46 PATHOGENIC BACTERIA. 



in distilled water to which the smallest amount of organic 

 matter has been added; others require so concentrated a 

 medium that only blood-serum can be used for their 

 cultivation. Sometimes a species with a preference for a 

 particular culture-medium can gradually be accustomed 

 to another, though immediate transplantation causes the 

 death of the transplanted organism. Sometimes the ad- 

 dition of such substances as glucose and glycerin has a 

 peculiarly favorable influence upon bacteria, causing, for 

 example, the tubercle bacillus to grow upon agar-agar. 



(c) Moisture. — A certain amount of water is always 

 necessary for the growth of bacteria. The amount can 

 be exceedingly small, however, so that the Bacillus pro- 

 digiosus is able to develop successfully upon crackers and 

 dried bread. Materials used as culture-media should not 

 be too concentrated; at least 80 per cent, of water should 

 be present. Most bacteria grow best in liquid media; 

 that is, they form the longest threads, and diffuse them- 

 selves throughout the 4iquid so as to be present in far 

 greater numbers than when on solid media. 



The statement that certain forms of bacteria can flour- 

 ish in clean distilled water seems to be untrue. When 

 transferred to such a medium the organisms soon die and 

 undergo a granular degeneration of their substance. If, 

 however, in their introduction a good-sized drop of cul- 

 ture-material is carried with them, the distilled water 

 ceases to be such, and becomes a dilute bouillon fitted to 

 support life for a time. 



(d) Reaction. — Should the pabulum supplied to bacte- 

 ria contain an excess of either alkali or acid, the growth 

 of the organisms is inhibited. Most true bacteria grow 

 best in a neutral or feebly alkaline medium. There are 

 exceptions to this rule, for the Bacillus butyricus and the 

 Sarcina ventriculi can grow well in strong acids, and the 

 Micrococcus urea can tolerate excessive alkalinity. Acid 

 media are excellent for the cultivation of moulds. 



(e) Light. — Most species of bacteria are not influenced 

 in their growth by the presence or absence of light. The 



