An amino acid substitution in PBP-3 in Haemophilus influenzae associate with the invasion to bronchial epithelial cells

SummaryHaemophilus influenzae is a common pathogen of respiratory infections. We examined whether β-lactamase-negative ampicillin-resistant (BLNAR) strains that are known to have ampicillin resistance due to a substitution of amino acid of penicillin binding protein (PBP)-3, differ from β-lactamase-negative ampicillin-susceptible strains with regard to invasion of bronchial epithelium. After 3 h incubation of each of 34 β-lactamase-negative ampicillin-susceptible and 57 BLNAR strains in the presence of BEAS-2B cells, a human bronchial epithelium cell line, extracellular bacteria were killed using gentamicin and intracellular bacteria numbered. All nine strains in which the efficiency of invasion was 1% or higher were BLNAR strains. The rate of invasion was significantly greater in strains with PBP-3 amino acid substitution (Met377 to Ile, Ser385 to Thr, Leu389 to Phe, and Asn526 to Lys) (n=34) than in those with no amino acid substitution. Electron microscopy showed that high invasive BLNAR strains were observed in cytoplasm of BEAS-2B cell layer. The injured cells were 9.44±1.76% among attaching cells examined by trypan blue staining after 6 h. These data may suggest that the amino acid substitution of the PBP in BLNAR strains may at least partly play roles in macropinocytosis, leading to the invasion and injury to epithelial cells.