232 LABORATORY EXERCISES IN BACTERIOLOGY. 



which sugar in small proportions has been added, is explained the frequently observed 

 phenomenon of an early acid reaction in such growths, followed later and overcome 

 to an alkaline reaction by the alkaline substances produced in the splitting of the 

 nitrogenous molecules into ammoniacal products. These fermentations are probably 

 also due to enzymes. It seems probable that in the ordinary alcoholic fermentation 

 this enzyme is retained in the bodies of the ferment cells. Such an enzyme by high 

 pressure has been obtained from yeast cells and is known as zymose; and it seems 

 likely that a similar or identical substance exists in bacterial cells capable of the same 

 process. In effect, in such processes the fermentescible substance probably goes through 

 the bacterial body and the process is closely related to metabolism. It probably is 

 performed for acquirement of some of the energy (heat) set free in the splitting process. 

 Closely related to these last considerations is the power of 



4. Gas Production. A number of gases may be produced by various bacteria 

 in different media. In the sugar-containing media in the conversion of the 

 sugar molecule into alcohol and carbon dioxide, the latter, as well perhaps as a few 

 other gases, as hydrogen and methane, are produced ; and in the albuminous media 

 ammoniacal gases and sulphuretted hydrogen may be encountered. For the purpose 

 of observation of gas production solid media may be utilized, note being taken of gas 

 bubbles produced in the media which have been inoculated by bacteria capable of 

 generating such gases. Or liquid media in the fermentation tubes described in a pre- 

 vious section may be employed. 



Exercise 63. A fermentation tube is filled without bubbles with a 

 bouillon containing one per cent, of glucose or other sugar, sterilized in the 

 autoclave, and after cooling is inoculated with Bacillus coli and put aside 

 for twenty-four to forty-eight hours in the incubator, when gas will be 

 found to have collected in the closed end of the tube. This gas is largely 

 composed of carbon dioxide from sugar destruction. There is usually 

 active sulphuretted hydrogen formation as well. To recognize the latter, 

 the following procedure may be practised. A slip of paper moistened with 

 a solution of acetate of lead is fixed in the bulb of the tube alongside of the 

 cotton plug and a dark rubber cap (free from sulphur) tightly fitted over 

 the opening. By tilting the tube the gas is now permitted to pass to the 

 bulb, and after allowing it to stand for a time, the lead will be found black- 

 ened into the sulphide. 



If now in a similar tube the amount be marked by wax-pencil mark on 

 the glass, and the bulb be filled with a ten per cent, solution of sodium 

 hydrate, the thumb being tightly placed over the opening and the tube 

 inverted and shaken so as to bring the soda solution and the gas together, 

 and the gas thereafter returned to the closed end of the tube, the amount 

 will be found diminished. This loss represents the proportionate amount 

 of carbon dioxide which was present, and which has been absorbed by the 

 sodium hydrate. In determining the amount of gas generated in these 

 tubes proper corrections should be made for atmospheric pressure and 



