Comparative in vitro antimicrobial procedural efficacy for susceptibility of Staphylococcus aureus, Escherichia coli and Pseudomonas species to chloramphenicol, ciprofloxacin and cefaclor

The present study assesses the reliability of the in vitro susceptibility tests E-test, disc diffusion and Alamar blue-based microbroth dilution assays for evaluating the efficacy of chloramphenicol, ciprofloxacin and cefaclor against clinical and reference isolates of Staphylococcus aureus, Escherichia coli and Pseudomonas spp. for use in empirical therapy. Ciprofloxacin showed 82% agreement between E-test and microbroth dilution by the visible dye reduction method against all organisms. This figure was 67% for cefaclor and 60% for chloramphenicol. E-test and microbroth dilution showed excellent correlation against S. aureus with all three antibiotics; however, correlation was not observed in Pseudomonas spp. between E-test and microbroth dilution by the percentage dye reduction method. E. coli did not show significant correlation, indicating the presence of heteroresistance. Chloramphenicol, being bacteristatic in nature, did not show clear agreement between the susceptibility test methods used. This study indicates that E-test provides an indication of minimum inhibitory concentration (MIC) of a panel of antibiotics against clinical isolates; however, microbroth dilution (dye reduction) is the most sensitive method for the determination of MIC due to intracellular enzymatic reduction of the dye to formazan, which only occurs in viable organisms. The study highlights the need for harmonisation between E-test and microbroth dilution methods in clinical trials of new antibiotics and in monitoring the drug resistance patterns in community and healthcare settings.