BIOLOGY OF BACTERIA. 51 



Zymogenic, or bacteria of fermentation. 

 Saprogenic, or bacteria of putrefaction. 

 Chromogenic, or color producers. 

 Photogenic, or phosphorescent bacteria. 

 Aerogenic, or gas producers. 

 Pathogenic, or disease producers. 



The parasitic organisms alone possess much interest to 

 the physician, but as in their growth the saprophytes ex- 

 hibit many interesting vital manifestations, it is not well 

 to exclude them or their products from the following 

 consideration of the 



Results of Vital Activity in Bacteria. — 1. Fermenta- 

 tion. — The alcoholic fermentation, which is a familiar phe- 

 nomenon to the layman as well as to the brewer and the 

 chemist, is not the work of a bacterium, but of a yeast- 

 plant, one of the saccharomyces fungi. The acetic-acid, 

 lactic-acid, and butyric-acid fermentations are, however, 

 caused by bacilli. A considerable number of bacilli seem 

 capable of converting milk-sugar into lactic acid, some- 

 times associating this with coagulation of milk, some- 

 times not. The production of coagulation in milk is not 

 always associated with acid-production, but with the pro- 

 duction of a curdling ferment similar to that belonging 

 to the gastric juice. There seems to be no real specific 

 micro-organism for the lactic-acid fermentation, although 

 the Bacillus acidi lactici seems to be the most powerful 

 generator of the acid. There may also be several bac- 

 teria which produce the acetic fermentation, though it is 

 generally attributed to a special common form, the Myco- 

 derma aceti or Bacillus aceticus. The butyric fermenta- 

 tion is generally due to the Bacillus butyricus, though it 

 also may be caused by other bacilli, the one named sim- 

 ply being the most common. (For an exact description 

 of the chemistry of the fermentations reference must be 

 made to text-books upon that subject, as their considera- 

 tion here would occupy too much space.) 



2. Putrefaction. — This process is in many respects sim- 



