12 BACTERIOLOGY. 



influence The influence of temperature is most important. 



ronment Bacteria thrive best at 37.5 C, and as the temperature 

 varies above or below this point growth is retarded. 

 A temperature of 62 C. will kill most bacteria. Low 

 temperatures are not so destructive, for by experiments 

 it has been proven that a temperature of 200 degrees 

 below zero (centigrade) will not kill all bacteria. 



Moisture is essential for the growth of bacteria, 

 as the food material upon which bacteria thrive must 

 be in solution. The reaction of the food material is 

 of considerable moment, for bacteria will not grow if 

 too much acid or alkali is present. A neutral or 

 slightly acid reaction gives the best growth. 



cuitiva- l n order to cultivate bacteria, substances must be 



tion 



used from which they can obtain the proper nutriment. 

 Such substances when made artificially are called cul- 

 ture media, and they may be solid or fluid. The solid 

 media are employed when a, surface growth is desired, 

 the bacteria being rubbed on the surface. The com- 

 mon kinds of solid media are agar-agar, gelatin, and 

 coagulated blood-serum. Fluid media are used for 

 the determination of acid formation, fermentation, 

 coagulation, and motility; those most often used are 

 milk and bouillon. The media are prepared in the 

 laboratory. After the ingredients have been dissolved 

 by boiling, the whole is filtered, run into test-tubes, 

 plugged with cotton, and finally sterilized by steam 

 under pressure in order that no bacteria may develop 

 in it except those introduced for the purpose of study. 

 The distribution of bacteria in nature is prac- 



