Escherichia coli mazEF-mediated cell death is triggered by various stressful conditions

Abstract

mazEF is an Escherichia coli suicide module specific for a stable toxin and a labile antitoxin. Inhibiting mazEF expression appeared to activate the module to cause cell death. Here we show that several stressful conditions, including high temperatures, DNA damage, and oxidative stress, also induce mazEF-mediated cell death. We also show that this process takes place only during logarithmic growth and requires an intact relA gene.

FIG. 1.

The effect of mazEF on bacterial viability after exposure to high temperatures. E. coli wild-type strains (▪) and their ΔmazEF derivatives (▴) were incubated for 10 min at various temperatures. Cell viability was determined by CFU counting on LB plates incubated at 37°C overnight. The survivor ratio was determined by comparing the number of CFU of treated cells to that of untreated cells. The results represent one experiment out of three independent similar experiments. (a) E. coli MC4100 relA⁺; (b) E. coli K38.

FIG. 2.

The effect of mazEF on bacterial viability after exposure to agents causing DNA damage. E. coli MC4100 relA⁺ (a) and K38 (b) and their ΔmazEF derivatives were treated for 10 min at 37°C under the following DNA-damaging conditions: nalidixic acid exposure (A), mitomycin C exposure (B), and UV irradiation (C). The cells were washed with saline and diluted in LB medium. For UV irradiation, a series of decimal dilutions were carried out and 10-μl drops were plated on LB plates, dried, and UV irradiated (25 J/m²). In all cases, the cells were plated on LB medium for CFU counting and the survivor ratio was determined by comparing the number of treated cells to the number of untreated cells as described in the legend to Fig. 1. The results represent one experiment out of three independent similar experiments. ▪, wild-type strains; ▴, ΔmazEF derivatives of the wild-type strains.

FIG. 3.

The effect of mazEF on bacterial viability after exposure to oxidative stress. E. coli strains MC4100relA⁺ (a) and K38 (b) and their ΔmazEF derivatives were incubated in the presence of various concentrations of H₂O₂ at 37°C for 10 min. Cell viability and survivor ratios were determined as described in the legend to Fig. 1. The results represent one experiment out of three independent similar experiments. ▪, wild-type strains; ▴, ΔmazEF derivatives of the wild-type strains.

FIG. 4.

The effects of the growth stage (A) and ppGpp (B) on the induction of mazEF-mediated cell death. (A) Logarithmic (Log) and stationary (Sta) E. coli MC4100 relA⁺ cells were either left untreated (a) or submitted for 10 min to high temperature (50°C) (b), UV irradiation (c), rifampin exposure (20 μg/ml) (d), or the overproduction of MazF (e). (B) Logarithmic E. coli MC4100relA⁺ and MC4100relA1 cells were submitted for 10 min to high temperature (50°C) (a), UV irradiation (b), nalidixic acid (Nal) exposure (2 mg/ml) (c), H₂O₂ exposure (15 mM) (d), rifampin (Rif) exposure (20 μm/ml) (e), chloramphenicol (Cam) exposure (50 μg/ml) (f), or the overproduction of MazF (g). Cell viability and survivor ratios were determined as described in the legend to Fig. 1. The results represent one experiment out of three independent similar experiments.