Detecting methicillin-resistant Staphylococcus epidermidis--disc diffusion, broth breakpoint or polymerase chain...

Area of Science:

• Microbiology
• Clinical Diagnostics
• Molecular Biology

Background:

• Methicillin resistance in staphylococci necessitates careful phenotypic testing to prevent false susceptibility results.
• Growth conditions significantly impact the expression of methicillin resistance.

Purpose of the Study:

• To develop rapid, simple phenotypic tests for methicillin resistance in staphylococci.
• To ensure test results correlate completely with the presence of the methicillin resistance gene (mecA).

Main Methods:

• A simplified polymerase chain reaction (PCR) method was used to amplify a region of the mecA gene without prior DNA extraction.
• Phenotypic susceptibility testing involved disc diffusion tests using oxacillin, cephalexin, and cephradine, alongside a broth tube breakpoint test.
• The 5 µg cephradine disc was validated on 441 clinical isolates.

Main Results:

• The simplified PCR method showed complete agreement with the broth tube breakpoint test for 110 S. epidermidis strains.
• Disc diffusion tests with oxacillin, cephalexin, and cephradine clearly differentiated resistant from susceptible strains.
• The 5 µg cephradine disc test demonstrated that all resistant coagulase-negative staphylococci grew to the disc edge, while susceptible strains had inhibition zones ≥10 mm.

Conclusions:

• The 5 µg cephradine disc diffusion test is recommended for routine detection of methicillin resistance in staphylococci.
• Both the simplified PCR method and the broth tube breakpoint test are reliable reference methods for detecting methicillin resistance.