Many bacterial commensals and pathogens use the sialic acids as carbon and nitrogen sources. In Escherichia coli, the breakdown of these sugars is catalysed by gene products of the nan (Nacylneuraminate) operon; other microorganisms may use a similar catabolic strategy. Despite the known ligand and antirecognition functions of the sialic acids, the contribution of their catabolism to infection or host colonization has never been directly investigated. We addressed these questions with Haemophilus influenzae type b, which metabolizes relatively few carbohydrates, using the infant-rat infection model. The predicted H. influenzae homologue (HI0142) of the E. coli sialic acid aldolase structural gene, nanA, was subcloned and mutagenized by insertion of a kanamycin resistance cassette. Phenotypic investigation of the resulting H. influenzae aldolase mutants showed that: (i) HI0142 is essential for sialic acid degradation; (ii) the products of the open reading frames (ORFs) flanking HI0142 (HI0140, 41, 44 and 45) are likely to have the same functions as those of their counterparts in E. coli; (iii) sialylation of the lipooligosaccharide (LOS) epitope recognized by monoclonal antibody 3F11 is dependent on an environmental source of sialic acid; (iv) a nanA mutant hypersialylates its LOS sialyl acceptor, corresponding to an apparent increased fitness of the mutant in the infant-rat model; and (v) expression of the LOS sialyl acceptor is altered in cells grown without exogenous sialic acid, indicating the direct or indirect effect of sialic acid metabolism on LOS antigenicity. Taken together the data show the dual role of sialic acid catabolism in nutrition and cell surface modulation.