FIFTEENTH REPORT. 
72 
organisms which have a stimulating’ effect upon the growth of the lactic 
may also greatly inhibit or prevent its growth. A milk agar shake 
of these organisms if stroked with Bact. lactis acidi immediately or 
within 20 to 30 hours generally gives a stimulated growth of the lactic; 
but, if 3 to 5 days pass before stroking the surface with the lactic, the 
growth is retarded or prevented. A number of microorganisms will be 
retarded from the first. 
The growth of most of the organisms commonly found in milk may 
be stimulated or retarded by association with Bact. lactis acidi. At 
the beginning certain types of organisms growing in milk in associa- 
tion with a lactic exhibit a stimulated growth; while later, when the 
acid produced by the lactic has caused a rise of about 0.1% in the 
acidity of the milk, their growth is arrested. The greater rapidity of 
growth which manifests itself at first may occur because the first traces 
of acid or some metabolic products of tlie lactic act as a weak poison 
and stimulant. This action may be illustrated as follows: make a 
plate from a milk agar shake of Ihe organism and inoculate the surface 
with a stroke of the lactic. The picture which presents itself is a 
normal or stimulated growth of the lactic surrounded by a narrow zone of 
apparently no growth and surrounding this a copious growth of the 
organism. Another factor which may cause or aid in causing this stim- 
ulated growth of the organism is a stimulated proteolysis, giving a 
more abundant supply of available food. 
In association with a lactic the growth of many organisms is retarded. 
A retardation which likely is due to their inability to tolerate the in- 
creasing amounts of acid. Yet the growth of a number of organisms 
is inhibited and even prevented before the acid produced by the lactic 
is measurable by our present chemical methods. From this it seems 
that the metabolic products of the lactic even in small amounts have 
an inhibiting action. 
The lactic in association with a few organisms, especially some of 
the torula and yeasts, is not able through either the accumulation of 
its metabolic products or its maximum acid production to exert to any 
marked degree a retarding effect. 
The changes caused in milk by many microorganisms in pure culture 
are greatly retarded or prevented if at the beginning an equal number 
of Bact. lactis acidi is introduced: the organism alone producing its 
characteristic changes while in the association the changes produced 
are those characteristic of the lactic alone. If, however, the organism 
is uiven a lead before the lactic is introduced, it is able, usually, to 
make its changes detectable. A number of organisms known to be able 
to liquefy casein rapidly, when growing in association with a lactic are 
unable to make manifest their changes. And, too, a number of or- 
ganisms regarded as non-liquefiers of casein because they will grow in 
milk in pure cultures for three to four weeks or longer without causing 
anv visible proteolytic changes may become upon association with a 
lactic a rather rapid liquefier. It is safe to say that nearly every if 
not every microorganism during its life produces some type of proteoly- 
tic enzyme. If this type, however, be that resembling trypsin, then 
the presence of acid produced either by itself or by a lactic in associa- 
tion will prevent a visible manifestation of a proteolytic action. Hut 
if the enzyme resembles pepsin, the presence of a limited amount of 
