FOOD SUPPLY 17 



reproduced. Such bodies are often referred to as spores, but 

 they have not the same staining reactions nor resisting powers 

 of so high a degree as ordinary bacterial spores. Sometimes, too, 

 the protoplasm of the filaments breaks up into bacillus-like 

 elements, which may also have the capacity of originating new 

 individuals. In the streptothrix 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, or possibly of defensive nature. The 

 streptothrix group, though its morphology and relationships 

 are much disputed, may be looked on as 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 proto- 

 plasm 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 in the growth of bacteria which 

 must be considered, namely, food supply, moisture, relation to 

 gaseous environment, temperature, and light. 



Food Supply. — The bacteria are chiefly found living on 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. 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 complexity of 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 the natural 

 conditions. 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, when a species is sown on a mass of artificial 

 food medium, it does not in the great majority of cases go on 

 growing till the food supply is exhausted, but soon ceases to 

 grow. Effete products diffuse out into the medium and prevent 



