98 MICROBIOLOGY 



portant fact to be observed, in general sanitary work as well 

 as in the laboratory study of bacteria. 



Moisture. The presence of water is necessary for the 

 continuous growth of bacteria. The amount of drying that 

 bacteria will stand varies with the species. As a rule, the 

 vegetative forms do not endure long but spores will resist 

 drying for a considerable length of time. 



Temperature. For each species of bacteria there is a 

 temperature at which it grows best. There is also a maximum 

 temperature which is usually but a few degrees above the 

 optimum and a minimum which is usually many degrees be- 

 low. Most pathogenic bacteria develop best at a tempera- 

 ture of from 35-37 C. ; many of them will grow at a tempera- 

 ture of from 40-43 C. and at a minimum of 14 C. There 

 are a few species that favor the extreme temperatures. 



Relation of gaseous environment. The relation of bac- 

 teria to the oxygen of the air is very important. Some spe- 

 cies will not grow in its absence while others will not multi- 

 ply in its presence. Because of this requirement, bacteria 

 are divided into the obligatory and facultative aerobes and 

 anaerobes. 



Light. Direct sunlight will hinder bacterial develop- 

 ment and if continued it will destroy the vitality of the or- 

 ganisms. Diffused light is also injurious. Dark places are 

 more favorable for bacterial growth. A strong electric light 

 is said to be as fatal as sunlight, but the X-rays seem to be 

 without germicidal action. 



Cultivation. The cultivation of bacteria means their 

 growth on artificial media such as gelatin, agar, or in bouil- 

 lon, milk or other substances. 



A colony. A colony is a mass of bacteria that has de- 

 veloped on a solid medium from a single individual organ- 

 ism. Thus if a colon bacillus is planted on the surface or in 

 the depth of agar it will multiply in 24 to 36 hours, and will 

 exhibit a mass of organisms visible to the unaided eye, and 

 when examined under a low magnification will show certain 

 definite markings. 



