Antimicrobial susceptibility of Haemophilus influenzae strains isolated from invasive disease in Italy

Objectives: Haemophilus influenzae invasive disease is potentially life threatening and requires prompt antibiotic therapy. The aim of this study was to assess the antimicrobial susceptibility of H. influenzae strains isolated from invasive disease in Italy and to investigate ampicillin-resistant isolates by molecular biology techniques. Materials and methods: One-hundred and seventy-six invasive H. influenzae isolates, collected during 1998–2003, were analysed for susceptibility to ampicillin, azithromycin, chloramphenicol and ciprofloxacin. Ampicillin-resistant isolates were further tested against cefotaxime and imipenem. MICs were determined by Etest and interpreted according to NCCLS criteria. The ampicillin resistance genes, blaTEM and blaROB, were searched for by PCR. Genetic relatedness among ampicillin-resistant isolates was investigated by PFGE. Results: Overall, ampicillin resistance was 10.2% (all β-lactamase producer strains). The prevalence of ampicillin-resistant isolates increased from 6.9% in 1998/1999 to 19% in 2002/2003. Resistance to azithromycin and chloramphenicol was 6.8% and 1.7%, respectively. No strains were resistant to ciprofloxacin. Co-resistance between ampicillin and chloramphenicol and between ampicillin and azithromycin was observed in three and one isolates, respectively. All ampicillin-resistant isolates were susceptible to cefotaxime and imipenem and all harboured the blaTEM gene. PFGE demonstrated that most of the ampicillin-resistant isolates showed little genetic homology. Conclusions: An upward trend in resistance to ampicillin due to β-lactamase production was demonstrated In Italy. According to PFGE results, clonal dissemination of ampicillin-resistant isolates does not occur. Imipenem may represent an appropriate alternative for treatment of H. influenzae invasive disease caused by ampicillin-resistant isolates when third-generation cephalosporins cannot be used.