RELATION TO ENVIRONMENT CLASSIFICATION 29 



while chlorides, on the other hand, according to Proskauer l and Beck 

 are not absolutely essential. Sodium salts, as a rule, seem to be more 

 advantageous for purposes of bacterial cultivation than potassium salts. 



The uncombined sulphur, which is often a constituent of bacteria, 

 is usually supplied by soluble sulphates. In the case of the thiobacteria 

 of Winogradsky, however, free H 2 S is necessary for its formation. 2 



The iron contained in some higher bacteria is taken in as ferrous 

 compounds, and is oxidized in the bacterial into ferric compounds. 



The relative quantities of various nutrients in culture media are 

 important in so far as too high concentrations may inhibit growth. In 

 this respect, however, separate species vary. 



The development of bacteria is far oftener arrested by the accumula- 

 tion of waste products than by an exhaustion of nutrient materials. 



PARASITISM AND SAPROPHYTISM 



When we speak of bacteria as parasites or as saprophytes, we classify 

 them, primarily, according to their relationship to the bodies of higher 

 animals. ''Parasites" are those bacteria which are capable of living 

 and multiplying within the human or animal body, whereas the term 

 "saprophytes" refers to the multitude of microorganisms which are 

 unable to hold their own under the environmental conditions found in 

 the tissues of higher animals, but are found, almost ubiquitously, in air, 

 soil, manure, and water. The 'separation is by no means a sharp one 

 and carries with it other implications, which the use of these terms always 

 conveys. While parasites are usually very fastidious as to nutritional 

 and temperature requirements, most saprophytes are easily cultivated 

 upon the simplest media. Thus certain parasitic bacteria, such as the 

 bacillus of influenza, the gonococcus, and others, are dependent upon 

 specific forms of animal proteids for their food supply, while typical 

 saprophytes, like Bacillus proteus, may thrive and multiply upon even 

 the simplest organic proteid derivatives. 



Between the strict parasites and the saprophytes, there is a large 

 cla^s of bacteria, to which the majority of pathogenic varieties belong, the 

 members of which are capable of developing luxuriantly under both con- 

 ditions. These bacteria are often spoken of as facultative parasites. 



More recently the question of parasitism and saprophytism has 



1 Proskauer and Beck, Zeit. f. Hyg., xviii, 1895. 



2 Voges, Cent. f. Bakt., I, xviii, 1893. 



