A Comparison of Six Selective Media for the Enumeration and Isolation of Staphylococci ¹

Area of Science:

• Microbiology
• Food Safety
• Clinical Diagnostics

Background:

• Accurate isolation and enumeration of Staphylococcus species are crucial for food safety and clinical diagnostics.
• Various selective media are employed, but their efficacy can vary depending on the specific Staphylococcus species and potential contaminants.
• Baird-Parker agar and other selective formulations are commonly used, necessitating a comparative analysis of their performance.

Purpose of the Study:

• To evaluate the efficacy of six selective agar media for the isolation and enumeration of Staphylococcus species.
• To compare the growth support for a diverse range of Staphylococcus species and identify limitations of each medium.
• To assess the selectivity of these media against common non-Staphylococcus contaminants.

Main Methods:

• Tested six selective media: Baird-Parker agar, Baird-Parker agar with bacitracin, supplemented Baird-Parker agar (acriflavin, polymyxin, sulphonamides), KRANEP agar, mannitol salt agar, and Staphylococcus medium 110.
• Used bovine blood agar as a control medium.
• Inoculated media with 37 Staphylococcus strains (23 species) and 19 non-Staphylococcus strains (Bacillus, Corynebacterium, Micrococcus).

Main Results:

• None of the six selective media supported the growth of all tested Staphylococcus strains.
• Supplemented Baird-Parker agar (acriflavin, polymyxin, sulphonamides) only supported Staphylococcus aureus growth.
• Staphylococcus medium 110 supported the growth of all tested Staphylococcus species except S. warneri BG 647.
• Bacillus, Corynebacterium, and Micrococcus species grew on all six selective media, indicating limited selectivity against these genera.

Conclusions:

• No single selective medium is universally effective for the isolation and enumeration of all Staphylococcus species.
• The choice of selective medium significantly impacts the recovery of specific Staphylococcus species.
• Supplemented Baird-Parker agar offers high specificity for S. aureus but lacks broad applicability, while Staphylococcus medium 110 provides broader coverage for Staphylococcus species but is less selective against other bacteria.