Activity of retapamulin against Streptococcus pyogenes and Staphylococcus aureus evaluated by agar dilution,...

Area of Science:

• Microbiology
• Pharmacology
• Infectious Diseases

Background:

• Retapamulin is a novel antibiotic targeting common bacterial pathogens.
• Staphylococcus aureus and Streptococcus pyogenes are significant causes of human infections.
• Evaluating antimicrobial susceptibility testing (AST) methods is crucial for clinical application.

Purpose of the Study:

• To assess the in vitro activity of retapamulin against clinical isolates of S. aureus and S. pyogenes.
• To compare the performance of four different AST methodologies: agar dilution, broth microdilution, E-test, and disk diffusion.
• To determine the optimal incubation conditions for retapamulin susceptibility testing.

Main Methods:

• Agar dilution, broth microdilution, E-test, and disk diffusion were performed on 106 S. aureus and 109 S. pyogenes isolates.
• Testing followed Clinical and Laboratory Standards Institute (CLSI) guidelines where applicable.
• Incubation conditions varied, including ambient air and carbon dioxide (CO2).

Main Results:

• Retapamulin exhibited low minimum inhibitory concentrations (MICs) against both S. aureus (0.03-0.25 µg/ml) and S. pyogenes (0.008-0.03 µg/ml) with ambient air incubation.
• All tested methods showed good agreement with broth microdilution for S. aureus (95-99%) and S. pyogenes (76-100%).
• E-test MICs for S. pyogenes were significantly higher when incubated in CO2 compared to ambient air, necessitating ambient air incubation for accurate results.

Conclusions:

• Retapamulin is highly effective against S. aureus and S. pyogenes, including methicillin-resistant S. aureus (MRSA) and macrolide-resistant S. pyogenes.
• Agar dilution, broth microdilution, E-test, and disk diffusion are suitable methods for retapamulin susceptibility testing.
• Ambient air incubation is recommended for E-test to ensure accurate retapamulin MIC determination.