SUMMARY: Of 775 conjugative plasmids found in enterobacteria mediating antibiotic resistance, 24 (3·1%) were thermosensitive (ts); they were most common in . 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 of them mediated resistance to mercury (53·4%), arsenite (38·4%) and tellurite (79·5%) but not to copper, cobalt and silver. Fifty-eight belonged to incompatibility group H2 and 12 belonged to the H1 group. Resistance to mercury, arsenite and tellurite was common in strains containing H2 plasmids but not in H1 plasmids. The 73 plasmids transferred at high rates at 22 and 28°C and at lower rates at 15°C; they transferred at very low rates or not at all at 37°C. They could be divided into two sets according to whether they transferred at a high or at a low rate at 33°C. Unlike the prototype plasmid, Rts 1, they were solely or mainly ts for transfer and not for replication and only one of them brought about a marked reduction in growth rate of its host organism at 42°C. None of the 73 plasmids mediated colicin or haemolysin production. Three plasmids, all from , mediated utilization of lactose, two of sucrose and raffinose and three, all belonging to group H1, of citrate. None of the plasmids increased the pathogenicity of for chicks or k 12 for mice.


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