This model was supported in acidophilic bacteria  and archaea , where Cu2+ increases PPX activity and phosphate (Pi) efflux. Pit system in Escherichia coli includes PitA (encoded by pitA) and PitB (encoded by pitB) . van Veen et al.  have shown that Pit can reversibly transport Ca2+, Co2+ or Mg2+
phosphates in E. coli and Acinetobacter johnsonii. The uptake of a neutral metal-phosphate (MeHPO) complex is mediated by an electrogenic proton symport mechanism. Conversely, the excretion of the metal-phosphate complex via Pit generates a proton motive force in A. johnsonii. Copper is an essential nutrient required for many biochemical functions, AZD1480 manufacturer acting as a cofactor for several enzymes . However, copper selleck chemicals is also a toxic element able to catalyze free radicals formation, producing alteration of nucleic acids, lipids and proteins [14, 15]. Thus, cells ensure their viability by a tight regulation of copper levels, involving several homeostatic mechanisms. E. coli is equipped with multiple systems to ensure PCI-32765 supplier copper handling under varying environmental conditions. For instance, the Cu+-translocating P-type ATPase CopA is responsible for removing excess Cu+ from the cytoplasm. Multi-copper oxidase CueO and the
multi-component copper transport system CusCFBA appears to safeguard the periplasmic space from copper-induced toxicity [16–18]. In aerobic conditions, AMP deaminase E. coli usually tolerate copper concentrations in the μM range, although minimal inhibitory concentrations for metals depend on the growth media and the methodology used [17–20]. Stationary phase cells are particularly vulnerable to oxidative damage since they lack the energy and materials needed to repair or replace the damaged molecules. In our laboratory, it has been demonstrated that E. coli stationary cells presented high viability, low oxidative damage and elevated resistance to exogenous H2O2 when Pi concentration in the medium was above 37 mM . These events were related to the maintenance of high polyP level in late stationary phase . According
to the model proposed previously by Keasling , we examined here the involvement of polyP metabolism and Pit system components in E. coli copper tolerance in stationary or exponential phase cells. Our approach included the use of mutants in PPK, PPX, PitA and PitB encoding genes and the modulation of polyP levels by varying media phosphate concentration. Results Cu2+ tolerance of stationary phase cells grown in different phosphate concentration media The ability to tolerate Cu2+ of MC4100 wild-type (WT) cells, grown to stationary phase in media with different phosphate concentration, was evaluated by semiquantitative resistance assay (Figure 1A). Cells grown for 48 h in MT medium (sufficient Pi concentration) were sensitive to 0.25 mM Cu2+.