+24. Colon-aërogenes group.+--This group takes its name from a typical species, _Bacterium coli communis_, which is a normal inhabitant of the intestines of man and animals, and from _Bacterium coli aerogenes_, which is similar in many respects to _B. coli communis_. The initial presence of these bacteria in milk is indicative of fecal contamination or unclean conditions of production. These organisms, however, grow and develop in milk very rapidly at high temperatures of handling. The total acidity produced by these forms is less than that by the _Bacterium lactis-acidi_ group. Of the acid produced, less than 30 per cent is lactic acid; the other acids are formic, acetic, proprionic and succinic. The large percentage of these acids, with comparatively large amounts of certain alcohols, aldehydes and esters, invariably impart undesirable flavors and odors to the milk. Members of this group uniformly ferment the lactose with the production of the gases, carbon dioxide and hydrogen. The milk is coagulated into a lumpy curd, containing gas pockets. +25. Acid peptonizing group.+--These are often associated with colon organisms. The group includes those bacteria which coagulate milk with an acid curd and subsequently partly digest it. They grow and multiply rapidly at a temperature between 65° and 98° F. They impart undesirable flavors and odors to the milk, which appear to be due to the formation of acids other than lactic acid, and to action on the milk proteins. +26. Bacillus bulgaricus group.+--These organisms grow best at a temperature of 105° to 115° F. They will develop at lower temperatures, but not so rapidly. They survive heating to 135° F. without loss of vigor, as occurs in Swiss cheese-making. They produce from 1 to 4 per cent of acid in milk, which is practically all lactic acid. They do not produce gas. They impart no undesirable flavors to the milk. +27. Acid cocci or weak acid-producers.+--This group of organisms is not very well defined. It consists mostly of coccus forms, commonly found in the air and in the udder. Their presence in the milk may indicate direct udder contamination. These are regarded as of little importance, unless in very large number, and they have been only partially studied. They produce little or no lactic acid, and small amounts of acetic, proprionic, butyric and caproic acids. These forms rarely create enough acid to coagulate milk. +28. Peptonizing organisms.+--This group includes all bacteria which have a peptonizing effect on the milk. It includes the acid peptonizing organisms, although they are of primary importance in the acid type of bacteria, because the acid-producing power is greater than the peptonizing power. Some of the specific organisms in this class are _Bacillus subtilis_, _Bacterium prodigiosus_ and _Bacterium liquefaciens_. These are commonly found in soil water and in fecal material. The presence of these organisms denotes contamination from such sources.