The method described here enumerates total Staphylococci by surface plating on Baird-Parker media. A coagulase test can be used to determine if individual colonies are S. aureus. Canadian Food And Drug Act and Regulations permit up to 100 coagulase positive S. aureus in pasteurized milk cheese and up to 1,000/g in cheese made from unpasteurized milk.
Equipment and Materials
Glass spreaders (hockey stick-shaped glass rods)
Incubator set at 37C
- Pour plates of B.P. agar (15 ml/plate) and dry surfaces (using sterile laminar airflow cabinet -- 2 hrs).
- Pipette 0.1 ml of homogenate and of subsequent dilutions onto surface of agar and spread evenly with a sterile bent glass rod until surface appears dry. Prepare duplicate plates. Use 10-1, 1--2, and 1--3dilutions.
- Incubate at 37C for 48 hrs.
- Count the number of colonies in each of the following groups:
- convex, shiny, black, with or without narrow gray-white margin, surrounded by clear zone extending into opaque medium.
- convex, shiny, black, with or without narrow gray-white margin, surrounded by clear zone extending into the opaque medium with an inner opaque zone.
- convex, shiny, black, with or without narrow gray-white margin, >1 mm in diameter.
Pipette 1 ml or 0.1 ml of homogenate and of subsequent dilutions into petri dish.
Add approximately 10 ml Baird-Parker medium. Mix well. Let stand on bench.
When solidified, invert and put in incubator at 37C (for 48 hours).
Read the same as Procedure A.
Note: Add 5 ml of well mixed Ey Tellurite, enrichment, at 5C to Baird-Parker agar prior to pouring plates.