52 PATHOGENIC BACTERIA. 



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 u 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 being among those produced, 

 and even this list may not comprehend them all. As 

 the acidity due to the microbic metabolism progresses, it 

 impedes, and ultimately completely inhibits, the develop- 

 ment of the bacteria. The addition of litmus to the cul- 

 ture-medium is one of the best methods for detecting the 

 acids. Milk to which litmus is added is particularly con- 

 venient. Rosalie acid may also be used, the acid convert- 

 ing 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. 



6. Production of Gases. This seems, in reality, to be 

 a part of the process of decomposition and fermentation. 

 Among the gases due to bacterial action, CO 2 , H 2 S, NH 4 , 

 CH 4 , and others have been described. If the bacterium 

 be anaerobic and develop at the lower part of a tube of 

 gelatin, not infrequently a bubble of gas will be formed 

 about the colonies. This is almost constant in tetanus 

 and malignant edema. Ordinarily, the production or 

 liberation of gases passes undetected, the vapors escaping 

 from the surface of the culture-medium. 



7. Production of Odors. Of course such gases as H 2 S 

 and NH 3 are sufficiently characteristic to be described as 

 odors. There are, however, a considerable number of 

 pungent odors which seem dependent purely upon odor- 



