from an overnight LB culture were subcultured into fresh LB to an initial OD600 ~ 0.025 and grown to an OD600 of 0.8. Bacterial culture (1 ml) was centrifuged and then re-suspended in 1 ml of permeabilization buffer (10 mM Tris pH 7.5, 1 mM EDTA, 10 mM glucose). Then 2 ml of 5 mg ml-1 1,1′,3,3′-tetraethylbenzimidazolyl-carbocyanine iodine (JC-1) (Molecular Probes) was added and the preparation incubated for 30 min at room temperature. Cells were then centrifuged and re-suspended in 500 ml of permeabilization buffer. Micro- scope slides were prepared as follows: 80 ml of molten 0.8% agarose was pipetted onto a clean microscope slide, covered with a coverglass pre-treated with sigmacote (Sigma) and left to dry at room temperature for 30 min, prior to removing the coverglass. To obtain a thin layer of agarose, the microscope slide was left to dry overnight at 37°C. Fluorescent bacteria were examined using a Leica TCS-NT confocal microscope (Leica Microsystems, Germany) equipped with a krypton/ argon laser with an excitation wavelength of 485 nm. Leica confocal software (Leica Microsystems, Germany) was used to calculate the green/red ﬂuorescence emission ratio from 100 individual cells taken from technical duplicates of three independently grown cultures of each strain. The threshold to distinguish low pmf from high pmf was chosen based on the Dy (pmf under given experimental conditions) of the WT strain under normal (non-stress) growth conditions. Acknowledgements This work was supported by funding from the Wellcome Trust. We acknowledge P. Model, M. Russel, D. Ladant, D. Georgel- lis, J. Beckwith, J. Brown, H. Mori and A. Mayhew for provid- ing strains, plasmids and antibodies, and S. Amartey for help with anaerobic work. We especially would like to thank Dr P. Burrows, Dr N. Joly and Dr J. 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