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The effect of various stress treatments on the coagulase test in Staphylococcus aureus
Abstract:
Coagulase production of 77 coagulase-positive Staphylococcus aureus strains stressed before plating on Baird-Parker agar were studied directly from Baird-Parker agar and from BHI broth on strains subcultured first from baird-Parker agar. Reduced pH, heat treatment and repeated freezing and lyophilization caused false-negative coagulase reactions carried out directly from Baird-Parker agar. No false-negative reactions were observed after simple freezing. The different stress treatments had no effect on coagulase production carried out from BHI broth. Coagulase tubes inoculated directly from Baird-Parker agar have to be incubated for longer time for a positive reaction than tubes inoculated from BHI broth. In the case of foods not heavily processed, it is possible to reduce the time for identification of S. aureus by performing the coagulase test directly from Baird-Parker agar.
Insights
Stressing Staphylococcus aureus before plating can cause false-negative coagulase tests directly from Baird-Parker agar. Testing from BHI broth avoids these errors, ensuring accurate identification of S. aureus.
Area of Science:
- Microbiology
- Food Safety
- Bacteriology
Background:
- Accurate identification of Staphylococcus aureus is crucial in food safety.
- The coagulase test is a standard method for S. aureus identification.
- Baird-Parker agar and Brain Heart Infusion (BHI) broth are common media for bacterial culture.
Purpose of the Study:
- To investigate the impact of pre-plating stress on coagulase production in Staphylococcus aureus.
- To compare the reliability of coagulase testing directly from Baird-Parker agar versus subculturing to BHI broth.
Main Methods:
- Seventy-seven coagulase-positive Staphylococcus aureus strains were subjected to various stress treatments (reduced pH, heat, freezing, lyophilization).
- Coagulase production was tested directly from Baird-Parker agar and after subculturing to BHI broth.
- Incubation times for positive coagulase reactions were recorded.
Main Results:
- Reduced pH, heat treatment, and repeated freezing/lyophilization led to false-negative coagulase reactions when tested directly from Baird-Parker agar.
- Simple freezing did not cause false-negative reactions.
- Stress treatments did not affect coagulase production when tested from BHI broth.
- Coagulase tests directly from Baird-Parker agar required longer incubation periods compared to those from BHI broth.
Conclusions:
- Testing Staphylococcus aureus coagulase directly from Baird-Parker agar after certain stress conditions can yield false-negative results.
- Subculturing to BHI broth before the coagulase test ensures accurate results, unaffected by prior stress.
- For less processed foods, direct testing from Baird-Parker agar may be feasible with adjusted incubation times, but BHI broth offers greater reliability.