80 BACTEEIOLOGY. 



ber of acids, ferments and the like. The composition of 

 the nutrient medium will, of course, influence the kind of 

 products. Thus, grape-sugar is readily fermented by the 

 yeast-plant yielding alcohol, carbonic .acid and other pro- 

 ducts whereas milk-sugar is not affected. 



The several chemical compounds elaborated by a given 

 species are subject to variation depending upon the health 

 and vigor of the organism. If the latter has been weakened 

 by an abnormally high temperature, by sun-light, or by pois- 

 onous chemicals it will be found that this change in the phy- 

 siological condition of the cell will be evidenced by altered 

 waste-products. Thus, a normal species may give rise to a 

 bright-red pigment whereas the attenuated or weakened 

 variety may yield a perfectly colorless growth. Again, a 

 certain organism may normally curd le milk in a few hours, but 

 when altered it will do so very slowly or not at all. Numerous 

 other instances might be cited to show that the kind of pro- 

 ducts formed depends not only upon the composition of the 

 soil, but also upon the physiological condition of the cell. 



In the study of the chemistry of bacteria and allied 

 forms it will frequently be found that a given chemical 

 compound is produced by several distinct species. Alcohol 

 is said to be the product of the yeast-plant. More than a 

 score of species of the yeast-plant, however, are known to 

 possess the power of making alcohol. One species may, of 

 course, produce a much larger amount than another. More- 

 over, alcohol is produced by many bacteria and by moulds. 

 "What has been said of alcohol is equally true of acetic, lac- 

 tic, butyric acids and other compounds. In other words, 

 entirely different species of organisms may give rise to a 

 given chemical compound, though in different amounts. It 

 is evident, therefore, that alcoholic, acetic, etc., fermenta- 

 tions are not necessarily due to one species. 



When bacteria grow in a tube or flask the various waste- 

 products which they elaborate remain in the liquid in direct 



