SOP: Staphylococcus aureus Enumeration

Staph aureus itself is killed by cooking, but its heat-stable toxin is not, which is why we count the organism in foods handled heavily by people. High counts signal temperature abuse or poor hygiene, and the test is a fixture in catering and deli food monitoring.

Microbiological testing protects consumers from pathogens and tracks hygiene across the plant. Because living organisms are the analyte, aseptic technique and precise incubation conditions are not optional extras; they are the measurement itself. A single lapse in sterility or temperature control invalidates the result.

Purpose

To enumerate coagulase-positive staphylococci in foods by selective plating for hygiene assessment.

Scope

Applies to cooked meats, salads, bakery cream products, dairy, and other ready-to-eat foods.

Equipment and Materials

• Baird-Parker agar with egg yolk tellurite emulsion
• Coagulase test reagents (rabbit plasma with EDTA) or validated latex kit
• Brain heart infusion broth
• Incubator at 35 to 37 C
• Stomacher, diluent blanks, pipettes
• Colony counter

Procedure

1. Prepare the 1:10 homogenate and decimal dilutions as for total plate count.
2. Spread 0.1 mL of appropriate dilutions onto duplicate Baird-Parker plates. Alternatively use the pour plate method with 1 mL.
3. Incubate at 35 to 37 C for 24 to 48 hours.
4. Count typical colonies: black or grey, shiny, with clear zones (lipolysis) and opaque halos (proteolysis).
5. Select at least 5 typical colonies per plate for confirmation. Inoculate each into brain heart infusion broth and incubate 18 to 24 hours.
6. Perform the coagulase test: add 0.1 mL of culture to 0.3 mL of rabbit plasma, incubate at 37 C, and read for clot formation up to 6 hours. Any degree of clotting is positive.
7. Calculate the count from the proportion of confirmed colonies.

Recording Results

CFU per g equals (confirmed colonies divided by tested colonies) times plate count times dilution factor. Report to two significant figures. Counts above 10,000 per g suggest potential enterotoxin risk and require QA review.

Quality Control

Verify each new lot of culture media with reference strains before use: the target organism must show typical growth and morphology, and a non-target strain must be inhibited or show atypical growth as expected. Include a positive control (reference strain) and a negative control (sterile diluent) with every test batch. Incubate an uninoculated plate from each poured batch as a sterility check. Monitor incubator temperatures twice daily with a calibrated thermometer; do not use incubators found outside tolerance until corrected and re-verified. Track contamination rates in blanks; a rising blank contamination rate signals a systemic problem requiring investigation. Participate in proficiency testing at least annually and investigate any unsatisfactory scores with documented corrective action.

Troubleshooting

• Atypical colonies dominate: confirm a larger proportion; some strains are weak halo producers.
• Coagulase weak or delayed: extend reading to 6 hours; use fresh plasma within expiry.
• Spreader interference: use the spread plate method rather than pour plates for better colony isolation.

Safety Notes

• Staph aureus is a Biosafety Level 2 organism. Avoid aerosols when spreading plates.
• Autoclave plates and cultures before disposal.
• Cover cuts and avoid handling cultures with broken skin.