EFFECT OF TEMPERATURE ON BACTERIA. 33 
agree in cultural characters. We regard them simply as phy- 
siological varieties of the same species. They produce a small 
opaque red colony with minute spines onits edge. The colonies 
never grow on the surface, and grow better under a mica plate. 
The cultures in general grow better if they are not exposed to 
the air. Somme of the cultures which are grouped under this 
head are Streptococci, and thus clearly belong to different 
species from the others, although their reaction upon milk 
and other culture media appears to be identical with that of 
Bacterium lactis acidt, 
Group Il. Bacterium lactis acidi IJ. 'This, as mentioned 
in a previous report, differs from Group I. only in its colony. 
The litmus gelatin colony is extremely minute, hardly visible 
to the naked eye, more transparent than the colony of Group 
I., and never shows thespiny edge. Weare inclined to believe 
that it is only a type of Group I., and that the differences in 
colony are due to conditions not yet understood. Sometimes, 
for several months in succession, Group II. will be wholly 
wanting. | 
Groups I. and II. constitute the typical lactic bacteria which 
sour milk in the vast majority of cases. 
Group III. Bacterium lactis aerogenes, etc. This type 
produces a colony of large size, brilliantly red in litmus gelatin, 
growing on the surface as well as below, and frequently pro- 
ducing gas. When these colonies are isolated they prove to be, 
usually, Bacterium lactis aerogenes. Occasionally they are De 
coli communis, as shown by their possessing flagella. 
To differentiate between these two species by their colonies 
has hitherto been found impossible. In a very few instances 
colonies isolated as of this type have shown a slight power of 
liquefying gelatin. This is very unusual. The amount of gas 
produced in dextrose broth varies widely. In some few in- © 
stances the cultures fail to produce gas, whilein others the gas 
produced is enormous. These gas producing species cause great 
trouble in the dairy, and their development in milk is, there- 
fore, a matter of much interest, since any conditions that 
stimulate their growth are almost sure to injure the milk for 
dairy purposes. 
