VITAL PHE\".M1-:XA OF BACTERIA 89 



may be accomplished by the addition of disinfectants carbolic acid, 

 chloroform, ether, etc. to the cultures or by filtration. 



( HAH.U TKRISTICS OF FEKMKXTs. Ferments are non-dialyzable. 

 They withstand moderate dry heat, but are usually destroyed in 

 watery solutions by a temperature of over 70 C. They are injured 

 by acids, especially the inorganic ones, but are resistant to all alkalies. 

 A simple example of bacterial fermentation of carbohydrates produced 

 by an enzyme is that of grape-sugar: 



C 6 H 12 6 2C 2 H 6 + 2CO 2 



Grape-sugar. 2 Alcohol. 2 Carbon dioxide. 



Or, 



C 6 H 12 6 2C 3 H 6 S 



Grape-sugar. 2 Lactic acid. 



gj 2 g -> 42 



Grape-sugar. 3 Acetic acid. 



Far less common is oxidizing fermentation, as in the production of 

 acetic acid from alcohol. Here the energy is acquired not by the 

 decomposition but by the oxidation of the alcohol. 



The proteolytic or peptonizing ferments which are somewhat anal- 

 ogous to trypsin being capable of changing albuminous bodies into 

 soluble and diffusible substances are very widely distributed. The 

 liquefaction of gelatin, which is chemically allied to albumin, is due to 

 the presence of a proteolytic ferment or trypsin. The production of pro- 

 teolytic 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 cul- 

 tures of the property of liquefying gelatin as a positive diagnostic char- 

 acteristic. Most conditions which are unfavorable to the growth of 

 bacteria seem to interfere also with their liquefying power. 



Certain bitter-tasting products of decomposition are formed by 

 liquefying bacteria in media containing proteid, as, for example, in milk. 



Diastatic ferments convert starch into sugar. That these are pro- 

 duced by bacteria is shown by mixing starch paste with cultures to the 

 resulting mixture of which thymol has been added, and keeping the 

 digestion for six to eight hours in the incubating oven; then, on the 

 addition of Fehling's solution and heating, the reaction for sugar appears 

 the reddish-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 polysaccharides into 

 monosaccharides) 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 proteid. For more details as to the 

 action of ferments on sugars see chapter on the colon-typhoid groups. 



