Influence of general stress-response alternative sigma factors σS (RpoS) and σB (SigB) on bacterial tolerance to the essential oils from Origanum vulgare L. and Rosmarinus officinalis L. and pulsed electric fields

•The influence of σS and σB on bacterial tolerance to essential oils and PEF was studied.•σS and σB influenced the bacterial tolerance to oregano and rosemary essential oils.•σS and σB did not influence the tolerance to PEF.•Parental and σS- and σB-mutant cells did not develop direct- or cross-tolerance.•Mutant cells pre-exposed to essential oils showed lower tolerance than parental cells.

This study assessed the influence of general stress-response alternative sigma factors RpoS (σS) and SigB (σB) on tolerance of Escherichia coli (E. coli MG1655 and its isogenic mutant E. coli MG1655 ΔrpoS) and Listeria monocytogenes (L. monocytogenes EGD-e and its isogenic mutant L. monocytogenes EGD-e ΔsigB) to the essential oils (EOs) from Origanum vulgare L.—oregano (OVEO) and Rosmarinus officinalis L.—rosemary (ROEO), as well as the changes in tolerance of parental and ΔrpoS and ΔsigB mutant strains to OVEO, ROEO and pulsed electric fields (PEF) following overnight exposure to subinhibitory concentrations (1/2 × minimum inhibitory concentration—MIC) of each tested EO. MIC values of OVEO and ROEO against the mutant cells were usually lower than those found against the parental cells. Survivor curves showed that mutant cells were more sensitive to these EOs than parental cells. The recovery of survivors in selective media showed a greater proportion of cells sublethally injured at their cell envelopes in the mutant strains compared with the parental strains. Induction of increased direct-tolerance to OVEO and ROEO or cross-tolerance to PEF was not observed after pre-exposure of parental and mutant cells to EOs. Otherwise, parental and mutant cells of E. coli and L. monocytogenes pre-exposed to OVEO or ROEO showed decreased tolerance when further treated with the homologous stressing agent at 2 × MIC. Still, mutant cells pre-exposed to OVEO or ROEO showed lower tolerance to PEF than parental strains. These results showed the influence of σS and σB in tolerance of single strains of E. coli and L. monocytogenes, respectively, to OVEO and ROEO. Moreover, the deletion of σS and σB resulted in decreased tolerance to OVEO, ROEO or PEF in tested strains following exposure to OVEO or ROEO at a subinhibitory concentration.