Excerpt from Methods of Classifying the Lactic-Acid Bacteria

The grouping of bacteria according to their action on any one specific substance usually brings together bacteria related in that one characteristic only but entirely unrelated in other respects. It is, however, sometimes convenient from a technical standpoint to group bacteria in this way. The bacteria concerned in the souring of milk have been so grouped for so long that many people have come to consider them as a division by themselves and their relation to other bacteria has been little considered. The bacteria taking part in the souring of milk may be readily divided into four general groups.

Group I 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 bacillus in pairs or in chains of varying lengths. The bacteria of this group are the ones ordinarily designated as the lactic-acid bacteria and have been described under various names. They have a very general distribution and their presence in milk is so constant that they may be considered as normal inhabitants of this medium.

Group II 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 Heuppe. The members of this group are readily distinguished from those of Group I 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 presence in small numbers, but their number is increased by the influence of high temperatures or insanitary conditions under which the milk has been collected or held.

Group III 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 related to our Group I as well as to some closely connected with this type.

Group IV includes the high-acid-forming bacteria of which the Bacillus bulgaricus is the type.

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