30 BIOLOGY AND TECHNIQUE 



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 

 anthracis, which actually invade all the tissues of their host, while, by 

 the term "half parasites," he refers to microorganisms like the spirilum 

 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 of course implies the simul- 

 taneous presence of many species in all places where special conditions 

 have provided a favorable environment for growth. Thus bacteriologi- 

 cal 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. 



This behavior is due to the influence which various microorganisms 

 exert upon each other and is known as antagonism. Such antagonism 

 probably depends upon the fact that the metabolic products of the pre- 

 dominant species (the one or ones for whom the special cultural condi- 

 tions are most favorable) inhibit the growth of the less vigorous varieties. 

 Many examples, experimentally supported, of such antagonism, can be 

 given. Thus, the gonococcus is distinctly inhibited by the soluble pro- 

 ducts of Bacillus pyocyaneus, l while in the presence. of pyogenic cocci it 



1 Schafer, Fortschr. d. Med., 5, 1896. 



