Mercury resistance among soil bacteria: Ecology and transferability of genes encoding resistance

A diverse population of bacteria, all with the common property of resistance to mercuric ions (Hg²⁺), was isolated from root-associated, and other soils. Each isolate was screened for its resistance to a variety of antibiotics and heavy metals as well as organomercurial compounds. Several isolates showed resistance values to individual antimicrobial agents that were significantly higher than the average resistance values with respect to those agents for the genus to which the isolate belonged. Of 76 distinct isolates tested, 4 were able to transfer the Hg^r phenotype to sensitive cells at environmental temperatures. “Mobilization” of putative Hg-resistant non-conjugative plasmids with the wide host range plasmid RPl resulted in the co-transfer of the RPl markers and resistance to Hg²⁺ in 9 cases. Pl transduetion showed that 7 of these transfers were due, at least in part. to the formation of RPl-Hg^r co-integrates. This suggests that translocatable DNA elements encoding resistance to Hg²⁺ may be partly responsible for the dissemination and maintenance of the Hg^r phenotype in microbial populations.