380 MICROBIOLOGY OF MILIJ AND MILK PRODUCTS 



The most important properties of organisms employed in the fermen- 

 tation industries are the physiological rather than the cultural or mor- 

 phological, since the quality of the product is dependeTit on the by- 

 products of the fermentation. Hence in characterizing the groups of 

 acid-forming bacteriaj the biochemistry of each group will be empha- 

 sized rather than the cultural and morphological characteristics of the 

 members of the group. 



Characteristics of the Bact. Lactis Acidi Group.* — The organisms of 

 this group are widely distributed in nature, as is shown by the constancy 

 with which milk undergoes the characteristic fermentation produced by 

 the members of the group. 



The cells are oval in form, about 0.6^1 to ifi in length, and 0.5/* in 

 diameter. The shorter cells appear nearly spherical, which, together 

 with the fact that chains of cells often occur, has led some to classify 

 them among the cocci and Kruse has applied the name Strept. laciicus to 

 a member of the group. In mUk the cells are usually in twos, the outer 

 ends of the two cells being pointed. None of the group is motile; spores 

 are not formed and capsules are often noted. The members of the 

 group are Gram-positive. 



The optimum temperature for growth lies between 30° and 35°, the 

 minimum -growth temperature ranging from 10° to 12°, while the maxi- 

 mum is 1 2°. They are to be classed as facultative aerobes. The growth 

 on all culture media is marked by its meagemess; in the absence of a fer- 

 mentable carbohydrate, no growth usually occurs; peptone favors the 

 growth even in milk. In the case of .freshly isolated cultures, the 

 growth is almost invisible, on slopes of sugar agar, appearing as small 

 discrete colonies. On sugar agar plates the colonies are small, often 

 surrounded by a hazy zone, and always occur below the- surf ace of the 

 medium. In lactose-agar stab cultures growth occurs along the entire 

 line of inoculation, but there is no surface growth. No liquefaction of 

 gelatin occurs. In bouillon the medium is uniformly turbid or it re- 

 mains clear with a sUght sediment. On potato, growths is shght or is 

 absent. Milk is usually curdled within twenty-four hours at the opti- 

 mum temperature by members of. the group, although some fail to cur- 

 ' die the milk, since the maximum amount of acid produced is notsxiffi- 

 cient to cause this phenomenon. Still others cause curdling in the .pres- 

 ence of small amounts of acids, in which case a rennet-like enzyme.inay 



* Prepared by E. G. Hastings, 



