100 PRINCIPLES OF BACTERIOLOGY 



or in a special degree, possess the property of assimilating and combin- 

 ing nitrogen. It seems, moreover, to have been recently established 

 that, independently of the assistance of the legumes, certain nodule 

 bacteria exist free in the soil, which accumulate nitrogen by absorbing 

 it from the air (Stutzer). 



Formation of Acids from Carbohydrates. Free acids are formed by 

 many bacteria in culture media containing sugar; the production of 

 acid in ordinary bouillon takes place on account of the presence of 

 grape-sugar, which is usually derived in small quantities from the meat. 1 

 According to Theobald Smith, all anaerobic or facultative anaerobic 

 bacteria form acids from sugar; the strict aerobic species do not, or so 

 very slowly that the acid is concealed by the almost simultaneous 

 production of alkali. The formation of acid occurs sometimes with 

 and sometimes without the production of gas. Excessive acid produc- 

 tion may cause the death of the bacteria from the increase in acidity of 

 the culture media. 



If after the sugar is consumed not enough acid has been formed to 

 kill the bacteria, the acid is neutralized gradually and in the end the 

 reaction becomes alkaline. 



Among the acids produced the most important is lactic acid; also 

 traces of formic acid, acetic acid, proprionic acid, and butyric acid, 

 and not infrequently some ethyl-alcohol and aldehyde or acetone are 

 formed. Occasionally no lactic acid is present, and only the other acids 

 are formed. 



Various bacteria, as yet incompletely studied, possess the property 

 of producing butyric acid and butyl-alcohol from carbohydrates. 



Some bacteria also seem to have the power of decomposing cellulose. 



Formation of Gas from Carbohydrates and Other Fermentable Substances 

 of the Fatty Series. The only gas produced in visible quantity in sugar- 

 free culture media is nitrogen. If sugar is vigorously decomposed by 

 bacteria, as long as pure lactic acid or acetic acid is produced there may 

 be no development of gas, as, for instance, with the B. typhosus on 

 grape-sugar; but frequently there is much gas developed, especially in 

 the absence of air. About one-third of the acid-producing species also 

 develop gas abundantly, this consisting chiefly of CO 2 , which, according 

 to Smith, is always mixed with H. Marsh-gas is seldom formed by 

 bacteria, with the exception of those decomposing cellulose. 



In order to test the production of gas, a culture medium composed 

 of glucose-agar, containing about 1 per cent, grape-sugar, may be used. 

 At the end of eight to twelve hours in the incubator (or twenty-four 

 hours' room temperature) the agar will be seen to be full of gas-bubbles 

 or broken up into holes and fissures. 



For the determination of the quantity and kind of gas produced by a 

 given micro-organism the fermentation tube recommended by Theobald 

 Smith is the best. This is a bent tube, constricted greatly at its lowest 



1 According to Theobald Smith, 75 per cent, of the beef ordinarily bought in the markets contains 

 appreciable quantities of sugar (up to 0.3 per cent.). 



