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Problems with current recommendations for susceptibility testing of Haemophilus influenzae
P M Mendelman1, E A Wiley, T L Stull
1Department of Pediatrics, Children's Hospital and Medical Center, School of Medicine, University of Washington, Seattle 98105.
Antimicrobial Agents and Chemotherapy
|August 1, 1990
Summary
Haemophilus test medium (HTM) shows unreliable results for ampicillin resistance testing due to variable bacterial growth. This impacts the accurate identification of ampicillin-resistant, non-beta-lactamase-producing strains, making routine testing problematic.
Area of Science:
- Clinical Microbiology
- Antimicrobial Susceptibility Testing
- Bacterial Genetics
Background:
- Accurate antimicrobial susceptibility testing is crucial for guiding treatment decisions.
- Haemophilus species are common causes of various infections.
- Standardized testing media are essential for reliable results.
Purpose of the Study:
- To compare the performance of Haemophilus test medium (HTM) with other media for ampicillin susceptibility testing.
- To evaluate the reliability of HTM in detecting different phenotypes of ampicillin resistance in Haemophilus isolates.
- To assess the impact of inoculum size and medium preparation on test results.
Main Methods:
- Minimum inhibitory concentration (MIC) and disk diffusion susceptibility tests were performed.
- Seven ampicillin-resistant, non-beta-lactamase-producing (AmprNBLP) isolates were tested for MICs on HTM and supplemented brain heart infusion (sBHI) agar.
- Disk tests were conducted on 16 AmprNBLP, 18 ampicillin-susceptible (Amps), and 17 ampicillin-resistant, beta-lactamase-producing (AmprBLP) strains using five different media.
Main Results:
- Variable growth of Haemophilus strains was observed on HTM, with 37% not growing on laboratory-prepared HTM (PHTM) and 8% on commercial HTM (CHTM).
- AmprNBLP strains were frequently misclassified as susceptible by HTM-based tests, with 62% misclassified on PHTM and 33% on CHTM.
- Other media like Mueller-Hinton chocolate agar and enriched chocolate agar supported growth of all strains and provided more reliable susceptibility results.
Conclusions:
- Haemophilus test medium (HTM) exhibits strain-dependent growth variability, making it problematic for routine Haemophilus susceptibility testing.
- The use of HTM can lead to unreliable detection of ampicillin-resistant, non-beta-lactamase-producing isolates.
- Alternative media are recommended for accurate ampicillin susceptibility testing of Haemophilus species.