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.
We isolated 56 Haemophilus (Actinobacillus) pleuropneumoniae strains from the pneumonic porcine lung tissues and tested them for antimicrobial susceptibility. Two drug-resistant strains were obtained. One, named KH-265, was resistant to streptomycin (SM) and sulfonamide (SA), and the other, named KH-195, was resistant to tetracycline (TC). The minimum inhibitory concentrations (MICs) of drugs for resistant strains were 100 micrograms/mliters for SM, 3200 micrograms/mliters for SA, and 12.5 micrograms/mliters for TC. KH-265 possessed a 8.3Kb nonconjugative plasmid, pMS260, encoding SM and SA resistance, which was transformable to E. coli strains. pMS260 belonged to none of 14 incompatibility groups including Inc. P and Inc. Q, so far tested. It was mobilizable to various causative strains for respiratory infections, Pseudomonas aeruginosa, Bordetella bronchiseptica, Pasteurella multocida and Haemophilus pleuropneumoniae, by RP4 (Inc. P) plasmid.
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