Haemophilus influenzae: antibiotic susceptibility.

Needham, Cynthia A.

doi:10.1128/cmr.1.2.218-227.1988

Cited by 1 publication

(3 citation statements)

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“…Resistance was first noted in 1972, and by 1988, the overall rate of P-Lac-mediated resistance of H. influenzae was reported to be as high as 20% (9). Although other mechanisms of AMP resistance have been recognized, these isolates are extremely uncommon in the United States (6,16,17). Several methods for in vitro susceptibility testing of H.…”

Section: Methodsmentioning

confidence: 99%

“…influenzae, including the disk diffusion and broth dilution procedures, exist. Many bacteria, including ,B-Lac-producing strains of H. influenzae, exhibit an inoculum effect when tested for antimicrobial susceptibility (16). It is important, therefore, to carefully adjust the inoculum, preferably with a mechanical device, to consistently ensure accurate results (1).…”

Section: Methodsmentioning

confidence: 99%

“…Most laboratories reported that less than 5% of their isolates produced P-Lac prior to 1978. The incidence in Great Britain in 1981 had increased to 14% (16).…”

Section: Methodsmentioning

confidence: 99%

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

1992

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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 13-lactamase (13-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 .20 mm with AMP were considered sensitive. By using HTM and NCCLS standard M2-A4, strains showing zone sizes of .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, 13-Lac-producing strains of H. influenzae. Thirteen percent of the isolates gave at least one intermediate or resistant zone for AMP but were 13-Lac negative. MIC determinations with NCCLS standard M7-A2 were performed with resistant and intermediate strains. MICs for 13-Lacproducing strains of H. influenzae were .8.0 ,ug/ml. MICs for 13-Lac-negative strains were '1.0 ,ug/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, 13-Lac-negative, AMP-resistant or -intermediate strains be confirmed by the MIC procedure.

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