High prevalence of β-lactam-resistant Haemophilus influenzae type b isolates derived from respiratory tract specimens in Japanese patients

OBJECTIVE: Serotypeable strains of Haemophilus influenzae, which can cause invasive infections, are found in the respiratory tract at low frequencies. We compared the antibiotic resistance of the typeable and nontypeable strains of H. influenzae in respiratory tract specimens obtained in Japan. METHODS: We determined the serotypes and the antibiotic susceptibilities of 440 clinical H. influenzae strains isolated from respiratory tract specimens. We also examined the prevalence of genotypes that are associated with beta-lactam resistance. RESULTS: The majority of the strains were nontypeable (421 strains, 95.7%). The remainder belonged to serotypes b (10 strains, 2.3%), e (three strains, 0.7%), or f (six strains, 1.4%). The type b strains exhibited the expression of beta-lactamase and resistance mutations in penicillin-binding protein 3 with significantly higher frequencies than other strains. CONCLUSIONS: H. influenzae type b strains, which are associated with meningitis and bacteremia, derived from respiratory tract specimens, shared more beta-lactam-resistant mechanisms than nontypeable and other serotype strains.