ISOLATION OF STAPHYLOCOCCI FROM DRIED MILK1

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Abstract
Summary This study was made to evaluate, for use with dry milk, several media used in selective quantitative analyses for staphylococci in food products. The principal problems involved in culturing staphylococci from dry milk are (a) growth of Bacillus organisms and (b) obtaining good recovery of coagulase-positive staphylococci while eliminating growth of micrococci and coagulase-negative staphylococci, or at least providing a means of differentiating colonies of these organisms on the plates. Tellurite-polymyxin-egg yolk (TPEY) agar produced highest counts of coagulase-positive staphylococci for two-thirds of 40 strains tested in comparisons with Staphylococcus 110 (S110) and tellurite-glycine (TG) media. Highest counts of coagulase-negative staphylococci and of micrococci were found on S110 medium for 9 to 12 cultures. TG agar gave the poorest recovery in both experiments. TG and Vogel-Johnson (VJ) media were found too inhibitory to be used for staphylococcus counts on dry milk. Mannitol salt (MS) agar reacted very similarly to S110 agar. Colonies of Bacillus were easily discernible on TPEY medium but often were mistaken for staphylococci on the media which depend upon salt concentration for selectivity.

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