A novel lectin, DltA, is required for expression of a full serum resistance phenotype in Haemophilus ducreyi.

Haemophilus ducreyi, the causative agent of chancroid, is highly resistant to the complement-mediated bactericidal activity of normal human serum (NHS). Previously, we identified DsrA (for ducreyi serum resistance A), a major factor required for expression of the serum resistance phenotype in H. ducreyi. We describe here a second outer membrane protein, DltA (for ducreyi lectin A), which also contributes to serum resistance in H. ducreyi. Isogenic dltA mutants, constructed in 35000HP wild-type and FX517 dsrA backgrounds, were more susceptible to the bactericidal effects of NHS than each respective parent, demonstrating the additive effect of the mutations. Furthermore, expression of dltA in H. influenzae strain Rd rendered this highly susceptible strain partially resistant to 5% NHS compared to a vector-control strain. Although primary basic local alignment search tool analysis of the dltA open reading frame revealed no close bacterial homologue, similarity to the beta-chain of the eukaryotic lectin ricin was noted. DltA shares highly conserved structural motifs with the ricin beta chain, such as cysteines and lectin-binding domains. To determine whether dltA was a lectin, ligand blots and affinity chromatography experiments were performed. DltA was affinity purified on immobilized lactose and N-acetylgalactosamine, and N-glycosylated but not glycosidase-treated model glycoproteins bound DltA. These data indicate that DltA is a lectin with specificity for lactose-related carbohydrates (CHO) and is important for H. ducreyi serum resistance.