30 BIOLOGY OF BACTERIA. 



The amount needed varies considerably, and a few bac- 

 teria possess wonderful accommodative powers in this respect. 

 They can grow on bread which contains only a trace of 

 moisture. In laboratory work it will be found that bacteria 

 develop most rapidly in liquid culture-media. Solid culture- 

 media must contain at least 80 per cent, of water. Many 

 bacteria will grow in ordinary water provided it contains 

 some organic matter, even if it is only a trace. 



Nutriment : Inasmuch as only a few bacteria contain chlo- 

 rophyll, it is absolutely necessary that their food contain 

 organic matter. In the case of a few bacteria only a very 

 slight amount of organic matter is required, but as a rule 

 a great deal is necessary. Carbon and nitrogen must be 

 constituents of all culture-media, although a few organisms 

 can obtain these elements from the ammonium salts. When 

 characteristic growths are wished for, the carbon and nitrogen 

 must be supplied. By cultivating bacteria on different media, 

 it will be seen that many bacteria exhibit a decided preference 

 for a certain medium. This is an important feature in the 

 differentiation of species. It is possible, however, by frequent 

 transplantation to accustom an organism to a certain medium, 

 but this is done at the loss of its characteristic appearance 

 and growth. Bacteria which grow very poorly or not at all 

 on an ordinary culture-medium, will develop luxuriantly on 

 media to which a little glycerin or glucose has been added. 



Eeaction : With a few exceptions the culture-medium should 

 be either neutral or faintly alkaline. An excessively alkaline 

 or an acid medium inhibits bacterial development. A very 

 few germs require either a decidedly alkaline or a slightly acid 

 medium. Moulds thrive best in an acicl medium. 



Movement : It is a well-known fact that a rapidly flowing 

 body of water will purify itself. The same is true of bac- 

 teria in culture. A slight to-and-fro movement does not 

 interfere with their development, but a violent shaking either 

 hinders or prevents their growth. This is an important 

 factor in the purification of rivers when sewers empty directly 

 into them. By creating a rapid current the growth of the 

 contained germs is checked and the water again becomes 

 pure. 



