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Reproducibility of disc susceptibility tests with Haemophilus influenzae
A L Barry1, J H Jorgensen, D J Hardy
1Clinical Microbiology Institute, Tualatin, Oregon 97062.
Abstract:
Triplicate susceptibility tests were performed in each of three laboratories with 45 strains of Haemophilus influenzae and a single lot of Haemophilus test medium (HTM) agar plates. Using clarithromycin, erythromycin and chloramphenicol discs. zone sizes varied over ranges averaging only 4.3-5.8 mm. Ampicillin zone sizes were more variable, especially when beta-lactamase-producing strains were being tested, but the latter strains were consistently categorized as being resistant. A 94% interpretive consistency was observed with the two macrolides.
Insights
Susceptibility testing of Haemophilus influenzae showed consistent results for macrolides like clarithromycin and erythromycin, with 94% interpretive consistency. Ampicillin testing showed more variability, particularly with beta-lactamase-producing strains.
Area of Science:
- Microbiology
- Antimicrobial Susceptibility Testing
Background:
- Haemophilus influenzae is a significant human pathogen.
- Accurate antimicrobial susceptibility testing is crucial for guiding treatment decisions.
Purpose of the Study:
- To evaluate the reproducibility and consistency of antimicrobial susceptibility testing for Haemophilus influenzae.
- To assess the performance of specific antibiotic discs, including macrolides and ampicillin, on Haemophilus test medium (HTM) agar.
Main Methods:
- Triplicate susceptibility tests were conducted across three laboratories.
- Forty-five strains of Haemophilus influenzae were tested.
- Standard antibiotic discs (clarithromycin, erythromycin, chloramphenicol, ampicillin) were used on Haemophilus test medium (HTM) agar plates.
Main Results:
- Zone sizes for clarithromycin, erythromycin, and chloramphenicol showed limited variation (averaging 4.3-5.8 mm ranges).
- Ampicillin zone sizes exhibited greater variability, especially for beta-lactamase-producing strains.
- Beta-lactamase-producing strains were consistently classified as resistant to ampicillin.
- A high interpretive consistency (94%) was achieved with the two macrolides tested.
Conclusions:
- Antimicrobial susceptibility testing for Haemophilus influenzae using macrolide discs on HTM agar demonstrates good reproducibility.
- Ampicillin susceptibility testing requires careful interpretation, particularly in the presence of beta-lactamase production.
- The findings support the reliability of macrolides for susceptibility testing of H. influenzae.