Biotype of Haemophilus influenzae: correlation with virulence and ampicillin resistance.

Biotype and serotype were determined for 473 isolates of Haemophilus influenzae from 399 children. Patients with invasive disease usually had serotypable (86%) and biotype 1 (80%) isolates. Respiratory isolates from well children and children whose illness was not caused by H. influenzae were rarely serotypable (1%) or biotype 1 (8%). Respiratory isolates from children whose illness was possibly or probably related to H. influenzae and isolates from children with cystic fibrosis were more frequently biotype 1, serotypable, or both (P less than 0.01 for all comparisons). Ten (67%) of 15 children with acute otitis media due to H. influenzae had tympanocentesis isolates which were biotype 1; only one isolate was serotypable. Isolates from inflamed conjunctivae had biotypes and serotypes similar to respiratory isolates from well children. Ampicillin resistance was less frequent among biotype 1 isolates (9%) compared with other biotypes (19%) (P less than 0.01). The data suggest that H. influenzae is biochemically heterogeneous and that noncapsular factors, frequently predictable by biotype, contribute to virulence.