Bacterial resistance to toxic metals determined by extrachromosomal R factors

Bacterial resistance to toxic heavy metal ions is generally controlled by genes on extrachromosomal resistance (R) factors that also house the genes conferring resistance to antibiotics. These R factors can be mobilized from cell to cell by direct conjugation and thus afford a rapid mechanism for changing a population of microorganisms that is predominantly sensitive to antibiotics and toxic ions to a population that is predominantly resistant. After a brief review of the structure, function, genetics, and transfer of R factors, we shall consider the mechanisms of resistance to mercury and cadmium cations - determined by separate R factor genes. Resistance to both inorganic mercury and organomercurials results from the synthesis of intracellular enzymes that reduce Hg(II) or mercury in organomercurials to metallic Hg(O), which is volatile and lost from the system, allowing growth of the resistant bacteria. The genetic control and enzymatic basis of mercury(ial) resistance has been studied in some detail in Enterobacteriaceae such as Escherichia coli and in pseudomonads from soil, water, and clinical sources. The mechanism of cadmium resistance involves a decrease in cellular uptake by the resistant bacterial strains. No chemical transformation of cadmium by resistant bacteria has been found. Other toxic heavy metal resistances [AsO₄^3-, AsO₂^3-, Bi³⁺, Co²⁺, Pb²⁺, Sb³⁺, Ni²⁺, and Ag⁺] are also determined by R factor genes or sometimes by chromosomal genes.