344 MILK 



Kruse and, somewhat later independently, the writer proposed 

 a classification in two groups, namely: 1, those bacteria belonging 

 to the Bacillus coli group, and 2, those belonging to the Strepto- 

 coccus lacticus group. These two groups are always present in 

 raw milk, and also most generally in milk pasteurized by the 

 holding process at relatively low temperature. In this grouping 

 cognizance is taken only of acid-producing bacteria that actually 

 cause the "normal" souring. 



Jensen has classified lactic acid bacteria into three groups as 

 follows: 



1. Those that grow at 25 to 50 C. To this group belong the 

 long bacilli, which form usually levorotatory lactic acid, although 

 some produce racemic acid. 



2. Those that grow at a wide range of temperature, namely, 

 from 5-7 C. to 45-50 C. These are all streptococci. 



3. The genuine lactic acid bacteria that grow well at 10 to 

 40 C. This group does not include the coli-aerogenes group. 



Rogers and Davis, in a bulletin entitled "Methods of Classi- 

 fying the Lactic Acid Bacteria," say: "The bacteria taking part 

 in the souring of milk may be readily divided into four general 

 groups. 



"Group 1 includes those bacteria which sour milk without 

 peptonization or gas formation; they grow poorly on artificial 

 media and fail to liquefy gelatin. Morphologically, they show 

 some variation, usually appearing as a coccus or very short bacil- 

 lus in pairs or in chains of varying lengths. The bacteria of this 

 group are the ones ordinarily designated as the lactic-acid bac- 

 teria and have been described under various names. They have 

 a very general distribution, and their presence in milk is so con- 

 stant that they may be considered as normal inhabitants of this 

 medium. 



"Group 2 includes the bacteria forming an acid curd with 

 evolution of gas. This embraces varieties of Bacillus coli and 

 Bacterium aerogenes or the Bacillus acidi lactici of Hiippe. 

 The members of this group are readily distinguished from those 

 of Group 1 by their abundant growth on artificial media, the 

 vigorous evolution of gas, and the marked difference in their 

 morphology. An examination of milk usually reveals their pres- 

 ence in small numbers, but their number is increased by the influ- 

 ence of high temperatures or unsanitary conditions under which 

 the milk has been collected or held. 



"Group 3 includes those bacteria forming an acid curd which 

 is subsequently partially peptonized. The bacteria of this group 

 have been little studied in their relation to milk. It will be 

 shown that this description applies to varieties only distantly 



