EVIDENCE FOR CAPSULE GENE-SEQUENCES AMONG PHARYNGEAL ISOLATES OF NONTYPABLE HAEMOPHILUS-INFLUENZAE

Haemophilus influenzae is a common commensal organism of the human res piratory tract and is an important cause of localized and systemic dis ease. While isolates recovered from the respiratory tract are generall y nonencapsulated (serologically nontypeable), isolates from systemic sites typically express a polysaccharide capsule. To explore the possi bility that nontypeable strains evolved from encapsulated organisms, a series of serologically nontypeable isolates were examined for the pr esence of capsule gene sequences. Pharyngeal isolates (123) were colle cted from healthy 3-year-old Finnish children and examined by Southern hybridization with pU-038, a plasmid that contains one complete set o f cap genes from an H. influenzae type b strain. Twenty-four isolates (20%) demonstrated homology with capsule-specific sequences. Of these 24, 18 in addition to 14 others had evidence of one or more copies of IS1016, an insertion element that has been associated with encapsulati on in H. influenzae. These results support the hypothesis that nontype able strains of H. influenzae arose from an encapsulated ancestor. Pos sibly the selective pressure driving the loss of encapsulation relates to the disadvantage associated with encapsulation during respiratory tract colonization.