26 PRACTICAL DAIRY BACTERIOLOGY 



what distinct groups, more or less sharply separated from each 

 other. The more bacteria are studied, the more clear it be- 

 comes that these groups represent very distinct types, each one 

 of which may have a large number of varieties differing in 

 slight points. We shall, therefore, understand the lactic bac- 

 teria best if we briefly consider the characteristics of these 

 primary types. 



BACTERIUM LACTIS ACIDI (Streptococcus lactarius} 



This organism is the lactic acid bacterium of milk, par ex- 

 cellence, being the type that, in the vast majority of cases, pro- 

 duces the souring and curdling of market milk. It is common, 

 not only in milk from all sections of the United States, but also 

 in all the markets of Europe, and must, without doubt, be 

 looked upon as a common cause of the souring of milk. It is 

 interesting to note that this species was not the first lactic acid 

 bacterium found in milk, and described as the cause of milk 

 souring. One of the other types mentioned below was found 

 earlier, because it produces a large colony on gelatin. But 

 when milk is studied by our more modern methods, this Bact. 

 lactis acidi type is found to be by far the most common. 1 It is 

 not to be inferred from this statement, however, tnat it is al- 

 ways the cause of sour milk, for, while it is the cause in the 

 majority of cases, not infrequently samples of milk are found 

 that have been soured by one of the other types mentioned later. 



It will be noticed that two names have been given above to 

 this organism. These are not, by any means, the only names 

 that have been applied to it, for it has been found by many 

 bacteriologists, many of whom have given it distinct names, 

 not knowing that they were studying an organism already 

 named. The reason why we have given the above two names 



lEsten. Ann. Rep. Storrs. Exper. Sta., p. 44, 1896. 

 Kozai. Zeit. f. Hyg., xxxviii., p. 386, 1901. 

 Weigmann. Cent. f. Bact. II., v., p. 825, 1899. 

 Gunther and Thierfelder. Arch. f. Hyg. xxv., p. 164, 1895. 



