Environmental strains isolated from the source S appeared much more cytotoxic than Lp1 clinical strains, especially at 48 post-infection: actually environmental Lp1, Lp10 and Lp12 are characterized by a cytotoxicity of 100% whatever their pulsotype (PST1, PST2 and PST5) or their mip sequence (mip1, mip2 or mip3) (Figures 4a and 4b). The gene discussed is MIP; the disease is infection.