i86 



BACTERIA AND SOIL FERTILITY 



cently when Kellerman and McBeth isolated thirty-six species 

 from different sources. These were much more active than those 

 studied by earlier workers. They were all rod-shaped organisms 

 varying in length from .8 to- 3.5 [^. Involution forms have been 

 observed for only three species. Five species have been found to 

 produce spores. Twenty-seven species are motile; of these, seven 

 are pseudomonas and twenty are bacilli. A few are faculative 



Fig. 36. — Colonies of cellulose ferments. (After McBeth.) 



anaerobes, but most are true aerobes. The optimum temperature 

 lies between 26° and 33° C. They grow rapidly on solid media 

 such as beef agar, gelatin, starch, and potato. Nineteen species 

 liquefy gelatin. They rapidly decompose cellulose and other car- 

 bohydrates with the production of acids, but none of the organ- 

 isms so far studied produce a gas. 



Products. — Cellulose when treated with dilute acid takes up 

 water and falls to pieces. There results first the complex dex- 

 trins, which in turn break into the sugar maltose and then into 

 grape sugar. This when oxidized yields alcohol ; hence, it is pos- 

 sible for the chemist to change sawdust into sugar and then alco- 

 hol. Likewise, the cellulose ferments change the woody tissues 

 of plants into dextrins, sugar, alcohol, and finally into acids. 

 These products, in turn, are used by other microorganisms in 

 their metabolism. During this tearing-down process the microbe 



