[An agar medium for direct enumeration of Staphylococcus aureus: pork plasma medium for S. aureus (PPSA)]

Area of Science:

• Microbiology
• Food Safety
• Bacteriology

Background:

• Accurate enumeration of Staphylococcus aureus is crucial for food safety.
• Traditional methods like Baird-Parker agar can be less efficient with certain strains.
• Egg yolk-negative strains of S. aureus present identification challenges in food matrices.

Purpose of the Study:

• To introduce and evaluate a novel selective agar medium for direct enumeration of coagulase-positive staphylococci.
• To assess the efficacy of the pork plasma medium for S. aureus (PPSA) compared to existing methods.
• To improve the confirmation process for Staphylococcus aureus in food samples.

Main Methods:

• Development of a selective agar medium (PPSA) based on Baird-Parker agar, modified with pig plasma instead of egg yolk.
• Testing the PPSA medium for the direct enumeration of coagulase-positive staphylococci.
• Comparative analysis of PPSA against egg yolk-containing media, particularly for foods with high levels of egg yolk-negative S. aureus strains.

Main Results:

• The PPSA medium facilitates direct enumeration of coagulase-positive staphylococci.
• Colonies of Staphylococcus aureus on PPSA are characterized by a distinct halo of precipitated fibrin.
• PPSA demonstrates an advantage over egg yolk-containing media when dealing with egg yolk-negative S. aureus strains, reducing the number of suspect colonies requiring confirmation.

Conclusions:

• Pork plasma medium for S. aureus (PPSA) is an effective selective medium for direct enumeration of coagulase-positive staphylococci.
• PPSA offers improved efficiency in identifying Staphylococcus aureus in food samples, especially those containing egg yolk-negative variants.
• The PPSA medium simplifies the confirmation process, contributing to more streamlined food safety testing.