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Updated: Sep 4, 2025
Biosensor for Detection of Antibiotic Resistant Staphylococcus Bacteria
Published on: May 8, 2013
Simultaneously screening for methicillin-resistant Staphylococcus aureus and its susceptibility to potentiated
Xiaoliang Ba1, Claire L Raisen1, Zhen-Chao Zhou1,2
1Department of Veterinary Medicine, University of Cambridge, Cambridge, UK.
Abstract:
Introduction. We recently revealed that a significant proportion of clinical methicillin-resistant Staphylococcus aureus (MRSA) isolates are susceptible to pencillins and clavulanic acid (potentiated penicillins), including widely available combinations such as co-amoxiclav. These isolates also showed increased susceptibility to oxacillin on Iso-Sensitest Agar (ISA).Hypothesis/Gap Statement. The increased susceptibility to oxacillin displayed on ISA by these MRSA isolates may be used to distinguish them from the resistant ones.Aim. We aimed to develop a method to simultaneously screen a S. aureus clinical isolate for its susceptibility to methicillin and potentiated penicillins.Methodology. A double-disc diffusion method using 10 µg cefoxitin and 1 µg oxacillin discs on ISA was developed and tested against a panel of 120 whole genome-sequenced MRSA isolates. The sensitivity of the method was compared with that of previously published genotypic and phenotypic methods. In addition, double-disc diffusion was performed for all isolates on Müller-Hinton agar (MHA) following the European Committee on Antimicrobial Susceptibility Testing (EUCAST) protocol.Results. All isolates (120/120) were reconfirmed to be phenotypically MRSA, as indicated by the result of cefoxitin disc diffusion testing. All isolates (40/40) that had a pencillins and clavulanic acid (Pen-Clav)-resistant genotype were not inhibited by oxacillin, while 77/80 (96.3 %) isolates that had a Pen-Clav-susceptible genotype were inhibited by oxacillin on ISA. The results also showed that the EUCAST method using MHA correctly identified all isolates as MRSA but failed to distinguish the Pen-Clav-susceptible isolates from the Pen-Clav-resistant isolates.Conclusions. This double-disc diffusion method using ISA could be used to accurately screen for clinical MRSA isolates and determine their susceptibility to Pen-Clav simultaneously, rapidly identifying MRSA infections that might be suitable for treatment with potentiated penicillins.
Insights
A new method accurately screens methicillin-resistant Staphylococcus aureus (MRSA) for susceptibility to potentiated penicillins. This rapid screening identifies MRSA infections treatable with penicillin combinations like co-amoxiclav.
Area of Science:
- Clinical microbiology
- Antimicrobial resistance
- Diagnostic methods
Background:
- A significant proportion of clinical methicillin-resistant Staphylococcus aureus (MRSA) isolates are susceptible to potentiated penicillins, such as co-amoxiclav.
- These susceptible MRSA isolates exhibit increased susceptibility to oxacillin when tested on Iso-Sensitest Agar (ISA).
- Distinguishing these susceptible MRSA from resistant strains is crucial for appropriate treatment selection.
Purpose of the Study:
- To develop a method for simultaneously screening MRSA for susceptibility to methicillin and potentiated penicillins.
- To evaluate the efficacy of a novel double-disc diffusion method for this purpose.
Main Methods:
- A double-disc diffusion method was developed using cefoxitin and oxacillin discs on Iso-Sensitest Agar (ISA).
- The method was tested on 120 whole genome-sequenced MRSA isolates.
- Performance was compared against genotypic and phenotypic methods, including the European Committee on Antimicrobial Susceptibility Testing (EUCAST) protocol on Müller-Hinton agar (MHA).
Main Results:
- The developed method confirmed all 120 isolates as phenotypically MRSA via cefoxitin disc diffusion.
- On ISA, 96.3% of penicillin-clavulanic acid (Pen-Clav)-susceptible MRSA isolates were inhibited by oxacillin, while all Pen-Clav-resistant isolates were not.
- The EUCAST method on MHA identified all isolates as MRSA but could not differentiate Pen-Clav-susceptible from Pen-Clav-resistant strains.
Conclusions:
- The double-disc diffusion method using ISA accurately screens MRSA and determines Pen-Clav susceptibility simultaneously.
- This rapid method identifies MRSA infections potentially treatable with potentiated penicillins.
- It offers a valuable tool for optimizing antimicrobial therapy in MRSA infections.
