308 MICROBIOLOGY OF MILK AND MILK PRODUCTS. 



acid-forming bacteria, the biochemistry of each group will be emphasized 

 rather than the cultural and morphological characteristics of the mem- 

 bers 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 o.6/* to i/j. in length, and 0.5/4 in diame- 

 ter. 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. lacticus to a member of the 

 group. In milk 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 42. They are to be classed as facultative anaerobes. The growth 

 on all culture media is marked by its meagerness; 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 surface of the medium. 

 In lactose-agar stab cultures growth occurs along the entire line of inoc- 

 ulation, but there is no surface growth. No liquefaction of gelatin occurs. 

 In bouillon the medium is uniformly turbid or it remains clear with a slight 

 sediment. On potato, growth is slight or is absent. Milk is usually 

 curdled within twenty-four hours at the optimum temperature by mem- 

 bers of the group, although some fail to curdle the milk, since the maxi- 

 mum amount of acid produced is not sufficient to cause this phenomenon. 

 Still others cause curdling in the presence of small amounts of acids, in 

 which case a rennet-like enzyme may be present. No gas is produced in 

 the fermentation of lactose, hence the curd formed in milk is perfectly 

 homogeneous; it shows but little tendency to shrink and to express whey. 

 In litmus milk the color is discharged from the entire mass of medium 

 before curdling occurs, due to the reduction of the litmus to the colorless 



* Prepared by E. G. Hastings. 



