Introduction: Non-typeable Haemophilus influenzae (NTHi) is a significant cause of morbidity in cystic fibrosis (CF) patients, particularly in the first decade of life and treatment of these infections is often compromised by antibiotic resistance which is becoming more prevalent.1 The selective pressure of frequent antibiotic use in these patients suggests that CF NTHi isolates might have a higher prevalence of antibiotic resistance than isolates from other sources, although few studies have been done to demonstrate this. In addition, a recent study of over 100 CF NTHi isolates from a placebo-controlled azithromycin trial found that all isolates, including those from the placebo group, had at least one acquired macrolide resistance gene and all but one of the isolates were azithromycin non-susceptible, raising concern about the effectiveness of macrolide therapy.2
Objectives: To compare the prevalence of resistance to selected antibiotics or the presence of selected antibiotic resistance genes between CF and non-CF NTHi isolates.
Methods: 72 CF and 87 non-CF NTHi were tested for resistance to cotrimoxazole (EUCAST disc), ciprofloxacin (Etest-EUCAST breakpoint) and azithromycin (Etest-EUCAST breakpoint), for the presence of B-lactamase (nitrocephin), for altered PBP3/BLNAR genotype (PCR) and the presence of acquired macrolide resistance genes ermA, B, C and F and mefA (PCR).
Results: None of the isolates showed resistance to ciprofloxacin or were positive for ermB or ermF genes. All other types of resistance or resistance mechanisms were detected, but there was no statistical difference in prevalence between the CF and non-CF isolates. There was no correlation between the presence of the acquired macrolide resistance genes (ermA and C, and mefA) and the azithromycin MIC.
Conclusions: NTHi isolates from CF patients are not more resistant than non-CF isolates. The significance of acquired macrolide resistance genes and impact on azithromycin MIC require additional study.