title: Evaluation of disk diffusion methods to detect low-level β-lactamase-negative ampicillin-resistant Haemophilus influenzae.
abstract: We evaluated the efficacy of disk diffusion methods for detection of low-level β-lactamase-negative ampicillin-resistant (low-BLNAR) Haemophilus influenzae. Four hundred and seventy unselected, recent clinical isolates were tested with ampicillin (10 μg), cefaclor (30 μg) and cefuroxime (30 μg) on iso-Sensitest agar enriched with nicotinamide adenine dinucleotide (NAD) and horse blood [ST agar; Swedish Reference Group for Antibiotics (SRGA) guidelines], and on chocolate agar (in-house guidelines). Selected isolates (n = 147) were subjected to partial sequencing of the ftsI gene. Forty-seven strains (10.0%) were genotypically identified as low-BLNAR, which was confirmed by determination of minimal inhibitory concentration (MIC) using microbroth dilution method: only low level resistance to ampicillin was detected [MIC ≤1 μg/mL; MIC(50) = 0.5 μg/mL, implying susceptibility by Clinical and Laboratory Standards Institute (CLSI) and European Committee on Antibiotic Susceptibility Testing (EUCAST) interpretative criteria]. The MIC of cefuroxime varied between 1 and 4 μg/mL (MIC(50) = 2 μg/mL), indicating susceptibility to cefuroxime by CLSI but not by EUCAST guidelines. Disk diffusion methods were able to discriminate low-BLNAR H. influenzae from the wild-type population with sensitivities ranging from 87% to 98% and specificities from 96% to 99%. Cefaclor was found to be superior to cefuroxime and ampicillin. Cefaclor zone diameter breakpoints of 30/29 and 23/22 mm are suggested for ST agar and chocolate agar, respectively.
authors.affiliation: Department of Clinical Microbiology, Aarhus University Hospital Skejby, Aarhus N, Denmark. nielnoer@rm.dk
authors.forename: Niels
authors.initials: N
authors.lastname: Nørskov-Lauritsen
authors.forename: Winnie
authors.initials: W
authors.lastname: Ridderberg
authors.forename: Lise T
authors.initials: LT
authors.lastname: Erikstrup
authors.forename: Kurt
authors.initials: K
authors.lastname: Fuursted
mesh_terms: D000667:Ampicillin / Q000494:pharmacology; D000668:Ampicillin Resistance; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D002433:Cefaclor / Q000494:pharmacology; D002444:Cefuroxime / Q000494:pharmacology; D052978:Disk Diffusion Antimicrobial Tests / Q000379:methods*; D024881:Drug Resistance, Bacterial; D005838:Genotype; D006193:Haemophilus influenzae / Q000187:drug effects* / Q000235:genetics / Q000302:isolation & purification*; D008826:Microbial Sensitivity Tests; D012044:Regression Analysis; D001618:beta-Lactamases / Q000032:analysis*
chemical_list: D000900:Anti-Bacterial Agents; D002433:Cefaclor; D000667:Ampicillin; D001618:beta-Lactamases; D002444:Cefuroxime