A pilus-deficient mutant of Haemophilus ducreyi is virulent in the human model of experimental infection

Naemophilus ducreyi expresses tine tangled pill, which are composed predomi nantly of a major subunit (FtpA). Confocal microscopy showed that an FtpA-s pecific monoclonal antibody bound to bacteria in biopsy samples obtained fr om infected human volunteers. To test the role of pill in pathogenesis, an isogenic mutant (35000HP-SMS1) was constructed by insertionally inactivatin g ftpA. 35000HP-SMS1 did not express FtpA and was nonpiliated but was other wise identical to its parent, 35000HP. Seven healthy adults were challenged on the upper arm with the isogenic isolates in a double-blinded, escalatin g dose-response study. Sites inoculated with the mutant produced papules an d pustules at rates similar to the rates observed at sites inoculated with the parent. The recovery rate of H. ducreyi from cultures and the histopath ology of biopsy samples obtained from pustules inoculated with 35000HP or 3 5000HP-SMS1 were similar. Although pill are expressed in vivo, FtpA is not required for pustule formation in the human challenge model.