34 BIOLOGY AND TECHNIQUE 



which are unable to hold their own under the environmental condi- 

 tions 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 gono- 

 coccus, and others, are dependent upon specific forms of animal 

 proteins for their food supply, while typical saprophytes, like Bacil- 

 lus proteus, may thrive and multiply upon even the simplest organic 

 protein derivatives. 



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

 class of bacteria, to which the majority of pathogenic varieties 

 belong, the members of which are capable of developing luxuriantly 

 under both conditions. These bacteria are often spoken of as facul- 

 tative 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 para- 

 sites 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 spirillum of cholera which gain a foothold upon some part of the 

 body of the host, but do not actually penetrate into the general cir- 

 culation. 



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

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

 far more important task of breaking up organic matter outside of 

 the animal body, by putrefaction and fermentation. Absolute sepa- 

 ration 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 resist- 

 ance to bacterial invasion is at a low ebb. 



ANTAGONISM AND SYMBIOSIS OF BACTERIA 



The ubiquity of bacteria in nature of course implies the simulta- 

 neous presence of many species in all places where special conditions 

 have provided a favorable environment for growth. Thus bacterio- 



