Ampicillin disk diffusion susceptibility testing of Haemophilus influenzae

Area of Science:

• Microbiology
• Antimicrobial Resistance
• Clinical Diagnostics

Background:

• Haemophilus influenzae is a significant pathogen.
• Beta-lactamase production confers resistance to ampicillin.
• Accurate susceptibility testing is crucial for effective treatment.

Purpose of the Study:

• To characterize beta-lactamase production and ampicillin minimum inhibitory concentrations (MICs) in Haemophilus influenzae strains.
• To evaluate disk diffusion susceptibility testing methods for ampicillin against H. influenzae.
• To establish reliable zone diameter interpretive criteria for ampicillin susceptibility.

Main Methods:

• Characterization of 114 Haemophilus influenzae strains for beta-lactamase production.
• Determination of ampicillin MICs for all strains.
• Disk diffusion susceptibility tests using 10 and 2 microgram ampicillin disks on Mueller-Hinton agar with supplements (CHOC-MHA) and enriched chocolate agar (CHOC).

Main Results:

• 41 strains produced TEM-type beta-lactamase with ampicillin MICs ≥ 2.0 µg/ml.
• 54 strains lacked TEM-type beta-lactamase activity with ampicillin MICs ≤ 0.5 µg/ml.
• 19 strains were beta-lactamase negative but had ampicillin MICs ≥ 2.0 µg/ml.
• Four disk diffusion combinations (CHOC-MHA and CHOC media with both disk concentrations) showed equivalent performance in identifying susceptible and resistant strains.

Conclusions:

• Established zone diameter interpretive criteria for ampicillin susceptibility testing of H. influenzae.
• Disk diffusion methods on both CHOC-MHA and CHOC agar are reliable for predicting ampicillin resistance and susceptibility.
• Testing accuracy is maintained irrespective of beta-lactamase production by the H. influenzae strains.