Structure, Growth and Distribution. 1 



animals are called parasitic bacteria. To the latter 

 group belong those especially important forms, the pa- 

 thogenic bacteria, which produce the transmissible dis- 

 eases of plants and animals. Some species ordinarily 

 lead a saprophytic life in the soil, but may, if accident 

 introduces them into the bodies of animals, grow and 

 produce serious trouble. These forms are not to be 

 looked upon as parasites since their natural habitat is- 

 not the body of a living animal. 



The saprophytic as also the parasitic bacteria vary 

 widely as to the kind of food best adapted to their dif- 

 ferent needs. Some kinds of parasitic bacteria grow 

 only in the body of man, others only in certain animals; 

 while still other types grow in the bodies of a large 

 number of different kinds of animals. Some of the soil 

 bacteria grow only on certain kinds of food substances, 

 while others flourish on the widest variety of materials. 



Condition of the food. Since the food must be ab- 

 sorbed by the bacterial cell before it can be used, it is 

 necessary that it be in solution. Some forms of bac- 

 teria are able to develop digesting substances, known as. 

 enzymes, by means of which they render insoluble ma- 

 terial soluble, utilizing the same in part as food. 



Bacteria can not, as a rule, grow well in acid sub- 

 stances, a fact which is widely applied in the preserva- 

 tion of human and animal foods. In this respect they 

 differ from most of the fungi which thrive preferably 

 on acid rather than on neutral or alkaline substances. 

 Jellies and plant juices, therefore, spoil from mold de- 

 velopment while blood and animal products as broths 

 and soups undergo bacterial decomposition. 



Oxygen supply of bacteria. Every living thing, even 

 every living cell of the growing plant or animal, must 



