Limited genetic diversity of recent invasive isolates of non-serotype b encapsulated Haemophilus influenzae

Invasive infections caused by non-type b encapsulated Haemophilus influenzae have increased in frequency in the last decade. This change prompted us to characterize the genetic relationships of 48 recently isolated invasive H. influenzae type a (Hia), e (Hie), and f (Hif) strains by comparison of restriction digest patterns (RDPs). Recent Hia isolates exhibited moderate genetic diversity, with the majority segregating into two major clonotypes. Recent Hie and, especially, Hif strains displayed considerably restricted genetic diversity. In particular, all but one Hif strain segregated into a single clonotype, and half of these isolates had identical RDPs. These results are consistent with the hypothesis that the increased incidence of disease due to non-type b encapsulated H. influenzae reflects the emergence of hypervirulent clones, especially in the case of Hif. Alternatively, it is possible that non-type b encapsulated H. influenzae strains have limited overall genetic diversity.