Appraisal of disk sensitivity tests with mecillinam

Area of Science:

• Microbiology
• Clinical Pharmacy
• Antimicrobial Susceptibility Testing

Background:

• Mecillinam is an antibiotic used to treat infections caused by Gram-negative bacteria.
• Accurate susceptibility testing is crucial for effective antibiotic therapy.
• Standardized disk diffusion methods are widely used for determining antibiotic susceptibility.

Purpose of the Study:

• To compare different disk susceptibility testing methods for mecillinam against Enterobacteriaceae.
• To establish reliable breakpoints for mecillinam disk susceptibility testing.
• To evaluate the predictive value of disk diffusion tests for mecillinam efficacy.

Main Methods:

• Disk susceptibility tests were performed on 167 hospital isolates of Enterobacteriaceae using eight variations of mecillinam disk testing.
• Three-way analysis of variance and Duncan multiple range tests were used to compare inhibition zone sizes.
• Semilogarithmic regression was used to correlate zone sizes with minimum inhibitory concentrations (MICs).
• Data from Vancouver and New York centers were pooled to determine breakpoints.

Main Results:

• The Association of Clinical Pathologists' method yielded significantly larger zones than the Kirby Bauer method (p = 0.0002).
• 10 microgram mecillinam disks from Mast Laboratories produced larger zones than BBL equivalents (p = 0.0004).
• Differences between agar media were not statistically significant (p = 0.061).
• A 16-mm breakpoint for 10 microgram mecillinam disks was identified using the Kirby Bauer method with BBL components.

Conclusions:

• Different disk susceptibility testing methods and disk manufacturers significantly impact mecillinam zone sizes.
• A 16-mm breakpoint using the Kirby Bauer method with BBL disks and media provides a reliable prediction of mecillinam response.
• Standardization of disk susceptibility testing for mecillinam is recommended for consistent clinical interpretation.