The germicidal property of milk

Abstract

Judged by the number of colonies that develop upon agar plates the bacteria in milk first diminish, then increase in number. The so-called germicidal property of milk occurs only in the fresh raw fluid. For the most part our work plainly shows that no actual reduction in the number of bacteria occurs. However, when compared with the controls a restraining action is evident. The phenomenon, therefore, appears to be that of a weak antiseptic rather than that of a true germicide. When milk is kept warm (37° C.), the decrease is pronounced within the first eight or ten hours. After this time the milk has entirely lost its restraining action. The decrease in the number of bacteria is largely apparent, being due at least in part to agglutination. The bacterial clusters may, to a certain extent, be shaken asunder. This fact goes far to reconcile the discordant results of the various investigations upon the germicidal properties of milk. Those who used dilution methods with vigorous agitation broke up the bacterial clusters and thus obtained a larger number of colonies upon agar plates than those who plated directly with different technique. Some of the polymorphonuclear leucocytes in milk seem to possess the power of phagocytosis, judged by microscopical preparations. Phagocytosis, however, plays no essential part in the “germicidal” action of milk, for the decrease in numbers is quite as marked in the cell-free serum as in the sediment rich in leucocytes. The germicidal action of milk is specifie. For instance, one sample restrained typhoid and Staphy Zococcus pyogenes aureus but not paratyphoid A or B. Dilution experiments demonstrate the enfeeblement of agglu-tinins rather than the presence of a germicidal substance in solution.