Effects of pili rigidity and energy availability on conjugative plasmid transfer in aquatic environments

Conjugal transfer frequencies of nonconjugative plasmid pCE325 associated with either the conjugative plasmid R388 (rigid pili) or R100-1 (flexible pili) were measured in waste water and seawater between two strains of Escherichia coli K12. These strains were selected from three strains after estimating (i) their survival capacity in the two water types and (ii) the maintenance and expression of plasmid-located genes in the different strains. Mobilization of plasmid pCE325 was always below the detection limit, but increased when organic matter was added to the microcosms. This mobilization was not related to cell growth, but to the availability of energy conditioning the physiological state of the cells. The transfer frequency was higher when the conjugative plasmid encoded flexible pili.