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 nore 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 
