34 GENERAL BIOLOGY 



bacteria in the alimentary tract, but some of those 

 present are beneficial in effect. They gain access in the 

 food and air. They assist in making fats more easily 

 assimilable, and they destroy some of the pathogenic 

 bacteria. Incidentally it might be added that their 

 activity in producing excessive putrefaction and fer- 

 mentation may be harmful to the body in general. 



Bacteria require for their life moisture, some degree 

 of heat, and a variety of foodstuffs. 



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 



