KELATION TO ENVIRONMENT— CLASSIFICATION 29 



tion as water and together with the carbon and nitrogen containing 

 substances. 



Salts. — ^The phosphatic constituents of the bacterial body are taken 

 in, chiefly, as phosphates of magnesium, calcium, sodium, or potassium. 

 The phosphates seem to be necessary constituents of culture media, 

 while chlorides, on the other hand, according to Proskauer '■ 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 a constituent of the bacterial body 

 in many cases, is usually supplied by soluble sulphates. In the case of 

 the thiobacteria of Winogradsky, however, the presence of free HjS is 

 necessary for its formation.^ 



The iron contained ia some of the higher bacteria is taken in in the 

 form of ferrous compounds, and is oxidized in the bacterial body into 

 ferric compounds. 



The relative quantities of the various nutritive substances in culture 

 media are of importance only in so far as too high concentrations may 

 have a distinctly inhibitory influence. In this respect, however, separate 

 species may show widely divergent tastes. 



The development of bacteria in any given medium, it may be noted, 

 is far oftener arrested by the accumulation 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 " sapro- 

 phytes " refers to the multitude of microorganisms which are unable to 

 hold their own under the environmental conditions found in the tis- 

 sues 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 



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

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



