Capsulation loci of non-serotype b encapsulated Haemophilus influenzae

Invasive infections caused by non-type b encapsulated Haemophilus influenza e have increased recently. Because capsule is a major virulence factor, cap sulation of 62 recently isolated non-serotype b encapsulated strains was ex amined. Repeated serotyping confirmed only 69.0% of isolates. The combinati on of slide agglutination and cap genotyping confirmed 78.9% of type a, 100 % of type e, and 86.4% of type f strains. Seven nonencapsulated strains may have lost capsulation through homologous recombination. Three strains that could not be serotyped or genotyped failed to hybridize with any cap probe and are probably nontypeable H. influenzae. Of isolates that retained an i ntact cap locus, 62.2% had evidence of cap amplification. The epidemiology of non-type b encapsulated H. influenzae infections is complicated by the p oor specificity of available serologic reagents and by spontaneous capsule loss. Recently isolated invasive non-type b encapsulated H. influenzae freq uently have cap amplification, which may contribute to their virulence.