the invention provides diastereomeric peptides with a net positive charge greater than + 1 , and cyclic analogues thereof , having at least 15 amino acid residues , wherein said residues are selected from : leucine and lysine ;, leucine , lysine and arginine ; glycine , serine , spargine , threonine , or glutamine optionally present at the n - terminus ; and / or lysine , arginine , glycine , or serine optionally present at the c - terminus . the diastereomeric peptides are suitable for treatment of cancer and for treatment , particularly topical treatment , of infections caused by pathogenic organisms such as bacteria and fungi . in addition , they may be used for controlling , mycoplasma infection in cell cultures and for food preservation as food supplements and as alternative to antibiotics for animal nutrition .