THE KINDS OF MICRO-ORGANISMS IN MILK 365 



means of differentiation. As stated by Rogers, streptococci can 

 be grouped according to certain tests or test substances, but the 

 grouping varies according to the tests selected by the investigator. 

 In the present state of our knowledge we must be contented to 

 assume that Streptococcus lacticus and Str. pyogenes are varia- 

 tions of one parent form, and that characteristics which are ac- 

 quired may be more or less transient. Buchanan has shown that 

 impressed variations on Str. lacticus do not persist. If, however, 

 acquired characteristics are impressed for very long periods, as 

 may be assumed possible with a strain inhabiting milk for a long 

 time, it is not impossible that these characteristics persist for a 

 considerable length of time. Thus a saprophytic streptococcus 

 may become pathogenic, and a pathogenic one lose its virulence. 

 Str. lacticus should be considered a close relative of Str. pyogenes, 

 but not identical, even though our present means of differentia- 

 tion do not reveal very marked and permanent differences. 



The third group of lactic acid bacilli according to Lohnis' 

 classification is an important one, hardly less so than the two 

 previous groups. The collective name of lactobacilli^ is sometimes 

 used to designate this class of bacteria, and they have attracted 

 considerable attention because they are the chief agents in the 

 production of some fermented milk. The Bulgarian bacillus 

 especially has been advertised broadly as the "bacillus of long 

 life," and has gained much notoriety through the publications of 

 Metchnikoff and some of his associates. Furthermore, this group 

 of bacilli has recently been recognized as playing an important 

 role in the final ripening stages of some types of cheese, and pure 

 cultures can be obtained for this purpose. 



Many varieties of bacilli belonging to this group have been 

 described in the literature, for example : Bacillus bulgaricus, Strep- 

 tobacillus lebenis, B. acidophilus, B. acidophil-aerogenes, B. bi- 

 fidus, Boas-Oppler bacillus, B. panis fermentati, B. casei, Bac- 

 terium caucasicum, Kornchen bacillus, Leptothrix buccalis, and 

 others. We have here another illustration of the multiplicity of 

 names assigned to a group of closely related organisms which 

 differ from each other in minor properties and which possibly are 

 but varieties of one type, whose characteristics have been modified 

 by environmental conditions. 



An organism closely resembling Bacillus bulgaricus was found 

 by Hefferan and the writer in cornmeal, and was held to be the 

 active agent for the production of lactic acid in salt-rising bread. 



^ The term "lactobacilli" for the group of lactic acid bacteria under 

 discussion seems preferable to terms which are not infrequently encountered, 

 such as "aciduric bacilli," "acidophil bacilli," or "bacilli of the Bacillus 

 bulgaricus group." Such terms are suggestive of some characteristic which 

 may not be applicable to some strains. 



