Serum resistance in Haemophilus ducreyi requires outer membrane protein DsrA

Haemophilus ducreyi is resistant to killing by normal serum antibody and co mplement. We discovered an H. ducreyi outer membrane protein required for e xpression of serum resistance and termed it DsrA (for "ducreyi serum resist ance A"). The dsrA locus was cloned, sequenced, and mutagenized, An isogeni c mutant (FX517) of parent strain 35000 was constructed and characterized, and it was found to no longer express dsrA. FX517 was at least 10-fold more serum susceptible than 35000. DsrA was expressed by all strains of H. ducr eyi tested, except three naturally occurring, avirulent, serum-sensitive st rains. FX517 and the three naturally occurring dsrA-nonexpressing strains w ere complemented in trans with a plasmid expressing dsrA. All four strains were converted to a serum-resistant phenotype, including two that contained truncated lipooligosaccharide (LOS). Therefore, serum resistance in H. duc reyi does not require expression of full-length LOS but does require expres sion of dsrA. The dsrA locus from eight additional H. ducreyi strains was s equenced, and the deduced amino acid sequences were more than 85% identical . The major difference between the DsrA proteins was due to the presence of one, two, or three copies of the heptameric amino acid repeat NTHNINK. The se repeats account for the variability in apparent molecular mass of the mo nomeric form of DsrA (28 to 35 kDa) observed in sodium dodecyl sulfate-poly acrylamide gel electrophoresis. Since DsrA is present in virulent strains, is highly conserved, and is required for serum resistance, we speculate tha t it may be a virulence factor and a potential vaccine candidate.