MICEOBES— INFECTION— IMMUNITY 455 



ments, while the photobacteria pofssess phosphorescent 

 properties. 



3. ParatrojMc bacti-ria are true {ohligatory) parasites 

 which can only exist in living animals, and under natural 

 conditions do not develop outside the body. Some of them 

 may be cultivated under certain conditions and on suitable 

 material. They act chiefly by their toxins, the chemical 

 nature of which has not yet been determined. Soluble 

 toxins may be excreted by the living organism, while others 

 keep to the cell and only escape after its death; such 

 extractive poisons are Tuberculin and Mallein. 



Infection. 



Pathogenic organisms are either obligatory parasites — viz., 

 can only exist in living animals, for example glanders — or 

 facultative parasites — viz., capable of growing outside the 

 body, such as the microbe of quarter-evil. The latter class 

 of parasites live in a metatrophic way, and when they gain 

 access to the body may develop there. 



Sajjropliytes live on dead organic matter. It is possible 

 that saprophytes may acquire pathogenic properties, just 

 as, on the other hand, pathogenic bacteria may lose their 

 virulence and live as saprophytes. As a rule saprophytes 

 are harmless, notwithstanding which the products of some 

 of them may be pathogenic when they gain access to the 

 body. 



Saprophytes may be so altered artificially as to grow 

 as parasites. A good example of this is the sapro- 

 phytic bacterium of potatoes, B. megatherium, which under 

 ordinary circumstances is harmless, but may, by being 

 grown in a collodion bag in the abdomen of the guinea- 

 pig, be converted into a pathogenic organism capable of 

 killing the same animal. 



This experiment explains that pathogenic bacteria may 

 at one time have been saprophytes, and owing to the 

 adaptability become true parasites. Conversely, it is possible 

 to tame some highly pathogenic organisms, as for instance 

 B. anthracis, so as to acquire purely saprophytic habits. 



Digitized by Microsoft® 



