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Assessment of the oxacillin disk screening test for determining penicillin resistance in Streptococcus pneumoniae
G V Doern1, A B Brueggemann, G Pierce
1Clinical Microbiology Laboratories, University of Massachusetts Medical Center, Worcester 01655, USA.
Abstract:
The 1 microgram oxacillin disk diffusion screening test was performed on 1516 recent clinical isolates of Streptococcus pneumoniae obtained in a 1994-1995 U.S. surveillance study and the results compared to penicillin MICs determined using a standardized broth microdilution method. The oxacillin disk screening method failed to distinguish penicillin-resistant strains from those that were intermediately susceptible. Furthermore, a high percentage (11.1%) of penicillin-susceptible strains, for which MICs of penicillin were usually 0.06 or 0.03 microgram/ml, yielded zone diameters of < or = 19 mm with the oxacillin screen test and thus would have been falsely categorized as being resistant to penicillin.
Insights
The oxacillin disk diffusion test is unreliable for screening Streptococcus pneumoniae penicillin resistance. This method inaccurately categorizes many susceptible strains as resistant, compromising accurate antimicrobial susceptibility testing.
Area of Science:
- Microbiology
- Antimicrobial Resistance
- Clinical Diagnostics
Background:
- Accurate antimicrobial susceptibility testing is crucial for guiding treatment decisions.
- Streptococcus pneumoniae is a significant human pathogen.
- Penicillin resistance in S. pneumoniae necessitates reliable screening methods.
Purpose of the Study:
- To evaluate the efficacy of the 1 microgram oxacillin disk diffusion screening test for identifying penicillin-resistant Streptococcus pneumoniae.
- To compare oxacillin disk diffusion results with penicillin minimum inhibitory concentrations (MICs).
Main Methods:
- A total of 1516 clinical isolates of Streptococcus pneumoniae were tested.
- Oxacillin disk diffusion screening was performed.
- Penicillin MICs were determined using standardized broth microdilution.
Main Results:
- The oxacillin disk screening test could not differentiate between penicillin-resistant and intermediately susceptible strains.
- 11.1% of penicillin-susceptible S. pneumoniae strains were falsely identified as resistant by the oxacillin screen.
- MICs for susceptible strains were typically 0.06 or 0.03 microgram/ml.
Conclusions:
- The 1 microgram oxacillin disk diffusion test is not a reliable method for screening penicillin resistance in Streptococcus pneumoniae.
- This screening method leads to a high rate of false-positive results for penicillin resistance.
- Alternative or confirmatory testing methods are required for accurate penicillin susceptibility assessment in S. pneumoniae.