to the surface of the milk as it stands in a vessel. The most
important is the well-known _B. prodigiosus_. Another form found at
times in milk possessing low acidity[73] is _B. lactis erythrogenes_.
This species only develops the red color in the dark. In the light, it
forms a yellow pigment. Various other organisms have been reported at
different times.[74]

~Blue milk.~ Blue milk has been known for many years, its communicable
nature being established as long ago as 1838. It appears on the surface
of milk first as isolated particles of bluish or grey color, which
later become confluent, the blue color increasing in intensity as the
acidity increases. The causal organism, _B. cyanogenes_, is very
resistant toward drying,[75] thus accounting for its persistence. In
Mecklenberg an outbreak of this sort once continued for several years.
It has frequently been observed in Europe in the past, but is not now so
often reported. Occasional outbreaks have been reported in this country.

~Other kinds of colored milk.~ Two or three chromogenic forms producing
still other colors have occasionally been found in milk. Adametz[76]
discovered in a sample of cooked milk a peculiar form (_Bacillus
synxanthus_) that produced a citron-yellow appearance which precipitated
and finally rendered soluble the casein. Adametz, Conn, and List have
described other species that confer tints of yellow on milk. Some of
these are bright lemon, others orange, and some amber in color.

Still other color-producing bacteria, such as those that produce violet
or green changes in the milk, have been observed. In fact, almost any of
the chromogenic bacteria are able to produce their color changes in milk
as it is such an excellent food medium. Under ordinary conditions, these
do not gain access to milk in sufficient numbers so that they modify the
appearance of it except in occasional instances.

~Treatment of abnormal fermentations.~ If the taint is recognized as of
bacterial origin (see p. 57) and is found in the mixed milk of the herd,
it is necessary to ascertain, first, whether it is a general trouble, or
restricted to one or more animals. This can sometimes be done by
separating the milk of the different cows and noting whether any
abnormal condition develops in the respective samples.

~Fermentation tests.~ The most satisfactory way to detect the presence of
the taints more often present is to make a fermentation test of one kind
or another. Thes