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

Ravin Hettiarachchi; Julie Allerton; Christopher McIver; Dianne Rafferty; Peter Taylor; Robert Stevens

doi:10.1016/j.pathol.2024.12.636

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

Pathology. 2025 Aug;57(5):630-636. doi: 10.1016/j.pathol.2024.12.636. Epub 2025 Feb 22.

Authors

Ravin Hettiarachchi¹, Julie Allerton², Christopher McIver³, Dianne Rafferty², Peter Taylor⁴, Robert Stevens⁴

Affiliations

¹ Department of Microbiology, New South Wales Health Pathology, St George Hospital, Sydney, NSW, Australia; School of Biomedical Sciences, University of NSW, Sydney, NSW, Australia. Electronic address: Ravin.Hettiarachchi@health.nsw.gov.
² Department of Microbiology, New South Wales Health Pathology, St George Hospital, Sydney, NSW, Australia.
³ Department of Microbiology, New South Wales Health Pathology, St George Hospital, Sydney, NSW, Australia; St George and Sutherland Clinical School, University of New South Wales, Sydney, NSW, Australia.
⁴ Department of Microbiology, New South Wales Health Pathology, St George Hospital, Sydney, NSW, Australia; School of Biomedical Sciences, University of NSW, Sydney, NSW, Australia.

PMID: 40155262
DOI: 10.1016/j.pathol.2024.12.636

Abstract

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.

Keywords: Staphylococcus aureus; antibiotic susceptibility testing; penicillin; polymerase chain reaction.

Publication types

Comparative Study

MeSH terms

Anti-Bacterial Agents* / pharmacology
Humans
Methicillin-Resistant Staphylococcus aureus / drug effects
Methicillin-Resistant Staphylococcus aureus / genetics
Microbial Sensitivity Tests / methods
Penicillin Resistance
Penicillins* / pharmacology
Phenotype
Polymerase Chain Reaction / methods
Staphylococcal Infections / drug therapy
Staphylococcal Infections / microbiology
Staphylococcus aureus* / drug effects
Staphylococcus aureus* / genetics

Substances

Penicillins
Anti-Bacterial Agents