A comparison of phenotypic methods to demonstrate penicillin susceptibility in Staphylococcus aureus

The aim of this study was to assess the performance of phenotypic methods to demonstrate penicillin susceptibility in Staphylococcus aureus isolates using polymerase chain reaction (PCR) as a reference. A total of 126 S. aureus isolates were categorised as penicillin-susceptible S. aureus (PSSA), methicillin-susceptible S. aureus (MSSA) and methicillin-resistant S. aureus (MRSA) using blaZ and mecA PCR. The minimum inhibitory concentration (MIC) of penicillin for each isolate was determined by broth microdilution (BMD) and agar dilution (AD) methods. Qualitative susceptibility was determined by blinded disc diffusion testing using Calibrated Dichotomous Sensitivity (CDS), Clinical Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) methods. Categorical agreement (CA) rate, major error (ME) rate and very major error (VME) rate were calculated. These isolates were characterised into 66 PSSA, 38 MSSA and 22 MRSA by molecular testing. High rates of VME (9.6%) were encountered using BMD and AD, with 20% of penicillin-resistant S. aureus isolates having a penicillin MIC ≤0.125 mg/L. There was no VME using disc diffusion methods with high level of agreement of interpretation by three separate plate readers. CA rate using CLSI was 98.6% with 1.4% ME rate, for CDS CA rate was 97.6% with 2.4% ME rate, and for EUCAST CA rate was 93.8% with 6.2% ME rate. All methods improved with consensus reporting after considering interpretation by the plate readers. Laboratories can confidently report penicillin susceptibility in the majority of clinical S. aureus isolates based on disc diffusion testing using CLSI, EUCAST or CDS methodologies. Consensus reporting by independent plate readers may improve CA of disc diffusion testing. Penicillin MIC determination should not be used to demonstrate penicillin susceptibly in S. aureus isolates without confirmation by disc diffusion or molecular methods.