SUMMARY: R factors from Enterobacter strains isolated outside Japan were transferred to Salmonella typhimuriumLT 2 and their resistance traits transduced by phage P 22. Of five fi+ factors conferring resistance to tetracycline (Tc), streptomycin (Sm), sulphonamides (Su) and chloramphenicol (Cm), three, one of which conferred also resistance to kanamycin (Km), behaved like previously reported factors in that: Tc was transduced by itself but never with any other trait; all other traits were usually co-transduced; no transductants could transmit resistance by conjugation, even after infection with F'-13 lac. The other two fi+ factors, which conferred also resistance to benzylpenicillin (Pn), behaved similarly, except that: Pn was transduced only at a low rate, and was never co-transduced with any other trait; and a few transductants when given F'-13 lac became able to transmit. Four fi− factors, two of type Tc, Sm, Su, Pn and two of type Tc, Su, Pn differed in that: nearly all Tc transductants acquired also all the other traits; Pn was usually co-transduced with the other traits; and some transductants could transmit by conjugation, even without F'-13 lac. The various transductant classes obtained by treatment with P 22 grown on LT 2 carrying both an fi+ (Tc, Sm, Su, Cm, Km) and fi− (Tc, Sm, Su, Pn) factor could be accounted for by transduction of fragments of either one or the other of these factors. The sorts of transductants produced by lysates of a strain possessing Tc, Sm, Su, Cm, Km determinants obtained by growing a strain carrying an fi+ (Tc, Sm, Su, Cm) factor with one carrying an fi+ (Km) factor suggested that the strain carried two fi+ factors, not a recombinant factor. Both fi− and fi+ factors conferring resistance to benzylpenicillin conferred also resistance to cephalothin and cephaloridine and caused the constitutive production of a β-lactamase, active on both benzylpenicillin and on cephalothin. All four fi− and one of the five fi+ factors resembled coll factors in that they protected strain LT 2 against the bactericidal effect of ultraviolet irradiation.
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