Correlation between phenotypic disc diffusion test and PCR molecular test for detection of methicillin resistance in Staphylococcus epidermidis and Staphylococcus hominis

Introduction. Coagulase-negative staphylococci (CoNS), particularly Staphylococcus epidermidis, are leading opportunistic pathogens in neonatal intensive care units, frequently associated with late-onset sepsis and medical device colonization.Gap statement. Cefoxitin disk diffusion has been adopted internationally as a surrogate marker for mecA, but its accuracy in CoNS is heterogeneous; high resistance rates and erratic phenotypic expression in species like Staphylococcus hominis indicate that the existing phenotypic-only approach does not transfer reliably between species and cohorts.Aim. Evaluate the diagnostic accuracy of oxacillin agar dilution (AD) and cefoxitin disc diffusion against mecA PCR in 664 coagulase-negative staphylococcal isolates collected over an 8-year period (2006-2014) at a tertiary-level hospital.Methodology. Phenotypic detection of methicillin resistance was performed by oxacillin AD with 4% NaCl/24 h and by cefoxitin disk diffusion (≤24 mm), using mecA PCR as the reference standard; inducible clindamycin resistance (iMLS_B) was screened by D-test. All isolates were stratified by species (S. epidermidis, S. hominis) and cohort (neonatal versus adult), with concordance quantified using Cohen's κ coefficient under the Landis and Koch framework.Results. S. epidermidis accounted for 87.2% and S. hominis for 8.6% of isolates, with 83.1% originating from neonates. In neonatal S. epidermidis, mecA prevalence reached 80%. Oxacillin AD demonstrated superior performance with 87.1% sensitivity (neonates) and 78.3% (adults), with moderate agreement in both subgroups (κ=0.44-0.54, respectively). The D-test further revealed a 20.8% prevalence of the inducible clindamycin resistance phenotype (iMLS_B) in neonatal S. hominis, indicating a high risk of therapeutic failure despite an apparently susceptible routine antibiogram, a prevalence nearly 20-fold higher than that observed in neonatal S. epidermidis (1.0%, n=5), identifying S. hominis as a particularly problematic inducible-clindamycin-resistance reservoir in the NICU setting.Conclusion. High resistance rates and erratic phenotypic expression in species like S. hominis, together with the unequal accuracy of phenotypic methods between neonates and adults, require a species-specific diagnostic approach. While oxacillin AD remains a robust predictor for both species and cohorts in this setting, systematic D-test implementation and mecA molecular confirmation are essential to ensure effective antimicrobial therapy.