Liquefaction of Gelatin 63 



XT 



bacteria. Smith has designated this relation of -„-- as the 'gas formula.' The 



H 2 

 colon bacillus has a gas formula corresponding to p^r- = -• Other aerogenic 



CU2 I 



bacilli sometimes show a formula ^^^ = -•" 



CUa 2 



Liquefaction of Gelatin. — As certain organisms grow in gelatin, 

 the medium becomes partly or entirely liquefied. This peculiarity 

 is apparently independent of any other property of the organisms, 

 and is manifested alike by pathogenic and non-pathogenic forms. 

 The Hquefaction is supposed to be dependent upon a form of pepto- 

 nization. Bitter* and Sternberg f have shown that if from a culture 

 in which liquefaction has taken place the bacteria be removed 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 " tryptic 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 Hquefy the gelatin in such a pecuUar and characteristic 

 manner as to make the appearance a valuable guide for the differen- 

 tiation of species. 



Production of Acids and Alkalies. — Under the head of "Fermen- 

 tation" the formation of acetic, lactic, and butyric acids has been dis- 

 cussed. Formic, propionic, baldrianic, palmitic, and margaric acids 

 also result from microbic metaboUsm. As the acidity progresses, 

 it impedes, and ultimately completely inhibits, the activity of 

 the organisms. The cultivation of the bacteria in milk to which 

 litmus or lacmoid has been added is a convenient method for de- 

 tecting changes of reaction. Rosohc 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 turning it yellow. 



The quantitative estimation of changes in reaction can be best 

 made by titration, and the fermentation-tube culture can be em- 

 ployed for the purpose. The contents of the bulb and branch should 

 be shaken together, a measured quantity withdrawn, and titration 



with ^— sodium hydroxid, or — hydrochloric acid, performed. 



The alkaU most frequently formed by bacterial growth is ammo- 

 nium, which is set free from its combinations, and either flies off as a 

 gas or forms new combinations with acids simultaneously formed. 

 Some bacteria produce acids only, some alkaUes only, others both 

 acids and alkalies. Both acids and the alkalies, when in excess, 

 serve to check the further activity of the micro-organisms. 



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



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



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



