BACTERIA 33 



individuals retain their vitality for months and even years, especially 

 at low temperature. The respiratory quotient remains constant in 

 the same species under identical conditions and is a measure of rate 

 of growth and multiplication. 



All living cells must depend on the pabulum within their reach. 

 This must be digested or broken up into particles which will fit into 

 the bacterial molecule. This is accomplished by agents which we 

 designate as ferments or enzymes. Each species of bacteria elaborates 

 its own specific enzymes, the functions of which are to cut the 

 pabulum into proper blocks and to place these blocks in the structure 

 of the cell. These enzymes are specific in two senses. First, they are 

 specific in origin; the ferments produced by typhoid bacilli are not 

 identical with those of anthrax bacilli. Secondly, they are specific 

 in their action. Given the same pabulum, the shape and size of the 

 blocks into which it is split differ with the ferment acting on it. 

 Besides, a given ferment cannot act on all pabulum. What is food to 

 one bacterium may be of no value to another. 



Pathogenitity. Why is it that one bacterium is capable of induc- 

 ing disease while another is harmless? The answer to this is quite 

 clear. Some bacteria can feed only on dead matter. Their ferments 

 will not prepare the fluid and tissues of the animal body for the 

 sustenance of their cells; or it may be that the ferments of the body 

 cells digest these bacteria and prevent their growth. These bacteria, 

 which compose the great majority of existing species are known as 

 saprophytes. There are other bacteria which can digest, absorb and 

 eliminate the constituents of the fluids and tissues of the animal body 

 and which are not digested by the body cells. These are parasites and 

 their growth and reproduction in the body cause disease and conse- 

 quently such bacteria are said to be pathogenic. Such an organism 

 may be able to grow in one species of animal and not in others. It 

 is pathogenic only to those animals in which it can grow. It may be 

 able to grow in certain individuals of a species and not in others. To 

 the man who has had smallpox or has been properly vaccinated, the 

 smallpox virus is not pathogenic. Vaccination has developed in the 

 body cells the function of producing a ferment which digests small- 

 pox virus and consequently this cannot grow and multiply in the body 

 of the vaccinated man. 



