No abstract

Combination of two types of transmissible drug-resistance factors in a host bacterium. J. Bacteriol. 84:9-16. 1962.-When two types of R factor, R(TC) and R (CM.SM. SA), or R(TC) and R (CM), were brought together in a host bacterium by superinfection with both factors, loss of either one or both factors was found. In the imperfectly stable existence of both factors in a host bacterium, both factors were transmitted separately by conjugation. As the result of interaction between the two types of R factor present in a host bacterium, recombinant factors were formed, R25 (TC CM. SM. SA) and R31 (CM. TC). The recombinant factors were able to transfer their resistance by conjugation. They were also transduced as one unit into Escherichia coli K12 by Plkc phage in the same fashion as the original Ri, (TC. CM. SM. SA) and R14 (CM. TC) factors independently isolated from dysenteric patients. The first isolation of multiply resistant Shigella was reported by Kitamoto et al. (1956). This organism was resistant to four drugs: tetracycline (TC), chloramphenicol (CM), streptomycin (SM), and sulfanilamide (SA). Escherichia coli strains resistant to these four drugs were also isolated in an epidemic of S. flexneri 3a resistant to the same agents (Matsuyama et al., 1958). S. flexneri 2a and E. coli, which were resistant to CM, SM, and SA, were also isolated in another epidemic in 1958 (Mitsuhashi, Harada, and Hashimoto, 1960a). E. freundii and E. coli, which were resistant to TC, CM, SM, and SA, were isolated in 1959 from a dysenteric patient (Harada et al., 1959). Many shigellae isolated from human cases of dysentery in Japan have 10 J. BACTERIOL.

Nontransferable drug resistance was acquired by conjugal transmissibility when mixed cultivation was applied to many isolates of the Enterobacteriaceae. The genetic element able to confer transmissibility on nontransferable drug resistance, was henceforth termed the transfer (T) factor. Genetic studies showed that there are three mechanisms involved in the acquisition of transferability of otherwise nontransferable drug resistance;(1) transfer of the drug resistance determinant on chromosome along with the transfer of host chromosome by T factor, (2) transfer of the nontransferable R factor by complementation with T factor, (3) formation of recombinant between the T factor and the drug resistance (r) determinant. The recombinant Tr factor was transferred as a single unit by the conjugal process, and was capable of conferring drug resistance. It was transduced jointly with bacteriophage Pl as a single unit in Escherichia coli.The authors reported previously [4] that the nontransferable drug resistance (r) determinant, obtained by transduction of R. factor with phage epsilon, acquired transferability by the formation of recombinants with F', F and R factors [5,6,7]. In the transduction of the R factor with phage Pl, a derivative of Pl phage (Plchl) was isolated in which the chloramphenicolresistance determinant of the R factor is specifically associated with the genome of phage Pl [11]. By the interaction of Plchl with F factor, we obtained a recombinant Fchl in which the chloramphenicol-resistance (chi) determinant of R factor is associated with the genome of F factor [12]. These results strongly suggest that the resistance (r) determinants of R factors have a wide range of affinity for other episomic elements and R factors are formed by recombination between r determinants and other genetic elements responsible for the replication and transmission of bacterial episomes.Anderson and Lewis [1] demonstrated that the transfer factor, which they referred to as delta (4 is present in Salmonella strains and nontransferable resistance determinants or Col E factor will transfer only when associated with the delta factor. The frequency of transfer of a given resistance determinants is governed by the number of resistance determinant-delta complexes available during the period of conjugation. Further they noted that the complex of resistance determinant-delta was dissociable. When the resistance determinant is closely associated with the delta factor, its 255

The R21(TC) factor, obtained by transduction of the R1o(TC. CM. SM. SA) factor with phage e to group E Salmonella, is not transferable by the normal conjugal process. However, when R21(TC)+ transductants are infected with the F13 factor, the nontransferable R21(TC) factor acquires transmissibility by conjugation. R21(TC)+ conjugants of Escherichia coli K-12, to which only the R21(TC) factor was transmitted by cell-to-cell contact from an F' R4 donor, were still unable to transfer their R21(TC) factor by conjugation. In crosses between Hfr and F-E. coli K-12 strains containing R21(TC), the gene responsible for tetracycline resistance was located on the E. coli K-12 chromosome between lac and pro, near lac.

Noninfectious drug-resistance determinants acquired conjugal transmissibility by the formation of recombinants with transfer factors, suggesting the origin of R factors.