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Updated: Sep 19, 2025
Biosensor for Detection of Antibiotic Resistant Staphylococcus Bacteria
Published on: May 8, 2013
Reliability of disc diffusion testing and molecular epidemiology of penicillin-susceptible Staphylococcus aureus
Pernilla Kihlberg1, Thor Bech Johannesen2,3, Marc Stegger2,3,4,5
1Department of Infectious Diseases, Faculty of Medicine and Health, Örebro University, Örebro, Sweden.
Background:
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.
Objectives:
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.
Methods:
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.
Results:
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.
Conclusions:
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.
Insights
Penicillin-susceptible Staphylococcus aureus (PSSA) is common in bloodstream infections, and disc diffusion testing reliably detects penicillin resistance. Molecular analysis reveals diverse PSSA strains and clonal lineages.
Area of Science:
- Medical Microbiology
- Infectious Diseases
- Molecular Epidemiology
Background:
- Increasing global prevalence of penicillin-susceptible Staphylococcus aureus (PSSA).
- Concerns regarding the accuracy of disc diffusion testing for penicillin resistance detection.
- Limited molecular epidemiological data on PSSA compared to MRSA.
Purpose of the Study:
- Evaluate the reliability of disc diffusion testing for identifying penicillin resistance in S. aureus.
- Determine the prevalence and molecular epidemiology of PSSA in bloodstream infections.
Main Methods:
- Analysis of 258 bacteraemic S. aureus isolates from Sweden (2018-2019).
- Disc diffusion testing for penicillin resistance.
- Whole genome sequencing to assess blaZ gene prevalence and PSSA molecular epidemiology.
Main Results:
- High concordance (98%) between phenotypic penicillin susceptibility and absence of the blaZ gene.
- PSSA prevalence of 35.1% in bacteraemic patients, emphasizing the need for susceptibility testing.
- Significant genetic diversity within PSSA, with CC5 and CC45 as dominant lineages, but a notable prevalence of CC12 in the study region.
Conclusions:
- The EUCAST disc diffusion method is reliable for detecting penicillin resistance in S. aureus.
- PSSA is highly prevalent in bloodstream infections, necessitating routine susceptibility testing.
- Molecular epidemiology reveals diverse PSSA clonal lineages, highlighting the importance of local surveillance.
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