54 PA THOGENIC BACTERIA . 



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 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 phenolphthalein 

 and litmus to the culture-medium is one of the best 

 methods for detecting the acids. Milk, to which litmus 

 is added, is particularly convenient. 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 ammo- 

 nium. 



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 

 fn.ni ilu- surface of the culture-medium. 



To determine the gas production where it is suspected 

 but not apparent, the ordinary fermentation-tubes can be 



