MICHIGAN ACADEMY OF SCIENCE. 
71 
THE 
INFLUENCE OF BACT. LACTIC 
CHANGES CAUSED IN MILK BY 
ACID l UPON THE 
SOME OF THE 
COMMON MILK MICROORGANISMS. 
BY CIIAS. N. BROWN. 
We know that in fresh milk bacteria of the Boot, lactis acidi type are 
usually greatly in minority: and, too, that in milk standing for 24 
to 3G hours under the usual temperature environment common to most 
market milks the Bact. lactis acidi type gains majority. It can not be 
said that this transition is due entirely to their ability to multiply more 
rapidly than any of the other types, because it is not uncommon to 
find in milk other types which when transplanted to sterile milk will 
multiply at temperatures between 15° to 20°C. more rapidly than will 
Bact. lactis acidi. 
The growth of Bact. lactis acidi may be stimulated or retarded by as- 
sociation with other microorganisms. Stimulation both in rapidity 
and duration occurs if in the medium is present some acid destroying 
or acid retaining compounds, as insoluble carbonates, casein, etc. For 
example, upon a plate made from a milk agar shake (25% sterile milk 
added to a tube of melted agar and shaken to mix) to which some sterile 
powdered calcium carbonate is added before pouring into the Petri 
dish and inoculated with a stroke on the surface will develop a super- 
abundant growth, while upon a similar plate without the calcium car- 
bonate will occur a very meager growth. The casein in a milk culture 
when compared with a whey culture, acts in a like manner. 
A number of the types of microorganisms commonly met in milk pro- 
duce, in pure milk cultures, compounds which react alkaline to litmus 
and phenolphthalin : and the lactic in association with any of these 
types usually manifests a stimulated growth. A visible stimulation may 
be seen upon a plate made from a milk agar shake heavily seeded with 
one of these types and inoculated 'with the lactic by stroking the surface. 
Several veasts isolated from milk and butter and inoculated into 
flasks of whey made 2 1 / 2 % acid by Hie addition of commercial lactic 
acid reduced the acidity in time to about 0.2% to 0.3% , thus showing 
that a number of microorganisms found in milk are in reality acid con- 
sumers. The growth and life of the lactic in association with these 
is greatly prolonged. Along this line considerable work has been done 
by Miss Zae Northrop of East Lansing. 
A factor which may stimulate the growth of Bact. lactis acidi when 
growing in association with liquefying organisms is an increased supply 
of food made available by the proteolytic changes. Dr. Rahn now 
of the University of Illinois has formed experiments which conclude 
that the addition of peptone to a pure milk culture stimulates the 
growth of some of the strains of Bact. lactis acidi. 
The metabolic products of many of the common milk organisms when 
growing in association with a lactic is not without an effect. Those 
