Poster Presentation Australian Society for Microbiology Annual Scientific Meeting 2014

Acquired Macrolide Resistance Genes in Nontypeable Haemophilus influenzae (#208)

Chris Atkinson 1 , Steve Tristram 1 , Dale Kunde 1
  1. School of Health Science, University of Tasmania, Launceston, Tasmania, Australia

Introduction: Nontypeable Haemophilus influenzae (NTHi) exhibits reduced susceptibility to macrolides due to an inherent efflux mechanism.1 Despite this, macrolides are commonly used to treat chronic respiratory infections where NTHi may be involved.2, 3 In a recent publication by Roberts (2011), a number of NTHi isolates were screened for common acquired macrolide resistance genes (AMRGs) using methods originally designed for Gram positive organisms. The findings suggest that these resistance determinants may be more common among NTHi isolates than previously described, although there was a poor correlation between gene carriage and increased minimum inhibitory concentration (MIC).4

Objectives: To screen a collection of NTHi isolates for select AMRGs as a follow-up to the study of Roberts.

Methods: A collection of 186 respiratory NTHi isolates was established. A number of these isolates were derived from a cohort with known prior population macrolide exposure, although no specific information was available about the macrolide exposure for any given isolate. Azithromycin MICs were determined by Etest. All isolates underwent PCR screening for the AMRGs mefA, ermA, ermB, ermC and ermF using the primers of Roberts.4 Amplicon identity was checked by sequencing the amplicons.

Results: Only 7/186 isolates were azithromycin resistant (MIC > 4 mg/L). Twenty isolates produced amplicons using the mefA primers, 7 using the ermA primers, and none using the remaining primers. Subsequent sequencing showed that the amplicons produced by the mefA and ermA primers did not identify as the target genes. Further investigation of the mefA amplicon revealed that it spanned 2 chromosomal genes (TAP and gyrB), and a gradient analysis revealed that all isolates will produce this amplicon when conditions are changed.

Conclusions: The above AMRGs are not widespread among NTHi isolates as described by Roberts. Caution should be taken when applying primers designed for a particular group of organisms to alternate organisms.

  1. Tristram S, Jacobs MR, Appelbaum PC. Antimicrobial Resistance in Haemophilus influenzae. Clin Microbiol Rev 2007; 20: 368-89.
  2. Serisier DJ. Risks of population antimicrobial resistance associated with chronic macrolide use for inflammatory airway diseases. The Lancet 2013; 1: 262-74.
  3. Altenburg J, de Graaff CS, Stienstra Y et al. Effect of azithromycin maintenance treatment on infectious exacerbations among patients with non-cystic fibrosis bronchiectasis: the BAT randomized controlled trial. JAMA 2013; 309: 1251-9.
  4. Roberts MC, Soge OO, No DB. Characterization of macrolide resistance genes in Haemophilus influenzae isolated from children with cystic fibrosis. Journal of Antimicrobial Chemotherapy 2011; 66: 100-4.