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During an epidemic, a strain of Salmonella panama resistant to ampicillin, kanamycin, tetracycline and which was colicinogenic was isolated. The three drug-resistance determinants were transferred singly, in pairs or en bloc to sensitive Escherichia coli k 12 strains. When the recipient was an Hfr strain, it remained sensitive to F-specific phage f 2, so the transferred resistance factors can be classified as fi –. The three resistance characters were always independently transduced by phage P 1-b, and remained transferable in subsequent conjugations. In contrast to closely related R factors or Col factors which cannot coexist stably in the same cell, the three R factors described here do not specifically interfere with one another. They are maintained together in the naturally occurring strain of S. panama and after successive transfer to E. coli k 12. These results support the hypothesis that three independent fi – resistance factors, R (Am), R (Km), R (Tc) and a Col factor coexist in S. panama and E. coli k 12 strains.
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