Specification of surface mating systems among conjugative drug resistance plasmids in Escherichia coli K-12

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Abstract
Representative plasmids for most incompatibility groups in E. coli K-12 were transferred to a bald strain to compare transfer frequencies for liquid and solid media. Standard broth matings were used for a liquid environment, but for solid surface mating, conjugation was allowed to take place on nutrient plates before washing off the cells for transconjugant selection on plates containing appropriate drugs. Plasmids that determine rigid pili transferred at least 2000 times better on plates than in broth. Some plasmids that determine thick flexible pili transferred 45-470 times better, whereas others transferred equally well in both environments, as did plasmids of the I complex, which determine thin flexible pili. These results clearly distinguished a number of surface mating systems where most plasmids were derepressed for transfer and determined conjugative pili constitutively. The temperature-independent IncH2 plasmid R831b transferred best on plates, but other IncH plasmids transferred equally well in broth. This inconsistency led to the reclassification of R831b as IncM.