A COMPARISON OF DIRECT PLATING AND ENRICHMENT METHODS FOR DETECTION AND ENUMERATION OF COAGULASE-POSITIVE STAPHYLOCOCCI IN FROZEN FEEDS OF ANIMAL ORIGIN1

Abstract

Direct surface plating on five selective and differential agar media and enrichment prior to plating were compared for detection and enumeration of coagulase-positive staphylococci in frozen feeds. The enrichment medium was trypticasc soy broth (TSB) fortified with 8, 10, and 12% sodium chloride. In general, direct plating on mannitol salt (MS) and staphylococcus 110 (S110) agars was accompanied by a higher recovery of coagulase-positive staphylococci than was obtained with tellurite polymyxin egg yolk (TPEY), Vogel-Johnson (VJ), and tellurite glycine (TG) agars. Suspected colonies picked from TPEY agar were most likely, and from TG agar were least likely, to be coagulase-positive staphylococci. Enrichment for 24 hr in TSB containing 8 or 10% sodium chloride yielded more staphylococci than did direct plating on TPEY or TG agars. Numbers obtained by the two procedures usually were more nearly equal when MS or S110 agars were used for direct plating. Enrichment in TSB containing 12% sodium chloride or incubation of broths for 48 instead of 24 hr, even if they contained 8 or 10% sodium chloride, were detrimental to the recovery of coagulase-positive staphylococci. Of the plating media tested, MS, TG, and VJ agars regularly and uniformly recovered coagulase-positive staphylococci from enrichment broths inoculated with highest dilutions of test samples. In contrast to this, erratic results were obtained when S110 and TPEY agars were used to recover these bacteria from the same broth cultures. Tests on feed-grade frozen meat by-product revealed the presence of 103 to 104 coagulase-positive staphylococci per gram in two samples and 104 to 105 per gram in two other samples. Examination of feed-grade frozen liver indicated the presence of 103 to 104 coagulase-positive staphylococci in one sample and 104 to 105 per gram in two samples.