30 BIOLOGY AND TECHNIQUE 



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, however, there is a 

 large class of bacteria, to which the majority of our pathogenic varieties 

 belong, the members of which are capable of developing luxuriantly 

 under both conditions. These bacteria are often spoken of as facultative 

 parasites. 



More recently the question of parasitism and saprophytism has 

 become closely interwoven with our conceptions of virulence. Bail 

 (see section on Aggressins) has classified parasites into strict parasites 

 and half parasites. By the first term he designates bacteria like Bacillus 

 anthraeis, which actually invade all the tissues of their host, while, 

 by the term "half parasites," he refers to microorganisms like the spiril- 

 lum of cholera which gain a foothold upon some part of the body of the 

 host, but do not actually penetrate into the general circulation. 



All pathogenic bacteria, therefore, must be grouped as parasites, 

 strict or facultative, while the saprophytes, as a class, perform the far 

 more thankful task of breaking up organic matter outside of the animal 

 body, by putrefaction and fermentation. Absolute separation between 

 the two classes, however, can not be maintained, since many ordinarily 

 saprophytic bacteria may display parasitic qualities if administered in 

 large numbers to animals or man in whom resistance to bacterial 

 invasion is at a low ebb. 



ANTAGONISM AND SYMBIOSIS OF BACTERIA 



The ubiquity of bacteria in nature naturally carries with it the simul- 

 taneous presence of many species in all places where special conditions 

 have provided a favorable environment for growth. Thus bacteriological 

 investigation of water, milk, manure, soil, or organic infusions, always 

 reveals the presence of a large number of different varieties within the 

 same substance. If the food supply in such a natural culture is at all 

 limited in quantity, or the removal of waste products is prohibited, it 

 will usually be found that gradually the numbers of varieties will dimin- 

 ish and a few species, or even only one, will prevail. In the case of milk, 

 for instance, after standing for three or four days at a suitable temper- 

 ature, two or three varieties will be found to have taken the place of 

 the twenty or thirty, which may have been present originally. 



