Frequency and properties of naturally occurring adherent piliated strains of Haemophilus influenzae type b

We found that 41 of 75 (55%) children with Haemophilus influenzae type b disease (70 cases of meningitis, 2 of cellulitis, 2 of septic arthritis, and 1 of epiglottitis) and 2 of 120 (1.7%) children with upper respiratory infection were colonized with H. influenzae type b in the nasopharynx (NP). Of these 43 NP strains from children with systemic H. influenzae type b disease, 7 (16%) adhered to human buccal epithelial cells. The strains isolated from the systemic site of all children, including children from whose NP adherent bacteria were isolated, did not adhere to buccal epithelial cells in vitro. Each adherent NP strain had biotype (I), serotype (b), and antibiotic susceptibility (sensitive) similar to that of the corresponding nonadherent systemic isolate. With one exception, all NP-systemic pairs had similar major outer membrane proteins. Six of the 7 NP strains had a protein band in the whole cell lysate preparation with a molecular weight between 22,000 and 23,000, which could not be seen in the nonadherent cerebrospinal fluid strains. Electron micrographs of all adherent strains showed that more than 95% of the organisms examined were highly piliated, whereas the nonadherent strains were not piliated. All piliated strains agglutinated human erythrocytes. Adherence to buccal epithelial cells and agglutination of erythrocytes could not be blocked by mannose or α-methyl-D-mannoside. We speculate that piliation is not important for NP colonization by H. influenzae type b and that the loss of pili may be required for host invasion.