Thermosensitive Antibiotic Resistance Plasmids in Enterobacteria

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Abstract
Of 775 conjugative plasmids found in enterobacteria mediating antibotic resistance, 24 (3.1%) were thermosensitive (ts); they were most common in Klebsiella pneumoniae. Ts plasmids were also found in all the samples of sewage and river water examined. Over half of 73 ts plasmids from unrelated sources mediated resistance to chloramphenicol in addition to several other antibiotics. Many mediated resistance to Hg (53.4%), arsenite (38.4%) and tellurite (79.5%) but not to Cu, Co and Ag. Fifty-eight belonged to incompatibility group H2 and 12 belonged to the H1 group. Resistance to Hg, arsenite and tellurite was common in strains containing H2 plasmids but not in H1 plasmids. The 73 plasmids transferred at high rates at 22.degree. and 28.degree. C and at lower rates at 15.degree. C; they transferred at very low rates or not at all at 37.degree. C. They could be divided into 2 sets according to whether they transferred at a high or at a low rate at 33.degree. C. Unlike the prototype plasmid, Rts 1, they were soley or mainly ts for transfer and not for replication and only 1 of them brought about a marked reduction in growth rate of its host organism at 42.degree. C. None of the 73 plasmids mediated colicin or hemolysin production. Three plasmids, all from K. pneumoniae, mediated utilization of lactose, 2 of sucrose and raffinose and 3, all belonging to group H1, of citrate. No plasmid increased the pathogenicity of Salmonella typhimurium for chicks or Escherichia coli K12 for mice.