Serum resistance of an invasive nontypeable H. influenzae

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Since the introduction of Haemophilus influenzae b polysaccharide-protein conjugate vaccines, the incidence of invasive H. influenzae disease dramatically decreased. Currently, invasive disease is caused by other capsular serotypes or strains lacking genes for capsule synthesis, and therefore nontypeable (NTHi). A common feature of invasive H. influenzae is the ability to replicate in human blood, due to resistance to the bactericidal acitivity of serum. Seeking to understand the mechanism of serum-resistance in an invasive NTHi (strain R2866) we studied the strains recovered from the blood of five-day-old rats. We hypothesized that there would be increased virulence and acentuated resistance to normal human serum (NHS) in passaged isolates. We found that the rat passaged strain caused more deaths in the infant rat model, invaded Chang cells better but became more susceptible to the bactericidal action of NHS. This phenotypic shift was associated with modification of the lipooligosaccharide (LOS): more gal-B(1-4)gln epitope was exposed, while less gal-a-(1-3) gal was reactive with the monoclonal antibody. GeneScan® analysis of tetrameric repeats in the R2866 and the passaged strain indicated that expression of a LOS glycosyltransferase was OFF in the rat isolates. We conclude that animal passage of an invasive NTHi selects for increased animal virulence through expression of alternative LOS structures.