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Penicillin susceptibility testing of penicillinase producing Neisseria gonorrhoeae by the E test: a need for caution
1Scottish Gonococcal Reference Laboratory, Department of Medical Microbiology, Edinburgh University Medical School, UK.
Abstract:
The penicillin MICs of 52 clinical isolates of penicillinase producing Neisseria gonorrhoeae (PPNG) and five reference strains were tested by agar dilution and the E test. All 52 PPNG were scored resistant (> or = 2 mg/L) by agar dilution whereas only 78.8% (41/52) and 82.7% (43/52) were classified as resistant by normal (0.016-256 mg/L) and low range (0.002-32 mg/L) E test MICs respectively. Isolates with an E test penicillin MIC > or = 0.19 mg/L should be checked by a specific test for beta-lactamase production.
Insights
Penicillinase-producing Neisseria gonorrhoeae (PPNG) showed varied resistance levels when tested with E tests compared to agar dilution. Further testing is recommended for isolates with specific E test minimum inhibitory concentration (MIC) values.
Area of Science:
- Microbiology
- Antimicrobial Resistance
- Sexually Transmitted Infections
Background:
- Neisseria gonorrhoeae is a major cause of sexually transmitted infections.
- Penicillinase-producing Neisseria gonorrhoeae (PPNG) strains exhibit resistance to penicillin antibiotics.
- Accurate susceptibility testing is crucial for effective treatment of gonorrhea.
Purpose of the Study:
- To evaluate the performance of the E test compared to agar dilution for determining penicillin Minimum Inhibitory Concentrations (MICs) in PPNG isolates.
- To identify potential discrepancies in resistance classification between the two methods.
Main Methods:
- Agar dilution and E test methods were used to determine penicillin MICs.
- Fifty-two clinical isolates of PPNG and five reference strains were analyzed.
- Resistance was defined as MIC ≥ 2 mg/L.
Main Results:
- All 52 PPNG isolates were resistant by agar dilution (MIC ≥ 2 mg/L).
- The E test classified 78.8% and 82.7% of isolates as resistant using normal and low-range MIC panels, respectively.
- A significant proportion of PPNG isolates were misclassified as susceptible by the E test.
Conclusions:
- The E test may underestimate penicillin resistance in PPNG isolates compared to agar dilution.
- Clinical laboratories should exercise caution when interpreting E test results for penicillin susceptibility in PPNG.
- Isolates with an E test penicillin MIC ≥ 0.19 mg/L warrant further investigation using a specific beta-lactamase detection test.