Evolution of heavy metal resistant transconjugants in a soil environment with a concomitant selective pressure

Conjugal transfer between Escherichia coli and Alcaligenes eutrophus of plasmid pDN705, containing genes encoding resistance against cadmium, zinc and cobalt (czc genes) occurred in heavy metal polluted soil. The selective pressure from heavy metals (especially Zn2+) resulted in an increased number of resistant transconjugants and higher respiratory activities in sterile soil. As filter matings showed no or even a negative effect of Zn2+ on plasmid transfer, the increase of the number of transconjugants in polluted soil was probably due to growth rather than stimulated transfer. High numbers of recipients inhibited extended growth of transconjugants in sterile unpolluted soil. This intraspecific competition was overcome in the presence of heavy metals. In non-sterile soil, such an effect of heavy metals was not always evident, and seemed to be related to the severity of the selective pressure and inversely to the overall biological competition.