Indiana University-Purdue University Indianapolis (IUPUI) / Haemophilus ducreyi is the causative agent of the genital ulcer disease chancroid, which has been shown to facilitate the transmission of HIV. H. ducreyi is likely exposed to multiple sources of antimicrobial peptides in vivo. APs are small, cationic molecules with both bactericidal and immunomodulatory functions. Because H. ducreyi is able to establish and maintain an infection in an environment rich with antimicrobial peptides, we hypothesized that the bacterium was resistant to the bactericidal effects of these peptides. Using a 96-well AP bactericidal assay, we examined H. ducreyi susceptibility to eight human APs likely to be encountered at the site of infection, including the α-defensins human neutrophil peptide-1, human neutrophil peptide-2, human neutrophil peptide-3, and human defensin 5, the β-defensins human β defensin-2, human beta defensin-3, and human beta defensin-4, and the human cathelicidin, LL-37. H. ducreyi survival was compared to the survival of Escherichia coli ML35, a strain known to be susceptible to several antimicrobial peptides. H. ducreyi was significantly more resistant than E. coli ML35 to the bactericidal effects of all peptides tested. Furthermore, we found that representative class I and class II strains of H. ducreyi were each resistant to APs of each functional category, indicating that resistance to antimicrobial peptides could represent a conserved method of pathogenesis for H. ducreyi as a species. The H. ducreyi genome contains a homolog for the Sap influx transporter. To study the role of the H. ducreyi Sap transporter in AP resistance, we generated an isogenic sapA mutant and used the 96-well AP bactericidal assay to compare the AP susceptibility profiles of wild-type H. ducreyi, the sapA mutant and the sapA trans-complement to α-defensins, β-defensins, and LL-37. We observed a 25% decrease in the survival of the sapA mutant when it was exposed to LL-37. These findings suggest that the H. ducreyi Sap transporter plays a role in H. ducreyi resistance to LL-37, but it is likely that other AP resistance mechanisms co-exist within the bacterium.