36 GENERAL BIOLOGY 



The reaction of the material upon which they are 

 growing is of no small importance. Nearly all bacteria 

 live best when the medium is about neutral or of 

 faintly alkaline or acid reaction. All need carbon, 

 oxygen, nitrogen, hydrogen, and salts. Some organisms 

 cannot live in the presence of free oxygen, but obtain 

 it as they need it by breaking up, or reducing, sub- 

 stances containing this element. These are called 

 anaerobic bacteria, such as the tetanus bacillus. Micro- 

 organisms that can live in the presence of atmospheric 

 oxygen are called aerobic. Most pathogenic forms have 

 this power. 



The foodstuffs presented to bacteria are seldom in 

 a pure state, so that the power of breaking up the 

 material on which they are existing into the elements 

 necessary for the life of the cell has to be done by some 

 process of cellular activity. To do this, bacteria form 

 what are called enzymes or ferments. An enzyme or 

 ferment is a product capable of changing a chemical 

 combination without itself entering into the product 

 of this change. The bacterial enzymes are comparable 

 to the enzymes found in the digestive juices of the 

 human alimentary canal. There are many kinds of 

 ferments, each having the power of breaking up certain 

 chemical substances. There are ferments splitting up 

 sugars and starches and fats and proteids, and the 

 result of this splitting is simpler in composition than 

 the substance split, thus making it easier of use as 

 food. The ferment activity of bacteria is just like 

 that of yeasts which are used in the industries, especially 

 that of spirituous liquor-making. In this case the 

 organisms and their enzymes are capable of splitting 



