Quantitative antibiogram typing using inhibition zone diameters compared with ribotyping for epidemiological typing of methicillin-resistant Staphylococcus aureus

Abstract

Antibiogram typing of methicillin-resistant Staphylococcus aureus with selected antibiotics was evaluated as a primary epidemiological typing tool and compared with ribotyping. Antibiograms were derived with the Kirby-Bauer disk diffusion method by using erythromycin, clindamycin, cotrimoxazole, gentamicin, and ciprofloxacin. For typing, antibiogram data were analyzed by similarity analysis of disk zone diameters (quantitative antibiogram typing). One hundred seventy-two isolates were typed. Reproducibility reached 98% for the quantitative antibiogram and 100% for ribotyping. With three selected restriction enzymes (EcoRV, HindIII, and KpnI), 40 epidemiologically unrelated isolates could be classified into 21 ribotypes, whereas quantitative antibiogram typing classified these isolates into 19 groups. To evaluate the discriminatory power of the methods, we calculated an index of discrimination from data obtained with these 40 isolates. This index takes into consideration both the number of types defined by the typing method and their relative frequencies. With both ribotyping and quantitative antibiogram typing, high discrimination indices (0.972 and 0.954, respectively) were obtained. When epidemiological links between patients (ward, period of hospitalization, and contacts between staff and patients) were compared with the results of ribotyping or the quantitative antibiogram typing method, it appeared that both methods were able to discriminate epidemiological clusters, with only a few discrepancies. In conclusion, quantitative antibiogram typing, although not necessarily based on genomic markers, is a simple method which enables a reliable workup of methicillin-resistant S. aureus epidemic when sophisticated molecular typing methods are not available.