A significant 19th century public health problem was that the inhabitants of many houses containing wallpaper decorated with green arsenical pigments experienced illness and death. The problem was caused by certain fungi that grew in the presence of inorganic arsenic to form a toxic, garlic-odored gas. The garlic odor was actually put to use in a very delicate microbiological test for arsenic. In 1933, the gas was shown to be trimethylarsine. It was not until 1971 that arsenic methylation by bacteria was demonstrated. Further research in biomethylation has been facilitated by the development of delicate techniques for the determination of arsenic species. As described in this review, many microorganisms (bacteria, fungi, and yeasts) and animals are now known to biomethylate arsenic, forming both volatile (e.g., methylarsines) and nonvolatile (e.g., methylarsonic acid and dimethylarsinic acid) compounds. The enzymatic mechanisms for this biomethylation are discussed. The microbial conversion of sodium arsenate to trimethylarsine proceeds by alternate reduction and methylation steps, with S-adenosylmethionine as the usual methyl donor. Thiols have important roles in the reductions. In anaerobic bacteria, methylcobalamin may be the donor. The other metalloid elements of the periodic table group 15, antimony and bismuth, also undergo biomethylation to some extent. Trimethylstibine formation by microorganisms is now well established, but this process apparently does not occur in animals. Formation of trimethylbismuth by microorganisms has been reported in a few cases. Microbial methylation plays important roles in the biogeochemical cycling of these metalloid elements and possibly in their detoxification. The wheel has come full circle, and public health considerations are again important

The perceived importance of tellurium (Te) in biological systems has lagged behind selenium (Se), its lighter sister in the Group 16 chalcogens, because of tellurium's lower crustal abundance, lower oxyanion solubility and biospheric mobility and the fact that, unlike Se, Te has yet to be found to be an essential trace element. Te applications in electronics, optics, batteries and mining industries have expanded during the last few years, leading to an increase in environmental Te contamination, thus renewing biological interest in Te toxicity. This chalcogen is rarely found in the nontoxic, elemental state (Te(0)), but its soluble oxyanions, tellurite (TeO(3)(2-)) and tellurate (TeO(4)(2-)), are toxic for most forms of life even at very low concentrations. Although a number of Te resistance determinants (Tel) have been identified in plasmids or in the bacterial chromosome of different species of bacteria, the genetic and/or biochemical basis underlying bacterial TeO(3)(2-) toxicity is still poorly understood. This review traces the history of Te in its biological interactions, its enigmatic toxicity, importance in cellular oxidative stress, and interaction in cysteine metabolism.

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