Altered channel properties of porins from Haemophilus influenzae: isolates from cystic fibrosis patients

Changes in amino-acid sequence of the unique pore-forming protein of H. influenzae (OmpP2; porin) have been associated with increased antimicrobial resistance in H. influenzae strains isolated from cystic fibrosis patients. From patients who were subjected to long-term antimicrobial therapy, H. influenzae strains 67d and 69a (patient 27) and strains 77a and 77f (patient 30) were isolated. Strains 67d and 77a were previously shown to have elevated values for minimal inhibitory concentrations of antibiotics compared to strains 69a and 77f. Porins were extracted from all four H. influenzae strains by detergent treatment and purified to homogeneity by ion exchange chromatography. By reconstitution of the clinical Hi porins into planar lipid bilayers, single-channel conductance, ionic selectivity, and voltage-gating characteristics were assessed. Porins 77a and 77f displayed similar single-channel conductance and ionic selectivity. Current-voltage relationships were determined for the different porins: porin 77f displayed substantial voltage gating at both positive and negative polarity; porin 77a gated at negative polarity only. Porins 67d and 69a showed substantial differences in their pore-forming properties: the single-channel conductance of porin 69a was significantly increased (1.05 nS) relative to porin 67d (0.73 nS). Porin 67d was twice as permeable to cations as porin 69a, and at both positive and negative polarities the extent of voltage gating was greater for porin 67d relative to porin 69a. Expression of the porins in an isogenic, porin-deleted H. influenzae background allowed for assessment of the contribution of each porin to the minimum inhibitory concentrations of various antimicrobial compounds. Porin 67d was found to have a decreased susceptibility to the antimicrobials novobiocin and streptomycin. This decreased susceptibility of porin 67d to novobiocin and streptomycin correlates with its decrease in single-channel conductance.