AN ISOGENIC HEMOLYSIN-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 Prot eus/Serratia family of pore-forming toxins. In order to assess the rol e of the haemolysin in virulence, we constructed an isogenic haemolysi n-deficient mutant of H. ducreyi strain 35000. This strain, designated 35000-3, lacks detectable haemolytic activity. We tested H. ducreyi s trains 35000 and 35000-3 for their cytopathic activity against human f oreskin fibroblasts (HFFs). We observed strong cytopathic activity whe n strain 35000 was co-cultured with HFFs. In contrast, cytopathic acti vity was not observed when strain 35000-3 was co-cultured with HFF cel ls. We also analysed the isogenic pair of H. ducreyi strains for cytop athic activity against HeLa cells and the keratinocyte cell line HaCaT . Strains 35000 and 35000-3 were strongly cytotoxic when co-cultured w ith HeLa cells. HaCaT monolayers were slightly damaged by cocultivatio n with strain 35000-3 but this damage was much less than that observed when HaCaT cells were cocultured with strain 35000. These results ind icate that the H. ducreyi haemolysin is responsible for the previously observed cytotoxic activity against HFF cells and is partially respon sible for the activity observed with HaCaT cells. The haemolysin, howe ver, is not responsible for the activity observed with HeLa cells.