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Susceptibility of Pseudomonas aeruginosa to gentamicin sulfate in vitro: lack of correlation between disc diffusion
Abstract:
Seventy-eight of 420 clinical isolates of Pseudomonas aeruginosa yielded zones of inhibition of less than 12 mm in diameter around 10-mug discs of gentamicin sulfate when tested by the standardized Bauer-Kirby disc diffusion method. Of 153 strains chosen from these isolates, one strain (0.65%) required 25 mug of gentamicin per ml for inhibition; the remainder (99.35%) were inhibited by 6 mug/ml or less of the antibiotic. It is recommended that those isolates of P. aeruginosa that yield zones of inhibition less than 12 mm in diameter be disc susceptibility-tested once more; those isolates that give zones of inhibition of less than 12 mm upon repeated examination should then be subjected to the broth dilution test before they are designated as sensitive or resistant to gentamicin.
Insights
Pseudomonas aeruginosa isolates showing small zones of inhibition to gentamicin require further testing. Repeat disc diffusion and broth dilution tests ensure accurate gentamicin susceptibility results for clinical isolates.
Area of Science:
- Clinical microbiology
- Antimicrobial resistance
Background:
- Pseudomonas aeruginosa is a significant opportunistic pathogen.
- Accurate susceptibility testing is crucial for effective treatment.
Purpose of the Study:
- To evaluate the accuracy of disc diffusion for gentamicin susceptibility testing of Pseudomonas aeruginosa.
- To determine appropriate methods for classifying gentamicin resistance in P. aeruginosa.
Main Methods:
- Standardized Bauer-Kirby disc diffusion method was used.
- Broth dilution tests were performed on selected isolates.
- Gentamicin sulfate discs (10-mug) were utilized.
Main Results:
- 78 out of 420 clinical isolates showed zones < 12 mm.
- One isolate (0.65%) required 25 mug/ml gentamicin for inhibition.
- 99.35% of tested strains were inhibited by ≤ 6 mug/ml.
Conclusions:
- Repeat disc diffusion testing is recommended for P. aeruginosa with zones < 12 mm.
- Broth dilution testing is advised for definitive gentamicin resistance classification.