362. Studies in the bacteriology of milk: I. The streptococci of milk

Abstract

The streptococci occurring in raw and pasteurized milks of varying purity have been investigated. The organisms were placed in five main groups by testing for the hydrolysis of arginine, growth at 45 and at 8–12° C, survival in milk at 63° C. for 30 min. (measured by a semi-quantitative plating method), action on litmus milk, and the ability to form CO2from glucose (tested by a cultural method). The groups thus distinguished are:(1) Organisms of bovine mastitis.(2)Str. bovisandStr. thermophilus. A clear-cut separation of these species was not obtained by tests for growth at 50° C, the hydrolysis of aesculin, or the fermentation of maltose or salicin; but, with suitable precautions, action on both maltose and salicin provides a differentiation. Certain strains ofStr. bovisdiffer from the typical form of that species in their ability to proliferate over a wider range of temperature and in higher concentrations of salt and methylene blue and in their failure to split aesculin.(3)Str. lactisandStr. cremoris, which were differentiated by tests for the hydrolysis of arginine and ability to grow at 40° C.(4)Str.faecalisand its varieties. Many of the strains are non-haemolytic but otherwise indistinguishable fromStr. durans. The formation of acetoin from citrate and from glucose are common but not universal properties in the enterococcus group.(5) Heterofermentative streptococci. The majority of the strains were identified asStr. kefir. Str. citrovorus, which was isolated only occasionally, was distinguished by its low maximum temperature (under 37° C), weak resistance to heat and by fermentation reactions.Raw milk was found to containStr. kefir, Str. lactisand the mastitis organisms more frequently than the other species.Str. thermophilusandStr. boviswere the dominant forms in freshly pasteurized milk. Pasteurized milk held at 10–22° C. until it became tainted yieldedStr. kefirandStr. faecalisand, at the higher temperatures,Str. bovisandStr. thermophilus.