Effect of mutations in lipooligosaccharide biosynthesis genes on virulence of Haemophilus influenzae type b

Chemical mutagenesis techniques and genetic transformation methods were used to construct isogenic mutants of Haemophilus influenzae type b (Hib) defective in the ability to synthesize lipooligosaccharide (LOS). A mutant (17B) which expressed a LOS molecule with an altered oligosaccharide was less virulent than the wild-type parent strain, as determined by measurement of the ability of these strains to produce bacteremia in infant rats after intranasal challenge. Further mutagenesis of this mutant strain yielded two new mutants with different LOS phenotypes. Mutant 7A was slightly sensitive to the bactericidal activity present in normal infant rat serum and was even less virulent than its immediate parent strain (17B) in the intranasal challenge model. However, both mutants 17B and 7A could produce bacteremia and meningitis when introduced into infant rats by the intraperitoneal route. The other LOS mutant (14A) derived from mutant 17B exhibited a level of virulence equivalent to that of the original wild-type strain. Genetic transformation of wild-type chromosomal DNA into the essentially avirulent mutant 7A and selection of transformants on the basis of their LOS antigenic characteristics resulted in the sequential restoration of full virulence to this mutant. These findings suggest that LOS is involved on at least two different levels in the ability of Hib to produce invasive disease in the infant rat model. Changes in LOS phenotype can independently affect the ability of Hib to produce bacteremia after intranasal challenge and the sensitivity of Hib to killing by normal infant rat serum. These results reinforce the significance of Hib LOS in the expression of virulence by this pathogen.