An antibacterial composition that comprises as active compound at least one light isotope of an element selected from the group which consists of 1H, 12C, 16O, 14N, 39K, 24Mg, 64Zn, 85Rb, 28Si, 54Fe, 92Mo, 74Se, 58Ni, 70Ge, 52Cr, 63Cu, 50V, or combinations thereof, wherein the composition is enriched for the at least one light isotope. A method of treating and preventing bacterial diseases in humans and non-human animals by administering the composition. The use of the said composition in human and veterinary medicine for the prevention and treatment of diseases in humans and non-human animals and also as an antiseptic and disinfectant.