Phase-variable lipopolysaccharide structures enhance the invasive capacity of Haemophilus influenzae.

Genes necessary for the expression and phase variation of lipopolysaccharide epitopes of a virulent Haemophilus influenzae type b isolate (RM.7004) are contained within two chromosomal loci designated lic-1 and lic-2. Mutations were introduced into both lic-1 and lic-2, and the virulence of the double mutant was compared with that of the wild type in infant rats. These mutations in RM.7004 resulted in a significantly reduced incidence of bacteremia following intranasal inoculation, although nasopharyngeal colonization was similar for the mutant and wild-type strains. In contrast, no differences in bacteremia were observed when the mutant and wild-type strains were inoculated intraperitoneally.