Investigation of ampicillin-intermediate strains of Haemophilus influenzae by using the disk diffusion procedure and current National Committee for Clinical Laboratory Standards guidelines.

It was noted in our laboratory that certain strains of Haemophilus influenzae yielded zone sizes interpreted as resistant to the ampicillin (AMP) disk on chocolate-Mueller-Hinton agar (CMH) but showed no evidence of beta-lactamase (beta-Lac) activity. Although it is known that a second mechanism of AMP resistance exists, strains with this mechanism are uncommon. To investigate this apparent discrepancy, a study of 100 consecutive clinical isolates of H. influenzae collected over a 6-month period was performed. Isolates were simultaneously tested against five antibiotics (AMP, chloramphenicol, cefotaxime, ciprofloxacin, and AMP-sulbactam) on CMH and on two brands of Haemophilus test medium (HTM) by using the disk diffusion procedure and National Committee for Clinical Laboratory Standards (NCCLS) standards. By using CMH and NCCLS standard M2-A3-S2, strains of H. influenzae showing zone sizes of greater than or equal to 20 mm with AMP were considered sensitive. By using HTM and NCCLS standard M2-A4, strains showing zone sizes of greater than or equal to 25 mm to AMP on HTM were considered sensitive. Intermediate strains had zone sizes of 22 to 24 mm. The majority of isolates (68%) were sensitive to all antibiotics. Two percent of the isolates were resistant to chloramphenicol. Seventeen percent of the isolates were AMP-resistant, beta-Lac-producing strains of H. influenzae. Thirteen percent of the isolates gave at least one intermediate or resistant zone for AMP but were beta-Lac negative. MIC determinations with NCCLS standard M7-A2 were performed with resistant and intermediate strains. MICs for beta-Lac-producing strains of H. influenzae were >/= 8.0 microgram/ml. MICs for beta-Lac-negative strains were </= 1.0 microgram/ml and were highly reproducible. If one uses the current NCCLS zone diameter interpretive criteria, results should be viewed with caution. Further investigation of zone size interpretive criteria is warranted. It is suggested that in the case of serious infections with H. influenzae, beta-Lac-negative, AMP-resistant or -intermediate strains be confirmed by the MIC procedure.