Enhancement of Agrobacterium tumefaciens Infectivity by Chlorpromazine Hydrochloride

The tumor-initiating ability of Agrobacterium tumefaciens was enhanced two- to threefold after a preinoculation exposure to the phenothiazine derivative chlorpromazine hydrochloride. The mean number of tumors initiated per viable bacterium on pinto bean leaves was greatest after a 1-hr exposure to 15 mug of the drug per ml. Chlorpromazine had no significant effect on tumor initiation when applied to previously inoculated leaves. Chlorpromazine at 10 mug/ml was bacteriostatic; concentrations of 20 mug/ml and higher were bactericidal. In contrast to mitomycin C, which also enhances A. tumefaciens infectivity, chlorpromazine failed to induce temperate phage in several lysogenic strains of A. tumefaciens. Replication of a virulent A. tumefaciens phage, however, was not inhibited by the presence of chlorpromazine.