Plasmid-borne mercury resistance in aquatic bacteria

Abstract Bacteria isolated from the River Mersey were analysed for their tolerance to mercury (HgCl₂). About 40% of the population was tolerant to mercury and in 13 of 52 mercury-tolerant isolates tested the mercury resistance (Hg^®) was transferred to Escherichia coli in conjugal matings. These 13 isolates represented a range of gram-negative genera and in each case mercury resistance was coded by a conjugative plasmid. These plasmids (75 kb to > 250 kb in size) all expressed mercury resistance of the narrow spectrum variety, volatilised HgCl₂ to elemental Hg° vapour and showed some degree of temperature sensitivity of transfer. None expressed resistance to nine different antibiotics. These 13 Hg^R plasmids were classified by restriction mapping into three distinct groups typified by pMER11, pMER327 and pMER610. The eight pMER610 group plasmids are identical and belong to the IncHI-2 group. Two of the four pMER327 group plasmids are closely related while the other two contain some common restriction fragments. pMER11 is quite distinct from the other groups. These results imply that within this aquatic environment plasmids play an important role in the response of bacteria to contaminating mercury and that there is widespread plasmid transfer and considerable genetic rearrangement.