There are but few micro-organisms which do not thrive in milk, and the majority of them produce fundamental changes. In this regard they may he classified as follows: (1) Those which produce Lactic Acid.—These decompose the milk sugar, with formation of lactic acid, acetic acid, formic acid, alcohol, etc.
(2) Those which produce Butyric Acid. These cause the milk-sugar or the lactic acids to ferment, with formation of butyric acid, propionic acid, hydrogen, and carbonic acid.
(3) Pro eolytic.—These decompose the protcids until the amino acids are reached, and, in so doing, usually coagulate the milk.
Besides these there are organisms that produce coloring matter,. others that make the milk slimy (by impairing the milk-sugar, or, more frequently, by altering the proteids); still others that make it ropy, saponaceous, bitter, or malodorous. The organisms that disintegrate the fat seldom become important in ordinary milk. This classification, how ever, is not to be understood as strictly differentiating the species. On the contrary some which produce lactic acid (e.g., B. coli) also have a proteolytic action, and rice versa. At first, with cow's milk under ordi nary circumstances, both the lactic acid producers and the proteolytics are active. but the former soon predominate. Indeed it is the Bacillus
acidi paralactici Kozai (a streptococcus, which is believed to be identi cal with Streptococcus enteritidis Hirsch, the Enterococcas Thiercelin, and the Bacterium lacks acidi Leichmann) which, in the formation and decomposition of lactic acid, sours the milk and uses up the hevolactic acid, so that dextrolactic acid remains. At the temperature of incu bation, on the contrary, the Bacillus acidi lwrolactici is active, and this acid soon predominates over the inactive mixture of hevolactic and dextrolactic. Finally, mildew bacilli destroy these acids and butyric acid bacilli decompose the remaining sugar. Room has now been made for the proteolytic species.
By boiling milk, the micro-organisms that produce lactic acid are destroyed, but not the spores of the butyric acid bacillus (Bacillus butyricus Botkin). Consequently, semi-sterilized milk in the course of time falls a prey to one or the other of these agents of decomposition and poisons are developed whose nature is still unknown. According to Conn-Esten (190-t), a low temperature is the best protection against the growth of the which produce lactic acid.