Sixty-five pure cultures of epilithic bacteria were examined for their ability to transfer mercury resistance to ; five isolates transferred plasmids encoding mercury resistance with frequencies ranging from 8.4 × 10 to 2.8 × 10 per recipient. Two of the plasmids, pQM3 and pQM4, encoded narrow-spectrum mercury resistance, pQM3 also encoded streptomycin resistance, and both plasmids were broad host range. Maximum transfer frequencies of epilithic plasmids from pure cultures occurred over the range 10-25 ° at 3.5 g C 1 and with donor to recipient ratios of 0.4-30. Transfer occurred over a range of pH values (pH 5.0-8.0) but the effect of pH was most significant at non-optimal temperature. Anaerobiosis inhibited transfer of one epilithic plasmid, pQM1, but not that of pQM3. Plasmids encoding mercury resistance were also transferred from mixed natural suspensions of epilithic bacteria (MNS) to spp. on agar in the laboratory. Transfer from MNS occurred over a wide range of environmentally relevant conditions with maximum frequencies (2 × 10 per recipient) after 24 h, at 25 °, pH 5.5-8.0 and on a medium containing 10 g C1. The optimal initial cell density of MNS and recipient was 1.7 × 10 c.f.u. cm and highest frequencies were obtained with donor to recipient ratios ranging from 1.2 × 10 to 1.7 × 10. Most of the plasmids (54%) from MNS transferred from their original transconjugants to a strain, with frequencies ranging from 1.1 × 10 to greater than 1.0 × 10 per recipient. The majority (80%) of the plasmids were larger than 300 kb and all of these large plasmids encoded UV resistance in addition to mercury resistance. Twenty-one plasmids > 300 kb were analysed by restriction digests and were shown to be similar, with only minor structural alterations. One of these alterations was associated with the acquisition of streptomycin resistance. Overall, these results suggest that the epilithic bacteria examined possess the potential to transfer mercury resistance within the epilithon under a wide range of environmentally relevant conditions.


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