BIOLOGY OF BACTERIA. 55 



These products are described as " tryptic enzymes" by 

 Fermi, who found that heat destroyed them. Mineral 

 acids seem to check their power to act upon gelatin. 

 Formalin renders the gelatin insoluble. As some of 

 the bacteria not only liquefy the gelatin, but do so in a 

 peculiar and constantly similar manner, the presence or 

 absence of the change becomes extremely useful for the 

 separation of different species. 



5. Production of Acids and Alkalies. — Under the head 

 of "Fermentation" the formation of acetic, lactic, and 

 butyric acids has been discussed. These, however, are 

 by no means all the acids resulting from microbic me- 

 tabolism. Ziegler mentions formic, propionic, baldrianic, 

 palmitic, and margaric as among those produced. As the 

 acidity due to the microbic metabolism progresses, it im- 

 pedes, and ultimately completely inhibits, the develop- 

 ment of the bacteria. Milk, to which litmus is added, is 

 particularly convenient for detecting the acids. Rosalie 

 acid may also be used, the acid converting its red into an 

 orange color. The same tests will also determine the 

 alkali-production, which occurs rather less frequently 

 than acid-formation and depends chiefly upon the salts 

 of ammonium. 



The quantitative estimation of the acids can be best 



made by titration, and the fermentation-tube culture 



can be employed for the purpose. The contents of the 



bulb and branch should be shaken together, a meas- 



n 

 ured quantity withdrawn, and titration with — sodium 



hvdroxid, or — hvdrochloric acid, performed. In rela- 



20 ' 



tion to the acid, it is always important to know through 

 the presence of what particular carbohydrate the acid was 

 formed. 



The alkali most frequently formed by bacterial growth 

 is ammonium, which is set free from its combinations, 

 and either flies off as a gas or forms new combinations 

 with acids simultaneously formed. Some bacteria pro- 



