, as shown
by injections into the milk cistern. They abound, however, on hay, in
dust, in the barn air, on the hairy coat of the animal, and from these
sources easily gain access to the milk. In this medium they find an
exceptionally favorable environment and soon begin a very rapid growth,
so that by the time milk is consumed, either in the form of milk or milk
products, they make up numerically the larger portion of the bacteria
present.

Another widely disseminated, although numerically less prevalent, type
is _B. lactis aerogenes_. This type forms gas in milk so that the soured
milk is torn by the presence of gas bubbles. It also grows more
luxuriantly in contact with the air.

Other types occur more or less sporadically, some of which are capable
of liquefying the casein of milk while at the same time they also
develop lactic acid. Conn and Aikman refer to the fact that over one
hundred species capable of producing variable quantities of lactic acid
are already known. It is fair to presume, however, that a careful
comparative study of these would show that simply racial differences
exist in many cases, and therefore, that they are not distinct species.

As a group these bacteria are characterized by their inability to
liquefy gelatin or develop spores. On account of this latter
characteristic they are easily destroyed when milk is pasteurized. They
live under aerobic or anaerobic conditions, many of them being able to
grow in either environment, although, according to McDonnell,[52] they
are more virulent when air is not excluded.

While growth of these lactic forms may go on in milk throughout a
relatively wide range in temperature, appreciable quantities of acid are
not produced except very slowly at temperatures below 50 deg. F.[53]

From the standpoint of frequency the most common abnormal changes that
occur in milk are those in which gases of varying character are
developed in connection with acids, from the milk sugar. Other volatile
products imparting bad flavors usually accompany gas production. These
fermentations are of most serious import in the cheese industry, as they
are especially prone to develop in the manufacture of milk into certain
types of cheese. Not often is their development so rapid that they
appear in the milk while it is yet in the hands of the milk producer,
but almost invariably the introduction of the causal organisms takes
place while the milk is on the farm. Numerous varieties o