# Reliability of disc diffusion testing and molecular epidemiology of penicillin-susceptible Staphylococcus aureus bacteraemia

## TL;DR

This study confirms the reliability of a common test for penicillin resistance in Staphylococcus aureus and finds high rates of penicillin-susceptible strains in blood infections.

## Contribution

Validates the EUCAST disc diffusion method for penicillin resistance and provides new insights into the genetic diversity of PSSA strains.

## Key findings

- Phenotypic susceptibility to penicillin strongly correlates with the absence of the blaZ gene.
- PSSA accounted for 35.1% of bacteraemic isolates, emphasizing the need for susceptibility testing.
- PSSA isolates showed high genetic diversity, with CC5 and CC45 being dominant lineages.

## Abstract

Recent studies have reported an increasing prevalence of penicillin-susceptible Staphylococcus aureus (PSSA) worldwide. The reliability of disc diffusion testing for detecting penicillin-resistance has been questioned, and the molecular epidemiology of PSSA has not been studied to the same extent as that of MRSA strains.

To investigate the reliability of the disc diffusion method for detecting penicillin-resistance in S. aureus, and to examine the prevalence and molecular epidemiology of PSSA in bloodstream infections.

A total of 258 bacteraemic isolates obtained from one geographic region in Sweden during 2018–2019 were analysed using the disc diffusion test to detect penicillin-resistance, and genome sequenced to examine the prevalence of the blaZ gene and the molecular epidemiology of PSSA.

Phenotypic susceptibility to penicillin correlated strongly with the absence of the blaZ gene, with nearly 98% concordance. The prevalence of PSSA among patients with bacteraemia was 35.1%, highlighting the need for penicillin-susceptibility testing. Additionally, population structure analyses revealed substantial genetic diversity, underscoring the complexity of the PSSA epidemiology. The PSSA belonged to diverse clonal lineages, with CC5 and CC45 dominating our cohort, similar to findings in Spain, Australia, and other parts of Sweden. However, our study revealed a higher prevalence of CC12 compared with other regions, underscoring the importance of local epidemiological surveillance.

These findings validate the reliability of EUCAST’s disc diffusion method, showing a high prevalence of PSSA, and provide insight into the genetic underpinnings of penicillin-susceptibility in S. aureus.