FOOD OF MICROORGANISMS. 99 



called " sulphur bacteria" certainly belong to one physiological group 

 which is very plainly denned by their ability to oxidize hydrogen sulphide 

 to sulphur and sulphuric acid. The official nomenclature distributes 

 them, however, into several genera. The "nitrate bacteria" are another 

 group having a very definite physiological character. In some groups 

 the morphological and physiological characters agree very largely, e.g., 

 in the butyric bacteria which are all spore-forming rods with a tendency 

 to show spindle form, and in the Azotobacter group which is morpho- 

 logically distinctly different from any other bacteria. 



With some other more general terms like " lactic bacterium," "putre- 

 fying organism," "acid producer," the quality indicated is very vaguely 

 expressed and therefore of little significance'. 



Another feature should be mentioned in discussing the relation of 

 microorganisms and their food, namely the fact that some organisms 

 prefer certain foods and live exclusively on a very few chemical com- 

 pounds, being unable to assimilate any others, while other organisms are 

 able to feed on nearly every organic substance. The nitrifying bacteria are 

 unable to use anything but ammonium salts. Certain pathogenic bacteria 

 require a very special medium. The invisible organisms cannot be culti- 

 vated on any medium except the living tissue. The fermenting yeasts can 

 grow on protein media without sugar, though their development is meager. 

 Some protozoa feed on many kinds of living bacteria, but cannot eat 

 dead bacteria. Of the scavengers which can live on all kinds of food, 

 the molds have many examples, living not only on protein, sugars, starch, 

 cellulose, fat, but also able to exist on inferior food like alcohol, acetic 

 or oxalic acids, with often no other nitrogen source than ammonium salt. 

 Such omnivorous species are also found among bacteria, especially among 

 sewage and soil bacteria. 



SYNTHETIC MEDIA. Recent investigations, especially by Gorham and 

 his associates, of the physiology of microorganisms have shown that 

 most organisms do not necessarily require media made from meat extracts, 

 peptone and similar unknown compounds. A large number even of 

 pathogenic bacteria which had been believed to specialize very particularly 

 in their food requirements can be grown on synthetic media containing 

 only compounds of well-known chemical composition, as amino-acids 

 (asparagin, glycocoll), urea, ammonium salts of tartaric, succinic or lactic 

 acids, dextrose, glycerin, and perhaps other organic substances, besides 

 the necessary mineral salts. The great advantage of these media is that 



