NEW SELECTIVE AND DIFFERENTIAL MEDIUM FOR COAGULASE-POSITIVE STAPHYLOCOCCI ALLOWING RAPID GROWTH AND STRAIN DIFFERENTIATION

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Abstract
A medium for isolation of virulent staphylococci is important in processing clinical specimens which might contain mixtures of organisms and in culturing the hospital environment where a wide variety of organisms is commonly encountered. Desirable characteristics in such a medium include (1) Selectivity[long dash]to avoid overgrowth or inhibition by other organisms, (2) Rapid and maximum growth of staphylococci (within 24 hours), (3) Good growth from small inocula, (4) Rapid pigmentation of staphylococci[long dash]helpful in recognizing strain differences, (5) Typical colonial morphology of staphylococci, (6) Rapid presumptive screening for coagulase-positive staphylococci, (7) Easy preparation, (8) Stability, (9) Results readily duplicated in any laboratory, (10) Relatively inexpensive, (11) Should be adaptable to pour plate as well as surface streak techniques. Currently available media generally fail in several of the above-listed desired characteristics. Studies in this laboratory demonstrate that nutrient agar (BBL) incorporating 75 gamma/ml of polymyxin B (base) and 400 gamma/ml of cycloheximide possesses all of the above-mentioned attributes. Virtually all strains of gram-negative bacilli other than Proteus are inhibited, and swarming of Proteus is prevented. One-hundred and fifteen strains of coagulase-positive staphylococci grew on 100 gamma/ml of polymyxin; however, one of these strains produced only small (0.1 -0.5 mm. diameter) colonies. Ninety coagulase-negative strains were inhibited by 50 gamma/ml or less of polymyxin. Cyclo-heximide is useful in preventing growth of fungi when plates are held several days. The polymyxin and cycloheximide do not interfere with the phosphatase reaction; however, this screening test for coagulase-positive staphylococci is not likely to add anything to such a selective medium. Staphylococcus-like colonies may be picked directly from these plates for slide and tube coagulase tests.