90 



ACTIVITIES OF BACTERIA. 



If sugar is broken up energetically by bacteria, gas- 

 formation may be absent, only lactic or acetic acid 

 being produced (for example, Bac. typhi on grape- 

 sugar), but very often an enormous production of gas 

 occurs, especially if air is excluded. About one-third 

 of the vigorous acid-forming varieties produce abundant 

 gas, which consists always of carbonic acid, with a 

 constant admixture, according to Smith (C. B. xvm, 1), 

 of hydrogen. Marsh-gas appears to be rarely formed 

 (aside from the bacteria causing fermentation of cellu- 

 lose). Compare in special part : Bact. brassicse acidae. 



To determine whether gas is formed, the shake-culture 

 in \°/o grape-sugar agar is very useful (Fig. 11). After 



Fig. 12. — Fermentation tube. 



twenty-four hours (if incubator temperature is available, 

 often after six to twelve hours) the agar is beset by gas 

 bubbles or cleft by numerous deep holes and cracks. If it 

 is desirable to collect and measure the gas, to investigate 

 the curve of the intensity of gas production, or to analyze 

 it, the gas is best collected after the method of Th. Smith in 

 a fermentation tube, such as has been long employed in 

 physiologic and pathologic chemistry (Fig. 12). 



The tubes, which preferably have the same form, are 

 filled with 1% grape-sugar peptone-bouillon (without air- 

 bubbles) and sterilized in the steam sterilizer. 



After inoculating with a platinum loop, they are kept 



