Quality control and interpretive criteria for the azlocillin disk diffusion susceptibility test

Area of Science:

• Microbiology
• Clinical Pharmacy
• Antimicrobial Resistance

Background:

• Accurate antimicrobial susceptibility testing is crucial for effective treatment.
• Disk diffusion methods offer a cost-effective approach for routine diagnostics.
• Azlocillin is a key antibiotic for treating infections caused by susceptible bacteria.

Purpose of the Study:

• To evaluate the performance of the 75-micrograms azlocillin disk diffusion test.
• To establish correlation between zone diameters and minimal inhibitory concentrations (MICs).
• To propose criteria for classifying bacterial isolates as susceptible or resistant.

Main Methods:

• Standardized disk diffusion tests were conducted using azlocillin (75 µg) disks.
• Control strains included Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, and Staphylococcus aureus ATCC 25923.
• Regression analysis correlated zone diameters with azlocillin MICs for clinical isolates.

Main Results:

• Test parameters for accuracy and precision were determined using control strains.
• Regression lines were established for gram-negative isolates, including Enterobacteriaceae, P. Aeruginosa, and Aeromonas hydrophila.
• The study identified correlations between zone size and azlocillin MICs.

Conclusions:

• The 75-micrograms azlocillin disk diffusion test demonstrates reliable performance for susceptibility testing.
• Proposed error-rate bound criteria can effectively distinguish susceptible from resistant gram-negative isolates.
• This method aids in optimizing azlocillin therapy and antimicrobial stewardship programs.