Limited genetic diversity of Haemophilus influenzae (type b).

170 strains of Haemophilus influenzae (serotype b), isolated largely from patients with invasive disease from differing temporal and geographic origins were characterized using the combined approaches of DNA hybridization and outer membrane protein classification. Hybridization of a DNA probe to a region of the chromosome involved in the expression of type b capsular polysaccharide revealed that 163 (96%) isolates had one of three distinct, but closely related, chromosomal restriction fragment length polymorphisms (RFLPs). Each polymorphism was associated with its own distinctive set of outer membrane protein subtypes, indicating that the majority of H. influenzae (type b) isolates have evolved from common ancestors, giving rise to globally distributed organisms that have clonal characteristics.