VITAL PHENOMENA OF BACTERIA, 65 



to albumin, is due to the presence of a proteolytic fer- 

 ment or trypsin. It is not pepsin, as pepsin acts only 

 in the presence of acid, and gelatin is liquefied with an 

 alkaline reaction only. The production of proteolytic 

 ferments by different cultures of the same varieties of 

 bacteria varies considerably, far more than is generally 

 supposed. Even among the freely liquefying bacteria, 

 such as the cholera spirillum and the staphylococcus, 

 poorly liquefying varieties have been repeatedly found. 

 These observations have detracted considerably from 

 the value in cultures of the property of liquefying 

 gelatin as a positive diagnostic characteristic. Most 

 conditions which are unfavorable to the growth of bac- 

 teria seem to interfere also with their liquefying power. 



Certain bitter-tasting products of decomposition are 

 formed by liquefying bacteria in media containing albu- 

 min, as, for example, in milk. 



Diastatic ferments convert starch into sugar. That 

 these are produced by bacteria is shown by mixing 

 starch paste containing 1 per cent, thymol with cultures 

 to which 1 to 2 per cent, thymol has been added, and 

 keeping the mixture for six to eight hours in the incu- 

 bating oven; then, on the addition of Fehling's solution 

 and heating, the reaction for sugar appears the red- 

 dish-yellow precipitate due to the reduction of the 

 copper. Bacteria may be directly tested for sugar also 

 by boiling potato-broth cultures and using the extract. 



Inverting ferments that is, those which convert cane- 

 sugar into grape-sugar are of very frequent occurrence. 

 Bacterial invertin withstands a temperature of 100 C. 

 for more than an hour, and is produced in culture media 

 free from albumin. 



Rennet ferments substances having the power of 



5 



