16 APPLIED BACTERIOLOGY 



further termed ' obligate ' parasites if they can only live on 

 this ' host.' If the bacteria draw their nourishment from 

 dead organic matter, they are called ' saprophytes.' These 

 are also divided into ' obligate ' and ' facultative ' sapro- 

 phytes. Thus, it will be seen that a parasite under certain 

 circumstances may readily become a saprophyte. It may 

 be said in general terms that at the present time know- 

 ledge of the life-history of facultative parasitic bacteria in 

 saprophytic conditions is relatively less than knowledge of 

 their parasitic existence. The importance of further in- 

 vestigation in this direction is nevertheless very great, and 

 its progress may lead to conclusions on many important 

 questions — such, for instance, as the persistence of infection 

 outside the human body, in regard to which existing data 

 are inconclusive. 



Some of the more important saprophytes play an im- 

 portant and useful part in our everyday life, such as, for 

 instance, in the phenomena of fermentation, and also as 

 the putrefaction agents which transform dead and de- 

 composmg organic matters into their simpler elements, thus 

 completing the great life cycle, and rendering the dead and 

 effete matter again ready for the vital processes. 



Amongst other life manifestations of the bacteria may be 

 mentioned those which have the property of generating 

 colouring matter, though not chlorophyll. The bacteria 

 themselves are colourless and transparent, and the pigment 

 is merely formed as a product of their metabolism, especially 

 under the influence of light. Many of the bacteria give 

 rise to various gases and odours, particularly the anaerobic 

 organisms which give rise to very foul putrefactive gases 

 (ammonia, sulphuretted hydrogen, etc.). The Bacillus 

 prodigiosus gives rise to a smell resembling that of tri- 

 methylamine. 



Micro-organisms have the property of producing various 



