An isogenic haemolysin‐deficient mutant of Haemophilus ducreyi lacks the ability to produce cytopathic effects on human foreskin fibroblasts

The haemolysin of Haemophilus ducreyi is the newest member of the Proteus/Serratia family of pore‐forming toxins. In order to assess the role of the haemolysin in virulence, we constructed an isogenic haemolysin‐deficient mutant of H. ducreyi strain 35000. This strain, designated 35000‐3, lacks detectable haemolytic activity. We tested H. ducreyi strains 35000 and 35000‐3 for their cytopathic activity against human foreskin fibroblasts (HFFs). We observed strong cytopathic activity when strain 35000 was co‐cultured with HFFs. In contrast, cytopathic activity was not observed when strain 35000‐3 was co‐cultured with HFF cells. We also analysed the isogenic pair of H. ducreyi strains for cytopathic activity against HeLa cells and the keratinocyte cell line HaCaT. Strains 35000 and 35000‐3 were strongly cytotoxic when co‐cultured with HeLa cells. HaCaT monolayers were slightly damaged by cocultivation with strain 35000‐3 but this damage was much less than that observed when HaCaT cells were cocultured with strain 35000. These results indicate that the H. ducreyi haemolysin is responsible for the previously observed cytotoxic activity against HFF cells and is partially responsible for the activity observed with HaCaT cells. The haemolysin, however, is not responsible for the activity observed with HeLa cells.