Metal resistance mechanism

In microbes without a permeability barrier, or when the barrier fails, a mechanism must be in place to export metals from the cytoplasm. These active transport systems involve energy-dependent, membrane-bound efflux pumps that can be encoded by either chromosomal- or plasmid-borne genes. Active transport is the most well-studied metal resistance mechanism. Some of these include the ars operon for exporting arsenic from E. coli, the cad system for exporting cadmium from Staphylococcus aureus, and the cop operon for removing excess copper from Enterococcus hiraeP i9A0... 

Some metals can be converted to a less toxic form through enzyme detoxification. The most well-described example of this mechanism is the mercury resistance system, which occurs in S. aureus,43 Bacillus sp.,44 E. coli,45 Streptomyces lividans,46 and Thiobacillus ferrooxidans 47 The mer operon in these bacteria includes two different metal resistance mechanisms.48 MerA employs an enzyme detoxification approach as it encodes a mercury reductase, which converts the divalent mercury cation into elemental mercury 49 Elemental mercury is more stable and less toxic than the divalent cation. Other genes in the operon encode membrane proteins that are involved in the active transport of elemental mercury out of the cell.50 52... 

Genome-Based Approaches to Investigate Microbial Metal Resistance Mechanisms... 

Silver, S., Plasmid-determined metal resistance mechanisms Range and overview, Plasmid, 27 (1), 1-3, 1992. 

Errzymatic conversion of the metal to a form which is less toxic for the bacterium, e.g. CHsHgand Hg (Misra 1992). Many of these metal-resistance mechanisms are encoded by genetic systems which have been extensively studied and are well understood. Perhaps the best-studied metal-resistance system is encoded by genes of the mer, or mercury resistance, operon. In this system, Hg(II) is transported into the cell via the MerT transporter protein, and detoxified by reduction to less toxic volatile mercury by an intracellular mercury reductase, MerA (see Osborn et al. 1997, Hobman etal. 2000). 

Alloys are solid metallic mixtures designed to meet specific needs (see Section 5.15). For example, the frames of racing bicycles can be made of a steel that contains manganese, molybdenum, and carbon to give them the stiffness needed to resist mechanical shock. Titanium frames are used, but not the pure metal. Titanium metal stretches easily, so much so that it becomes deformed under stress. However, when alloyed with metals such as tin and aluminum, titanium maintains its flexibility but keeps its shape. 

Ji, G. and Silver, S., Bacterial resistance mechanisms for heavy metals of environmental concern, J Ind... 

Rouch, D.A., Lee, B.T.O., and Morby, A.P., Understanding cellular responses to toxic agents A model for mechanism-choice in bacterial metal resistance, J Ind Microbiol and Biotechnol, 14 (2), 132-141, 1995. 

Polyethers. Acetal Resins. These stabilized polyoxymethy-lenes were introduced dramatically by DuPont and Celanese as engineering plastics to replace non-ferrous metals. Good mechanical strength, resilience, fatigue-resistance, lubricity, abrasion-resistance, heat distortion temperature, water and solvent-resistance can approach the behavior of metals on a volume basis, while processability, color possibilities, and corrosion-resistance are superior. Major weakness is sensitivity to thermal, oxidative, and ionic degradation. 

Metal toxicity to microorganisms and microbial resistance mechanisms... 

As a consequence of, or perhaps in spite of, metal toxicity, some microorganisms have developed various resistance mechanisms to prevent metal toxicity. The strategies are either to prevent entry of the metal into the cell or actively to pump the metal out of the cell. This can be accomplished by either sequestration, active transport or chemical transformation through metal oxidation or reduction. 

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