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QUALITATIVE ANALYSIS OF MARKET MILK. eye 
non-motile rod, producing lactic acid, but not readily curdling 
milk, and fermenting milk sugar with the production of gas. 
Upon our plates it produces a colony which is of good size, 4% 
mim., growing under the surface and on the surface and sur- 
rounded by an intensely red ring. ‘This acid ring is much 
more intense than that of either Group I. or Group II., in 
spite of the fact that the species does not commonly curdle 
milk. Frequently the colony has a gas bubble beneath it, and 
it sometimes grows vertically up from the surface of the gela- 
tin into a mound, which may be higher than broad. They are 
commonly the most striking colonies on the plate. One curious 
feature concerning the colonies is that although at first they 
are very acid and turn the gelatin in their vicinity a bright 
red, after a number of days the red color disappears, and the 
litmus turns blueagain. A plate inoculated with pure cultures 
of this organism becomes in two days very red; but after about 
a week some colonies become blue, and in a few days the whole 
plate is strongly blue again. To what this change in reaction 
is due we have not yet determined. 
This group, unlike the first two, includes several different 
- species. We have isolated and studied many colonies of the 
character described, and find that there are at least four differ- 
ent types. Besides the typical Bact. aerogenes, No. 208 of our 
list, there is a species similar in all respects except that it fails 
to ferment milk sugar with the production of gas—No. 223 of 
our list. This is about equally abundant with No. 208. Some 
of the colonies prove to be 2. cold communis, differing from the 
first two species in being motile. In our studies these have 
not been so numerous as the first two. Some colonies prove 
to be cocci instead of rods, but in other respects apparently 
identical with Bact. aerogenes—No. 224 of our list. Other 
colonies agree with this last except in failing to ferment milk 
sugar—No. 168 of our list. Ina few cases we have found that © 
these colonies prove to be different from any of the others. 
This group is therefore a somewhat miscellaneous one, which 
further study may enable us to differentiate; but the colonies 
produced by all the species are so nearly alike that at present 
we are unable to differentiate them on our plates, and are there- 
’ fore forced to include them in a single group. Practically this 
Group III. includes nearly all of the aerobic lactic bacteria, 
