D. H. Nies # Microbial heavy-metal resistance Received: 25 November 1998 / Received revision: 18 February 1999 / Accepted: 20 February 1999 ## Abstract We are just beginning to understand the metabolism of heavy metals and to use their metabolic functions in biotechnology, although heavy metals comprise the major part of the elements in the periodic table. Because they can form complex compounds, some heavy metal ions are essential trace elements, but, essential or not, most heavy metals are toxic at higher concentrations. This review describes the workings of known metal-resistance systems in microorganisms. After an account of the basic principles of homoeostasis for all heavy-metal ions, the transport of the 17 most important (heavy metal) elements is compared. ## Introduction: heavy-metal toxicity, tolerance and resistance Heavy metals are metals with a density above 5 g/cm³, thus the transition elements from V (but not Sc and Ti) to the half-metal As, from Zr (but not Y) to Sb, from La to Po, the lanthanides and the actinides can be referred to as heavy metals. Of the 90 naturally occurring elements, 21 are non-metals, 16 are light metals and the remaining 53 (with As included) are heavy metals (Weast 1984). Most heavy metals are transition elements with incompletely filled d orbitals. These d orbitals provide heavy-metal cations with the ability to form complex compounds which may or may not be redox-active. Thus, heavy-metal cations play an important role as "trace elements" in sophisticated biochemical reactions. At higher concentrations, however, heavy-metal ions form unspecific complex compounds in the cell, which leads to toxic effects. Some heavy-metal cations, e.g. Hg²⁺, Cd²⁺ and Ag⁺, form strong toxic complexes, which makes them too dangerous for any physiological function. Even highly reputable trace elements like Zn²⁺ or Ni²⁺ and especially Cu²⁺ are toxic at higher concentrations. Thus, the intracellular concentration of heavy-metal ions has to be tightly controlled, and heavy-metal resistance is just a specific case of the general demand of every living cell for some heavy-metal homoeostasis system.D. H. Nies # Microbial heavy-metal resistance Received: 25 November 1998 / Received revision: 18 February 1999 / Accepted: 20 February 1999 ## Abstract We are just beginning to understand the metabolism of heavy metals and to use their metabolic functions in biotechnology, although heavy metals comprise the major part of the elements in the periodic table. Because they can form complex compounds, some heavy metal ions are essential trace elements, but, essential or not, most heavy metals are toxic at higher concentrations. This review describes the workings of known metal-resistance systems in microorganisms. After an account of the basic principles of homoeostasis for all heavy-metal ions, the transport of the 17 most important (heavy metal) elements is compared. ## Introduction: heavy-metal toxicity, tolerance and resistance Heavy metals are metals with a density above 5 g/cm³, thus the transition elements from V (but not Sc and Ti) to the half-metal As, from Zr (but not Y) to Sb, from La to Po, the lanthanides and the actinides can be referred to as heavy metals. Of the 90 naturally occurring elements, 21 are non-metals, 16 are light metals and the remaining 53 (with As included) are heavy metals (Weast 1984). Most heavy metals are transition elements with incompletely filled d orbitals. These d orbitals provide heavy-metal cations with the ability to form complex compounds which may or may not be redox-active. Thus, heavy-metal cations play an important role as "trace elements" in sophisticated biochemical reactions. At higher concentrations, however, heavy-metal ions form unspecific complex compounds in the cell, which leads to toxic effects. Some heavy-metal cations, e.g. Hg²⁺, Cd²⁺ and Ag⁺, form strong toxic complexes, which makes them too dangerous for any physiological function. Even highly reputable trace elements like Zn²⁺ or Ni²⁺ and especially Cu²⁺ are toxic at higher concentrations. Thus, the intracellular concentration of heavy-metal ions has to be tightly controlled, and heavy-metal resistance is just a specific case of the general demand of every living cell for some heavy-metal homoeostasis system.