Susceptibility testing with disc diffusion for new cephalosporins: pre-diffusion revisited

Area of Science:

• Microbiology
• Clinical Pharmacy
• Antibiotic Resistance

Background:

• Accurate antimicrobial susceptibility testing (AST) is crucial for guiding antibiotic therapy.
• Disc diffusion is a common AST method, but pre-diffusion incubation of antibiotic discs may influence zone of inhibition interpretation.
• Third-generation cephalosporins, such as ceftazidime and ceftriaxone, are vital in treating infections caused by Gram-negative bacteria.

Purpose of the Study:

• To compare the efficacy of 20-hour pre-diffusion versus direct diffusion for third-generation cephalosporins.
• To evaluate the impact of these diffusion methods on regression analysis of zone sizes versus inhibitory concentrations.
• To assess the influence of bacterial growth characteristics on the interpretation of disc diffusion results.

Main Methods:

• Investigated 20-hour pre-diffusion and direct diffusion methods for ceftazidime and ceftriaxone.
• Utilized three bacterial groups with distinct agar growth characteristics: Escherichia coli, Enterobacter species, and streptococci.
• Constructed regression lines correlating zone sizes with minimum inhibitory concentrations determined by plate dilution.

Main Results:

• Twenty-hour pre-diffusion consistently produced larger zones of inhibition than direct diffusion.
• Pre-diffusion resulted in greater zone size variation, lower regression slopes, reduced residual variances, and higher correlation coefficients.
• Regression lines differed significantly among bacterial groups with direct diffusion, but not with 20-hour pre-diffusion.

Conclusions:

• Twenty-hour pre-diffusion is a superior method for interpreting disc diffusion zone sizes with ceftazidime and ceftriaxone.
• Pre-diffusion offers improved consistency and reliability in AST for these cephalosporins across different bacterial types.