THE USE OF GLYCEROL AS A PRESERVATIVE FOR MILK SPECIMENS TO BE EXAMINED FOR HEMOLYTIC STREPTOCOCCI

Abstract

W HEN epidemics of septic sore throat are investigated, samples of milk submitted for laboratory examination may be in transit several days. During this time, in summer particularly, saprophytic strains of bacteria may multiply so rapidly that hemolytic streptococci, if present, are difficult to isolate. The efficient preservative action of glycerol on material containing micro6rganisms of the enteric disease group' and on milk to be examined for B. abortus2 suggested its use also in milk to be examined for hemolytic streptococci. A one-third volume (approximately 30 per cent) of glycerol of tested purity was added to samples of milk which had been recently collected. The specimens were then inoculated with strains of hemolytic streptococci from cases of septic sore throat. After 7 days at room temperature or 10 days in the refrigerator, the micro6rganisrms could be easily isolated, while they were recovered with difficulty after 24 hours at room temperature from similarly inoculated portions of the milk samples which contained no glycerol. An equal volume of glycerol-50 per centproved less satisfactory since, under these conditions, the colonies of streptococci which developed on the plating medium were slightly fewer than those from specimens containing a 30 per cent concentration; 20 per cent of glycerol has usually sufficed to preserve the specimens for 2 days only, when they were kept at room temperature. During the past 2 years, approximately a one-third volume of glycerol has been added to samples of milk sent to be examined for streptococci; greenproducing as well as hemolytic streptococci have been easily isolated from such specimens. A simple outfit containing a measured amount of glycerol of tested purity is used. By means of a label pasted on the container to indicate the level to which the milk should be added, a 30 per cent concentration of glycerol is secured. After collection, the specimen container should be well shaken to insure thorough mixing of the glycerol and the milk.