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ORIGIN OF PROTOTROPHIC VARIANTS IN MIXTURES OF AUXOTROPHIC MUTANTS OF SERRATIA MARCESCENS

Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts

ABSTRACT

DUSHMAN, MARIAM B. (Massachusetts Institute of Technology, Cambridge). Origin of prototrophic variants in mixtures of auxotrophic mutants of Serratia marcescens. J. Bacteriol. 86:1173–1181. 1963.—The mechanism whereby mixed cultures of Serratia marcescens auxotrophs gave rise to prototrophs was shown not to be a genetic exchange by mating, since cell-to-cell contact was not required for the formation of new types. New types arose when the two "parental" strains were not mixed; they were kept separate by means of a Millipore membrane filter. In mixed cultures on media selective for new cell types, these new types arose as a result of reversions in one strain, the apparent recipient, during residual growth promoted by materials excreted by the other strain, the apparent donor or "feeder." Strains bearing one reverting and one nonreverting marker gave rise only to those types that could be accounted for by reversion. Ultraviolet irradiation of the feeder strain increased the frequency of formation of new types; streptomycin treatment of a streptomycin-sensitive feeder, even starting several hours after the onset of mixed cultivation, prevented the formation of apparent recombinants.