The complete absence of ROS, in anaerobiosis, is a condition where oxygen-sensitive cellular molecules are produced by the cell. Some bacteria able to reduce tellurite include phototrophs to heterotrophs, under both aerobic and anaerobic conditions, such as Shewanella, Staphylococcus epidermidis, Rhodobacter sphaeroides, Rhodobacter capsulatus, and Halomonas. In addition, it has been observed that both highly tellurite-resistant and tellurite-susceptible bacteria, can reduce it. At least in aerobic conditions, this tellurite reduction process produces ROS, so it would not constitute an entirely favorable mechanism for the cell. However, some doubts regarding tellurite reduction contribution to toxic resistance have emerged. BNF22 glutathione reductase are only a few examples of proteins that contribute to tellurite resistance. coli, Zymomonas mobilis, Streptococcus pneumoniae, Geobacillus stearothermophilus V and Aeromonas caviae, NDH-II from E. sphaeroides, the E3 component of pyruvate dehydrogenase complex from E. Nitrate reductases (NarG and NarZ) from E. To date, numerous NAD(P)H-dependent proteins that reduce tellurite have been identified. In vitro tellurite reduction has been determined by using crude extracts of bacteria such as Thermus thermophilus HB8, Shewanella oneidensis, and Aeromonas caviae. Such is the case of glutathione reductase and thioredoxin reductase and their reduced products glutaredoxins (A, B, and C, and thioredoxins (A and C, which are directly oxidized by the toxic oxyanion.
In addition, cellular systems related to the metabolism of RSH molecules can also be targeted by tellurite.
However, the intracellular concentration of free thiols molecules is very low, this reaction's progress must occur catalytically, possibly in collaboration with some of the tellurite reducing enzymes. A possible explanation for this phenomenon is that tellurite and the thiol groups experience a Painter-like reaction (RSH + TeO 3 2− → RS-Te-SR → RSSR + Te 0). Exposure to high concentrations of cysteine or reduced glutathione (GSH) triggers the reduction of tellurite in vitro, evidenced by the formation of a black precipitate of Te 0 in the assay. Several molecules with RSH groups are also targets of tellurite, which are likely to participate directly in the oxyanion reduction. Moreover, other important cellular processes affected by tellurite include inhibition of glycolysis, tricarboxylic acid cycle, respiratory chain, and glutathione metabolism, among others.
In addition, tellurite directly affects the synthesis of heme groups, producing the accumulation of the toxic intermediate protoporphyrin IX. Tellurite metabolization triggers protein oxidation (carbonylation), dismantling of centers of dehydratases, such as AcnB and FumA, and membrane lipoperoxidation. Tellurite, the most soluble and bioavailable tellurium oxyanion, exhibits high toxicity on Escherichia coli, exceeding the effect found on the minimum inhibitory concentration of other ions and toxic compounds such as AsO 2 −, AsO 4 3−, Hg 2+, SiO 3 2−, Pb 2+, SeO 3 2−, Cd 2+, Cu 2+, Ag +, and SeO 4 2−. Together, these results let us propose that tellurite reduction and the intracellular RSH content are related to the oxyanion bacterial resistance, this tripartite mechanism in an oxygen-independent anaerobic process. Furthermore, we observed that, when the bacterium exhibits less resistance to the oxyanion, a decreased tellurite reduction was seen, affecting the growth fitness. large availability of cellular RSH groups, results in a more significant reduction of tellurite. We demonstrated that in vivo tellurite reduction is related to the intracellular thiol concentration, i.e. This metabolization of tellurite directly contributes to the resistance of the bacteria to the oxyanion. The in vivo tellurite reduction is related to the intracellular concentration of total RSH, in the presence and absence of oxygen. This work shows that tellurite reduction to elemental tellurium is increased under anaerobic conditions in E. Nevertheless, the mechanisms implemented by bacteria for tellurite reduction and its role in resistance have not been evaluated in vivo. Additionally, in vitro experiments have suggested that several enzymes can reduce tellurite (IV) to its elemental form (0) where RSH present on their active sites may be responsible for the process. reduced glutathione (GSH)), enhancing the cellular redox imbalance. Moreover, it has also been observed that tellurite can target free cell thiols groups (RSH) (i.e.
Tellurium is a rare metalloid that exerts high toxicity on cells, especially on bacteria, partly due to reactive oxygen species (ROS) generation.