Genetics of Plasmids in Salmonella typhimurium

IN bacteria certain genetic determinants which may be gained or lost can multiply without incorporation in the bacterial chromosome. Some such plasmids 1 , the class of episomes 2 , may become reversibly integrated into the chromosome; for others, including the transmissible drug-resistance ( R ) factors 3 , integration has not been unequivocally demonstrated. We report observations on the transmission of an R factor and of colicine factors by conjugation and transduction in Salmonella typhimurium , and show that R factor genetic material may become attached to the bacterial chromosome—but only by becoming associated with an episome, prophage P 22.