Mercury Microbiology: Resistance Systems, Environmental Aspects, Methylation, and Human Health

Abstract

Mercury has no beneficial biological role, and is highly toxic to all forms of life. Bacteria are involved in the global environmental cycling of mercury, both by reducing Hg²⁺ to metallic Hg⁰, which is less soluble in aqueous systems and therefore less bioavailable, and by oxidizing and methylating Hg species, and in the process making Hg more bioavailable and more highly toxic. The most thoroughly studied bacterial biotransformation of mercury is reduction by the widely distributed mer resistance operons found on plasmids and transposons in Gram-negative and -positive bacteria. The products of these resistance operons transport ionic Hg²⁺ from outside the cell to the cellular cytoplasm, where mercuric reductase reduces divalent Hg²⁺ to Hg⁰, which is less toxic than Hg²⁺. Metallic mercury vapor, Hg⁰, is volatile under aerobic conditions, leaves the cell by passive diffusion, and is volatilized from the growth environment. Sometimes, additional gene(s) determine organomercurial lyase, the enzyme that cleaves organomercurial compounds to inorganic Hg²⁺, which is then reduced to Hg⁰. Two types of mer operons (“narrow spectrum” with inorganic Hg²⁺ resistance and “broad spectrum” with both organomercurial and inorganic mercury resistances) confer high levels of resistance on host bacteria. The expression of mer resistance genes is primarily regulated by the MerR protein, which is the prototype of an increasing family of metal and other effector-responsive transcriptional activators. Methylation of inorganic Hg²⁺ to CH₃Hg⁺ is thought to occur nonenzymatically (perhaps even extracellularly) with microbially synthesized S-adenosylmethionine as methyl donor.

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