Production of monoclonal antibodies specific for Haemophilus ducreyi: a screening method to discriminate specific and cross-reacting antibodies

Detection of Haemophilus ducreyi lipooligosaccharide by means of an immunolimulus assay

A murine monoclonal antibody (MAb) directed against a surface-exposed epitope of the lipooligosaccharide (LOS) of Haemophilus ducreyi strain 35000 was shown to be reactive with all 37 strains of this pathogen tested in a colony blot-radioimmunoassay. The LOS epitope bound by this MAb appeared to be stably expressed by H. ducreyi growing in vitro. The use of this MAb in the immunolimulus system revealed that it could detect purified H. ducreyi LOS at a level of 25 pg/ml. Similarly, this immunolimulus system could detect as few as 1000 colony forming units of in vitro-grown H. ducreyi cells per ml of buffer. When this MAb was utilized in the immunolimulus system together with lesion material from rabbits infected with two different H. ducreyi strains, a positive reaction was obtained with every sample tested, even when no viable organisms were present in the lesion material. In contrast, this MAb yielded consistently negative results when used in the immunolimulus system with lesion material from animals infected with Staphylococcus aureus.

Chancroid and Haemophilus ducreyi: an update

Haemophilus ducreyi is a fastidious gram-negative bacillus that causes the sexually transmitted infection chancroid. Chancroid is a major genital ulcerative disease in Africa, Southeast Asia, the Caribbean, and Latin America and is of increasing concern in the United States. Genital ulcerative disease and chancroid in particular have been associated with facilitating the transmission of human immunodeficiency virus. The diagnosis of chancroid based on the clinical appearance of the genital lesion or on the isolation of H. ducreyi on selective medium is relatively insensitive. However, recent advances in nonculture diagnostic tests have enhanced our ability to diagnose chancroid. There has been renewed interest in understanding the pathogenesis of H. ducreyi. In vitro and in vivo models have been developed to help identify important virulence determinants. Through the use of biochemical and molecular techniques, macromolecular components that may be important in virulence have been identified.

Use of electroporation to construct isogenic mutants of Haemophilus ducreyi

Little is known about the genetics of Haemophilus ducreyi, the etiologic agent of chancroid. To develop a method for constructing isogenic mutants of this organism that could be utilized in pathogenesis-related studies, electroporation techniques were evaluated as a means of introducing DNA into this organism. Electroporation of the plasmid shuttle vector pLS88 into H. ducreyi yielded approximately 10(6) antibiotic-resistant transformants per microgram of plasmid DNA. Studies of the feasibility of moving mutated genes into H. ducreyi were initiated by using NotI linker insertion and mini-Tn10kan mutagenesis techniques to introduce insertion mutations into cloned H. ducreyi genes encoding cell envelope antigens. In the former case, a gene encoding chloramphenicol acetyltransferase was then inserted into the NotI linker site created in the cloned H. ducreyi gene. The recombinant Escherichia coli strains containing these mutated plasmids no longer expressed the homologous H. ducreyi cell envelope antigens, as evidenced by their lack of reactivity with monoclonal antibody probes for these H. ducreyi proteins. Subsequent electroporation of both circular and linearized forms of plasmids carrying these mutated H. ducreyi genes into the homologous wild-type strain of H. ducreyi yielded antibiotic-resistant transformants which also lacked reactivity with the cell envelope antigen-specific monoclonal antibodies. Southern blot analysis confirmed that homologous recombination had occurred in these monoclonal antibody-unreactive transformants, resulting in the replacement of the wild-type allele with the mutated allele. Allelic exchange was most efficient when linear DNA molecules were used for electroporation. These results indicate that electroporation methods can be utilized to construct isogenic mutants of H. ducreyi.