60 Biology of Micro-organisms 



the mouth of the bulb. The intensity of the explosion varies with the amount 

 of air present in the bulb. The relative proportion of gases resulting from the 

 fermentation is frequently of importance for the differential diagnosis of related 



TT 



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



LU 2 



The colon bacillus has a gas formula corresponding to ^- = -, Other aerogenic 



LO2 i 



bacilli sometimes show a formula 7^- = -." 



LOz 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 liquefaction is supposed to be dependent upon a form of pepto- 

 nization. Bitter* and Sternbergf 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, f who found that heat de- 

 stroyed 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 a peculiar 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 metabolism. 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. 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 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 quantity withdrawn, and titration with. 



- sodium hydroxid, or hydrochloric acid, performed. 



The alkali 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 alkalies only, others both 



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



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



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



