VII] CHEMISTRY OF BACTERIA 125 



4. A further not uncommon phenomenon is the formation 

 of gas (methan gas or marsh gas). This is best shown by 

 making the inoculation into deep gelatine or by inoculating 

 the gelatine, then melting it, shaking it, and letting it again 

 solidify — " shake culture." On incubation every colony that 

 appears in the depth of the gelatine is associated with 

 a gas bubble. A shake culture of ordinary nutrient gelatine 

 after inoculation with bacillus coli gives a very character- 

 istic appearance, being in its deeper layers crowded with 

 small gas bubbles. After some days they become fewer, 

 most of them escaping to the surface. In the cultures in 

 deep sugar-gelatine of bacillus of Koch's malignant 

 oedema, of the bacillus of symptomatic anthrax, of tetanus, 

 the formation of gas is a conspicuous feature. Some species 

 of bacillus coli form copiously gas bubbles in deep 

 nutrient agar cultures and even in broth cultures as the 

 growth becomes conspicuous, e.g. after twenty-four to thirty- 

 six hours at 37° C. ; on watching the culture numerous small 

 gas bubbles are seen to ascend to the surface. 



5. A number of microbes have the power to produce in 

 special materials specific chemical changes representing 

 specific fermentations. The alcoholic fermentation of sugar 

 by yeast is the best-known and longest-established instance ; 

 the acid fermentation (oxidation of alcohol) by bacterium 

 aceti and mycoderma aceti, the change of lactic sugar into 

 lactic acid by various species of bacterium lactis and other 

 bacilli, is a widespread one ; so also is the formation of butyric 

 acid by bacillus butyricus (van Tighem). The hydration of 

 urea and conversion into ammonium carbonate by micre- 

 coccus ureae, the dextrose fermentation, the mannit fer- 

 mentation, are further instances. In this category must be 

 included the conversion of albumen into peptone, previously 

 described. In all these instances a particular substance, 



