Abstract
BackgroundMetal tolerance in bacteria has been related to polyP in a model in which heavy metals stimulate the polymer hydrolysis, forming metal-phosphate complexes that are exported. As previously described in our laboratory, Escherichia coli cells grown in media containing a phosphate concentration >37 mM maintained an unusually high polyphosphate (polyP) level in stationary phase. The aim of the present work was to evaluate the influence of polyP levels as the involvement of low-affinity inorganic phosphate transport (Pit) system in E. coli copper tolerance.ResultsPolyP levels were modulated by the media phosphate concentration and/or using mutants in polyP metabolism. Stationary phase wild-type cells grown in high phosphate medium were significantly more tolerant to copper than those grown in sufficient phosphate medium. Copper addition to tolerant cells induced polyP degradation by PPX (an exopolyphosphatase), phosphate efflux and membrane polarization. ppk−ppx− (unable to synthesize/degrade polyP), ppx− (unable to degrade polyP) and Pit system mutants were highly sensitive to metal even in high phosphate media. In exponential phase, CopA and polyP-Pit system would act simultaneously to detoxify the metal or one could be sufficient to safeguard the absence of the other.ConclusionsOur results support a mechanism for copper detoxification in exponential and stationary phases of E. coli, involving Pit system and degradation of polyP. Data reflect the importance of the environmental phosphate concentration in the regulation of the microbial physiological state.
Highlights
Metal tolerance in bacteria has been related to polyP in a model in which heavy metals stimulate the polymer hydrolysis, forming metal-phosphate complexes that are exported
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)
As a first step to elucidate the differential copper tolerance in cells grown in MT or MT + P for 48 h, assays using ppk−ppx− and ppx− cells were performed in these conditions
Summary
Metal tolerance in bacteria has been related to polyP in a model in which heavy metals stimulate the polymer hydrolysis, forming metal-phosphate complexes that are exported. The aim of the present work was to evaluate the influence of polyP levels as the involvement of low-affinity inorganic phosphate transport (Pit) system in E. coli copper tolerance. Resistance to cadmium can be chelated by polyP and/or regulate the activity of PPX, which would in turn degrade polyP, allowing the removal of metal-phosphate complexes possibly via lowaffinity inorganic phosphate transport (Pit) system [7]. This model was supported in acidophilic bacteria [8] and archaea [9], where Cu2+ increases PPX activity and phosphate (Pi) efflux.
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