32 BACTERIOLOGY 



SAPROPHYTES AND PARASITES. In the case of certain 

 bacteria, in fact, the majority, the source of food-supply 

 must of necessity be dead organic matters of either animal 

 or vegetable origin. They cannot exist in the presence of 

 living tissues. To the members of this group the designa- 

 tion saprophytic or metatrophic (A. Fischer) is given. To 

 that group that can exist only upon living organic matters, 

 and herein belong many (not all) of the disease-producing 

 bacteria, the appellation parasitic or paratrophic (A. Fischer) 

 is applied; while for the few species that either do not 

 require organic matters, or do not, so far as is known, have 

 the faculty of decomposing and assimilating proteid stuffs 

 at all, the name prototrophic is suggested by Fischer. In 

 the strict sense of the word, a parasite can exist only in the 

 body of a living host, and a saprophyte only upon lifeless 

 organic matters, and such obligate parasites and saphrophytes 

 are known, but in the majority of cases such nutritive con- 

 ditions are not obligatory, many of both parasites and 

 saprophytes having the power to adapt themselves to 

 conditions other than those for which they are by nature 

 best fitted. For instance, certain species that exhibit their 

 most important properties under conditions of parasitism 

 may, nevertheless, lead a saprophytic existence when cir- 

 cumstances demand it, and, on the other hand, particular 

 species usually saprophytic by nature may find conditions 

 favorable to their development in a living host. To such 

 adaptable species the designation "facultative" is given, 

 and, when employed, signifies that the species in question 

 has the faculty of adapting itself to environments other 

 than those in which it is usually encountered. In this sense 

 all of the disease-producing bacteria that can be cultivated 

 artificially are manifestly facultative saprophytes. 



