis. Certain anaerobes will grow when air 

 is admitted into the culture tube if cultivated with an ae'robe; 

 it is assumed that the ae'robe deprives the air of its oxygen con- 

 tent and thus renders conditions suitable for anaerobic growth. 



Effects of Chemicals. Chemical substances vary in their 

 bactericidal powers as do the various bacteria in their degree of 

 resistance. Just what their action really is, in many instances, 

 is not known. In the case of bichloride of mercury or formaldehyde 

 there appears to be a chemical union- between the disinfectant and 

 the cell protoplasm. Vegetative forms are affected more quickly 

 than spores. 



The following terms are frequently used to express the inimical 

 effect of chemicals or physical forces upon bacteria : 



Attenuation. Function diminished but not impaired. 



Antiseptic Action. Growth arrested but capable of recom- 

 mencement as soon as surroundings become suitable. 



Disinfection. Destruction of all disease-producing forms and 

 their products. 



Sterilization. Destruction of all forms, pathogenic and non- 

 pathogenic. 



Disinfectants which are very effective under certain circum- 

 stances may become almost inert under others. Milk of lime is 

 of use only while it remains milk of lime ; when the carbon dioxide 



