52 Biology of Bacteria 



malin renders the gelatin insoluble. Some of the bacteria 

 liquefy the gelatin in such a peculiar and characteristic 

 manner as to make the appearance a valuable guide for the 

 differentiation of species. 



Production of Acids and Alkalies Under the head 

 of "Fermentation" the formation of acetic, lactic, and 

 butyric acids has been discussed. Formic, propionic, baldri- 

 anic, palmitic, and margaric acids also result from microbic 

 metabolism. As the acidity progresses, it impedes, and 

 ultimately completely inhibits, the activity of the bacteria. 

 The cultivation of the bacteria in milk to which litmus or 

 lacmoid has been added is a convenient method for detecting 

 changes of reaction. Rosolic acid solutions may also be 

 used, the acid converting the red into an orange color. 

 Neutral red is also much employed for this purpose, the 

 acids turning it yellow. 



The quantitative estimation of changes in reaction can 

 be best made by titration, and the fermentation-tube culture 

 can be employed for the purpose. The contents of the bulb 

 and branch should be shaken together, a measured quan- 

 tity withdrawn, and titration with ^ sodium hydroxid, or 

 - hydrochloric acid, performed. 



The alkali most frequently formed by bacterial growth 

 is ammonium, which is set free from its combinations, 

 and either flies off as a gas or forms new combinations 

 with acids simultaneously formed. Some bacteria produce 

 acids only, some alkalies only, others both acids and alka- 

 lies. Both acids and the alkalies, when in excess, serve to 

 check the further activity of the micro-organisms. 



Chromogenesis. Bacteria that produce colored colo- 

 nies or impart color to the medium in which they grow are 

 called chromogenic; those producing no color, non-chromo- 

 genic. Most chromogenic bacteria are saprophytic and non- 

 pathogenic. Some of the pathogenic forms, as Staphylo- 

 coccus pyogenes aureus, are, however, color producers. It 

 seems more likely that certain chromogenetic substances 

 unite with constituents of the culture medium to pro- 

 duce the colors than that the bacteria form the actual 

 pigments ; but, as Galeotti * has shown, there are two 

 kinds of pigment, one being soluble, readily saturating the 

 culture medium, as the pyocyanin and fluorescin of Bacillus 



* " Lo Sperimentale," 1892, XLVI, Fasc. m, p. 261. 



