Resistance to Metals and Metalloids

There is concern over the toxicity of a nnmber of metals and metalloids, and their oxyanions. As for antibiotics, the genes for resistance are often plasmid-bome. There are several mechanisms that may operate—redaction, methylation, efflnx, and the synthesis of metal-binding metallothioneins. The following text illustrates aspects of these mechanisms. 

Redaction to less toxic forms, for example, Hg + to Hg°, Cr04 

The degradation of phenylmercuric acetate to benzene, methylmercuric chloride to methane, and ethylmercuric chloride to ethane and Hg + is apparently carried out by different enzymes from the plasmid-carrying Escherichia coli strain K12 (R831) (Schottel 1978) and Pseudomonas sp. Resistance to organic mercury compounds has also been found in clinical isolates of nontuber-culous, rapidly growing mycobacteria (Steingrube et al. 1991) and can present a challenge in the clinical environment. 

Although reduction of chromate Cr to Cr has been observed in a number of bacteria, these are not necessarily associated with chromate resistance. For example, reduction of chromate has been observed with cytochrome Cj in Desulfovibrio vulgaris (Lovley and Phillips 1994), soluble chromate reductase has been purified from Pseudomonas putida (Park et al. 2000), and a membrane-bound reductase has been purified from Enterobacter cloacae (Wang et al. 1990). The flavoprotein reductases from Pseudomonas putida (ChrR) and Escherichia coli (YieF) have been purified and can reduce Cr(VI) to Cr(III) (Ackerley et al. 2004). Whereas ChrR generated a semi-quinone and reactive oxygen species, YieR yielded no semiquinone, and is apparently an obligate four-electron reductant. It could therefore present a suitable enzyme for bioremediation. 

Arsenite is also an intermediate in the fungal biomethylation of arsenic (Bentley and Chasteen 2002) and oxidation to the less toxic arsenate can be accomplished by heterotrophic bacteria including Alcaligenes faecalis. Exceptionally, arsenite can serve as electron donor for chemolithotrophic growth of an organism designated NT-26 (Santini et al. 2000), and both selenate and arsenate can be involved in dissimilation reactions as alternative electron acceptors. 

---

Although knowledge on the biodegradation of these compounds is sparse, a number of them are important in industrial processes. Formation of methylated derivatives may take place in metals and metalloids belonging to groups 15 and 16 of the periodic table, and a few of group 14. These have been discussed in a critical review (Thayer 2002) and in Chapter 3, Part 4, and they have been noted in the context of the bacterial resistance to metals and metalloids. Since carbon monoxide has been considered as an organic compound (Chapter 7, Part 1), it is consistent to make brief comments on metal carbonyls. 

---