novel antimicrobial agents that can serve as replacements to conventional pharmaceutical antibiotics are disclosed . the antimicrobial agents comprise conjugatively transmissible plasmids that kill targeted pathogenic bacteria , but are not harmful to donor bacteria . two types of lethal transmissible plasmids are disclosed . one type kills recipient bacteria by unchecked replication in the recipient cells and is prevented from occurring in donor cells . another type kills recipient bacteria by expressing a gene that produces a product detrimental or lethal to recipient bacterial cells , that gene being prevented from expression in donor cells .