In vitro opsonin-independent interactions between cells of smooth and rough phenotypes of Flavobacterium psychrophilum and rainbow trout (Oncorhynchus mykiss) head kidney macrophages

► The cytotoxicity of Flavobacterium psychrophilum to fish macrophages was tested. ► The cytotoxic activity was differently expressed in smooth and rough phenotypes. ► The cytotoxicity is suggested to be contact-dependent and opsonin-independent. ► The cytotoxicity is initiated by binding of carbohydrates to macrophage lectins. ► The macrophage lysis is executed by different mechanisms in the two phenotypes.