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Screening pneumococci for penicillin resistance.

J M Swenson, B C Hill, C Thornsberry

    Journal of Clinical Microbiology
    |November 1, 1986
    PubMed
    Summary
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    Oxacillin disk diffusion effectively screens penicillin-susceptible pneumococci from resistant strains. However, it struggles to differentiate between resistant and relatively resistant strains using standard breakpoints.

    Area of Science:

    • Clinical Microbiology
    • Antimicrobial Resistance
    • Diagnostic Testing

    Background:

    • Penicillin resistance in Streptococcus pneumoniae is a growing public health concern.
    • Accurate and efficient laboratory methods are crucial for identifying resistant strains.
    • Disk diffusion tests are commonly used for antimicrobial susceptibility testing.

    Purpose of the Study:

    • To evaluate the effectiveness of oxacillin and methicillin disk diffusion tests in distinguishing penicillin-susceptible from penicillin-resistant pneumococci.
    • To compare the performance of different agar media and incubation atmospheres.
    • To assess the suitability of current breakpoints for classifying resistance levels.

    Main Methods:

    • Eighty-four pneumococcal isolates with varying penicillin minimum inhibitory concentrations (MICs) were tested.

    Related Experiment Videos

  • Disk diffusion tests were performed using oxacillin and methicillin on two different agar media.
  • Incubation was carried out in ambient air and increased carbon dioxide (CO2) conditions.

    • Main Results:

    • Oxacillin disk diffusion could differentiate susceptible from resistant strains but not resistant from relatively resistant strains using the 20 mm breakpoint.
    • Methicillin testing with a 20 mm breakpoint misclassified 12% of relatively resistant strains as susceptible.
    • Adjusting breakpoints for methicillin improved separation but still resulted in some misclassification of relatively resistant strains.

    Conclusions:

    • Oxacillin is recommended over methicillin as a screening agent for penicillin resistance in pneumococci.
    • Mueller-Hinton agar with 5% sheep blood incubated in ambient air is the preferred method.
    • A breakpoint of less than 20 mm for oxacillin disk diffusion should indicate resistance or relative resistance.