FOOD SUPPLY OF BACTERIA. 21 



capacity of originating new individuals. In the strepto- 

 thrix actinomyces there may appear a club-shaped swelling 

 of the membrane at the end of the filament, which has by 

 some been looked on as an organ of fructification, but which 

 is most probably a product of a degenerative change. The 

 streptothrix group as a whole is a link between the bacteria 

 on the one hand, and the lower fungi on the other. Like 

 the latter, the streptothrix forms show the felted mass of 

 non-septate branching filaments, which is usually called a 

 mycelium. On the other hand, the breaking up of the 

 protoplasm of the streptothrix into coccus- and bacillus-like 

 forms, links it to the other bacteria. 



GENERAL BIOLOGY OF THE BACTERIA. 



There are five prime factors which must be considered 

 in the growth of bacteria, namely, food supply, moisture, 

 relation to gaseous environment, temperature, and light. 



Food Supply. The great function performed by bacteria 

 in nature is the breaking up into simpler constituents of 

 the complicated organic substances which form the bodies 

 of dead plants and animals, or which are excreted by the 

 latter while they are yet alive. The natural food of 

 bacteria is therefore of an extremely complex nature. 

 Not only is it so to start with, but seeing that, as a 

 general rule, many bacteria grow side by side, the food 

 supply of any particular variety is, relatively to it, altered 

 by the growth of the other varieties present. It is thus 

 impossible to imitate the natural food environment of any 

 species. The artificial media used in bacteriological work 

 may therefore be poor substitutes for the natural supply. 

 In certain cases, however, the conditions under which we 

 grow cultures may be better than they naturally are. For 

 while one of two species of bacteria growing side by side 

 may favour the growth of the other, it may also in certain 

 cases hinder it, and, therefore, when the latter is grown 

 alone it may grow better. Most bacteria seem to produce 

 excretions which are unfavourable to their own vitality, for 



