Evidence for Capsule Gene Sequences among Pharyngeal Isolates of

Haemophilus influenzae is a common commensal organism of the human respiratory tract and is an important cause oflocalized and systemic disease. While isolates recovered from the respira-tory tract are generally nonencapsulated (serologically nontypeable), isolates from systemic sites typically express a polysaccharide capsule. To explore the possibility that nontypeable strains evolved from encapsulated organisms, a series of serologically nontypeable isolates were exam-ined for the presence of capsule gene sequences. Pharyngeal isolates (123) were collected from healthy 3-year-old Finnish children and examined by Southern hybridization with pU038, a plasmid that contains one complete set of 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 ofISI016, an insertion element that has been associated with encapsulation in H. influenzae. These results support the hypothesis that non typeable strains of H. influenzae arose from an encapsulated ancestor. Possibly the selective pressure driving the loss of encapsulation relates to the disadvantage associated with encapsula-tion during respiratory tract colonization. Haemophilus infiuenzae is a gram-negative coccobacillu