New recommendations for disk diffusion antimicrobial susceptibility tests for methicillin-resistant (heteroresistant)...

Area of Science:

• Clinical Microbiology
• Antimicrobial Resistance
• Bacterial Pathogenesis

Background:

• The agar disk diffusion susceptibility test is a common method for determining bacterial susceptibility to antibiotics.
• Accurate susceptibility testing is crucial for guiding effective antibiotic treatment, especially for challenging pathogens like Staphylococcus aureus.
• Current guidelines for disk diffusion tests may not align with updated minimum inhibitory concentration (MIC) breakpoints for certain antibiotics.

Purpose of the Study:

• To reevaluate the agar disk diffusion susceptibility test for differentiating susceptible and resistant strains of Staphylococcus aureus and coagulase-negative staphylococci.
• To propose revised disk potencies and interpretive zone diameters for methicillin, oxacillin, and nafcillin based on current MIC data.
• To assess the suitability of oxacillin as a primary penicillin for routine susceptibility testing.

Main Methods:

• Agar disk diffusion susceptibility testing was performed on 128 strains of Staphylococcus aureus and 19 strains of coagulase-negative staphylococci.
• Testing utilized methicillin, oxacillin, and nafcillin as antimicrobial agents.
• Results were correlated with minimum inhibitory concentration (MIC) breakpoints derived from a broth microdilution system using a salt-containing medium.

Main Results:

• Current disk diffusion test recommendations for disk potencies and zone diameters showed poor correlation with recommended MIC breakpoints.
• New parameters are proposed: for methicillin, a 10-micrograms disk with specific zone diameter breakpoints for resistance (≤11 mm) and susceptibility (≥15 mm).
• For oxacillin and nafcillin, 4-micrograms disks are recommended with proposed zone diameter breakpoints for resistance (≤12 mm) and susceptibility (≥16 mm).

Conclusions:

• The agar disk diffusion susceptibility test requires updated parameters for accurate detection of methicillin, oxacillin, and nafcillin resistance in staphylococci.
• Revised disk potencies and zone diameters are suggested to improve the reliability of this widely used susceptibility testing method.
• Oxacillin is recommended as the preferred agent for routine testing, although hospital-specific staphylococcal populations may influence this choice.