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Infection and Immunity, March 2000, p. 1608-1619, Vol. 68, No. 3
0019-9567/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Serum Resistance in Haemophilus ducreyi Requires Outer Membrane Protein DsrA

Christopher Elkins,1,2,* K. John Morrow Jr.,3 and Bonnie Olsen1

Departments of Medicine1 and Microbiology and Immunology,2 School of Medicine, University of North Carolina, Chapel Hill, North Carolina 27599, and Department of Cell Biology and Biochemistry, Texas Tech Health Science Center, Lubbock, Texas 794303

Received 22 September 1999/Returned for modification 3 November 1999/Accepted 12 December 1999

Haemophilus ducreyi is resistant to killing by normal serum antibody and complement. We discovered an H. ducreyi outer membrane protein required for expression of serum resistance and termed it DsrA (for "ducreyi serum resistance A"). The dsrA locus was cloned, sequenced, and mutagenized. An isogenic 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. ducreyi tested, except three naturally occurring, avirulent, serum-sensitive strains. FX517 and the three naturally occurring dsrA-nonexpressing strains were 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. ducreyi does not require expression of full-length LOS but does require expression of dsrA. The dsrA locus from eight additional H. ducreyi strains was sequenced, 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. These repeats account for the variability in apparent molecular mass of the monomeric form of DsrA (28 to 35 kDa) observed in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Since DsrA is present in virulent strains, is highly conserved, and is required for serum resistance, we speculate that it may be a virulence factor and a potential vaccine candidate.

* Corresponding author. Mailing address: Departments of Medicine and of Microbiology and Immunology, School of Medicine, Room 521 Burnett-Womack, Campus Box 7030, University of North Carolina, Chapel Hill, NC 27599. Phone: (919) 966-3661. Fax: (919) 966-6714. E-mail: chriselk{at}med.unc.edu.

Infection and Immunity, March 2000, p. 1608-1619, Vol. 68, No. 3
0019-9567/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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