The germicidal action of milk.

Abstract

The so called germicidal action of milk has given rise to much discussion and controversy as to its extent, significance and even its existence. All workers are in agreement that the germicidal effect, if such exists, is only slight in action and transitory in occurrence after the milk is drawn from the animal. Some doubt its existence at all; some believe it to be effective so far as some organisms are concerned but to have no effect upon the typical milk bacteria; others believe in a bactericidal action, but hold that it may occur in the milks of some cows and not in others, and may or may not appear in the milk of the same cow at different times.

Studies made by measuring the rate of increase in the natural fortuitous flora of fresh milk have, quite obviously, given contradictory results. When fresh milk is inoculated with known pure cultures, the results are still not definite as the germicidal effect is here complicated by the presence of the natural lag period, in the culture used as the inoculum, before rapid growth ensues.

It seemed to us that the problem could be got at more directly, and more definite information obtained, by the application of modern knowledge of the bacterial growth curve. As is well known, the freshly inoculated bacterial culture passes through a latent or “lag” phase before multiplication begins. It is also well known that transplants taken from a culture only a few hours old, while it is in the rapidly growing stage, will show no lag but continue multiplication at the same rate when seeded in suitable media. We therefore inoculated freshly & awn aseptic milk with young cultures in the period of rapid growth. For this purpose Strept. lactis, probably the most perfectly adapted milk organism, was used. Samples of milk were brought to the laboratory immediately after king drawn and placed in a water bath at 37° C. They were then inoculated from rapidly growing cultures three hours old at 37° C.; sterile milk king used as the growth medium from which the inoculum was taken. In these experiments control tests were also run by inoculating at the same time the same culture into sterilized milk (autoclaved) and incubating as in the case of the raw fresh milk.