Fortunately, this immense number of bacteria in milk need not especially alarm us, for they are not disease-germs and are harmless to the healthy person. Nevertheless, they are undoubtedly a nuisance in the milk. They can not grow there without producing some effect upon it. Commonly the first change noticeable is the appearance of the well-known odor and taste of sour milk, followed shortly by its curdling. This souring is undoubtedly the result of bacteria growth, and it was at first supposed that there was a single definite species which alone had this power of producing lactic acid. So thought Pasteur and Lister at first, and such a species they described. The species of bacterium studied by them certainly had this power, and it was named Bacterium lactis by Lister. In later years the name Bacillus acidi lactici has been given to it. By the work of the last six years we have learned that more than one species has the power of souring milk by the formation of lactic acid. Lactic-acid formation is the characteristic of a class of bacteria comprising many species, and even in the ordinary souring of milk under normal conditions it is not always the same species of bacteria which produces the mischief.
While it is true that any one of a number of species of bacteria may produce lactic acid by their growth in milk and thus cause its souring, in other respects these different species do not have the same effect. The formation of lactic acid is not the only change that occurs in the souring of milk. Sour milk has a well-known odor, but this is not due to the lactic acid, since lactic acid is odorless. The formation of such an odor tells us, therefore, that there are other changes going on in the souring of milk. The fact is, that a decomposition of the albuminoids and other substances in the milk is going on, and it is these decomposition products that give the odor. Now, the different species of bacteria do not all produce the same sort of decomposition products. All who are familiar with milk will recall that the character of sour milk is by no means uniform. It differs in the hardness of the curd, in the amount of the whey, in odor, and even in taste. When different specimens of milk are examined just before or just after souring, it is found that the species of bacteria are by no means the same in the different specimens. Each will contain some of the acid forming class, but the particular species which happen to be present in the different specimens will vary with the different conditions. Different localities and different methods of handling the milk will affect the variety of bacteria that it contains. It will sour in all cases, since all have some of the members of the acid forming class; but the other accompanying phenomena may be different. Thus we have learned to attribute all the differences in the different specimens of sour milk to the fact that the souring
