Acquired macrolide resistance genes in Haemophilus influenzae?

Objectives: The objective of this study was to determine the prevalence of specific acquired macrolide resistance genes previously reported as present in clinical isolates of Haemophilus influenzae.

Methods: A collection of 172 clinical respiratory isolates of H. influenzae, including 59 isolates from cystic fibrosis patients and 27 from non-cystic fibrosis bronchiectasis patients with significant prior macrolide use, was established. This collection was tested for azithromycin susceptibility using Etest and screened for the presence of erm(A), erm(B), erm(C), erm(F), mef(A) and mef(E) using locked nucleic acid dual-labelled hydrolysis probes.

Results: The azithromycin MICs ranged from 0.09 to > 256 mg/L, with 2 (1.2%) isolates susceptible, 163 (94.8%) intermediate and 7 (4%) resistant according to EUCAST breakpoints (susceptible, ≤ 0.12 mg/L; resistant, >4 mg/L). None of the acquired macrolide resistance genes erm(A), erm(B), erm(C), erm(F), mef(A) or mef(E) was detected in any of the isolates.

Conclusions: The specific acquired macrolide resistance genes are not widespread in H. influenzae and the high prevalence of these genes previously reported might be unique to the specific circumstances of that study.