310 MICROBIOLOGY OF MILK AND MILK PRODUCTS. 



These data signify that other compounds than lactic acid are formed in 

 the fermentation of lactose by these acid-forming bacteria. Acetic acid 

 (CH 3 .COOH); formic acid (H.COOH); propionic acid (C 2 H 5 .COOH); 

 traces of alcohols, aldehydes and esters have been found. The lactic acid 

 formed is the dextro modification. It is believed that the fermentation is 

 due to an enzyme, lactacidase, one of the intracellular enzymes that can 

 be demonstrated only with difficulty. 



Milk fermented by members of this group has a mild acid taste, an 

 agreeable odor, and the curd can be so finely divided by agitation as to 

 produce almost as perfect an emulsion as in raw milk. The organisms 

 are to be classed as desirable from the stand-point of the dairy manu- 

 facturer and the fermentation produced by them may be called a true 

 lactic fermentation. 



Characteristics of the B. Coli-aero genes Group* This group includes 

 a considerable variety of organisms, which differ in morphology, in cul- 

 tural characteristics and undoubtedly in the character and amounts of 

 their by-products. They are more distinctly bacilli than the members of 

 the preceding group; are mo tile or non-motile; none produces spores and 

 they are usually negative to Gram's stain. The optimum growth 

 temperature, 35 to 40, is somewhat higher than for the preceding group, . 

 the vegetation range being 15 to 45. They are to be classed as facul- 

 tative anaerobes. 



The conditions for development are less narrow than for the Bact. lactis 

 acidi group, growth occurring on all the ordinary culture media and in the 

 absence of carbohydrates. Indol and hydrogen sulphide are often formed 

 and nitrates are reduced. The growth is usually profuse, the colonies 

 large and surface growth occurring in stab cultures. Gelatin is not usually 

 liquefied. 



Lactose, dextrose and saccharose are fermented, with the production 

 of varying amounts of gas in which have been found carbon dioxide; 

 hydrogen, methane, and free nitrogen. The maximum amount of acid 

 produced in any culture medium is quite similar to that formed by the 

 members of the previous group. The relative proportions between the 

 non-volatile and volatile acids are far different, lactic acid comprising less 

 than 30 per cent of the total acid formed, while volatile acids as acetic and 

 formic make up the remainder. Traces of succinic acid (C 2 H 4 (COOH 2 )) 

 and alcohol have also been found. The lactic acid is of the laevo-form. 



* Prepared by E. G. Hastings. 



