Production of Acids and Alkalies 51 



4. Liquefaction of Gelatin. As certain bacteria grow 

 in gelatin, the medium becomes partly or entirely liquefied. 

 This peculiarity is apparently independent of any other 

 property of the bacterium, and is manifested alike by patho- 

 genic and non-pathogenic forms. The liquefaction is sup- 

 posed to be dependent upon a form of pepftonization. 

 Bitter * and Sternberg f first showed that if from a culture 

 in which liquefaction has taken place the bacteria be re- 

 moved by filtration, the filtrate will retain the power of 

 liquefying gelatin, showing the property is not resident in 

 the bacteria, but in some substance in solution in their 

 excreted products. These products were described as "tryp- 

 tic enzymes" by Fermi, J who found that heat destroyed 

 them. Mineral acids seem to check their power to act upon 

 gelatin. Formalin renders the gelatin insoluble. Some of 

 the bacteria liquefy the gelatin in such peculiar and char- 

 acteristic manner as to make the appearance an extremely 

 valuable guide for the differentiation of species. 



5. Production of Acids and Alkalies. Under the head 

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

 butyric acids has been discussed. Formic, propionic, baldri- 

 anic, palmitic, and margaric acids also result from microbic 

 metabolism. As the acidity progresses, it impedes, and 

 ultimately completely inhibits, the activity of the bacteria. 

 The cultivation of the bacteria in milk to which litmus or 

 lacmoid has been added is a convenient method for detecting 

 changes of reaction. Rosolic acid solutions may also be 

 used, the acid converting the red into an orange color. 

 Neutral red is also much employed for this purpose, the 

 acids of the colon bacillus turning it yellow. 



The quantitative estimation of changes in reaction 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 measured quan- 

 tity withdrawn, and titration with ^ sodium hydroxid, or 

 '- hydrochloric acid, performed. 



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 



* " Archiv fur Hygiene," 1886, Heft 2. 



t "Medical News," 1887, No. 14. 



t "Centralbl. f. Bakt.," etc., 1891, Bd. x, p. 401. 



