The phenomenon of paradoxical inhibition (p.i.) manifests itself as well-defined growth of a sensitive bacterium in the centre of the zone of inhibition overlying an area of growth of the inhibitory bacterium. Bacteriocin typing medium (BTM) for elicited p.i. for this group only. A medium for paradoxical inhibition (MPI) was devised which supported the development of p.i. among members of Enterobacteriaceae, and spp. at high frequency. The reacting pair of strains could belong either to the same or to different genera. Colonies of non-bacteriocinogenic strains derived from the central area of p.i. showed acquisition of bacteriocinogeny at high frequency. Acquisition was rapid in the beginning (up to 4 h) followed by a decline and virtual disappearance of bacteriocinogeny by 18 h on MPI, but was substantially retained on BTM. The overall frequency of acquisition among cell populations of different strains varied between 10 and 10, and the acquired characters were stable. Acquisition did not require either cell contact or participation of phages, but depended upon extracellular diffusable agents that were DNase sensitive and RNase resistant. A mutational basis for such acquisition could be ruled out, and the process of gene transfer was considered to be transformation. The extra-chromosomal nature of the determinants of bacteriocinogeny in the bacteria studied was suggested by effective elimination of these markers by sodium dodecyl sulphate and acridine orange.


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