Extracellular pH has been assumed to play little if any role in how bacteria respond to antibiotics and antibiotic resistance development. Here, we show that the intracellular pH of Escherichia coli equilibrates to the environmental pH following treatment with the DNA damaging antibiotic nalidixic acid. We demonstrate that this allows the environmental pH to influence the transcription of various DNA damage response genes and physiological processes such as filamentation. Using purified RecA and a known pH-sensitive mutant variant RecA K250R we show how pH can affect the biochemical activity of a protein central to control of the bacterial DNA damage response system. Finally, two different mutagenesis assays indicate that environmental pH affects antibiotic resistance development. Specifically, at environmental pH's greater than six we find that mutagenesis plays a significant role in producing antibiotic resistant mutants. At pH's less than or equal to 6 the genome appears more stable but extensive filamentation is observed, a phenomenon that has previously been linked to increased survival in the presence of macrophages.

Department of Biology Masaryk University Kamenice 5 A7 625 00 Brno Czech Republic

Department of Cancer Research and Molecular Medicine Norwegian University of Science and Technology Trondheim Norway

Department of Microbiology University of Oslo and Oslo University Hospital Rikshospitalet Oslo Norway

International Clinical Research Center Center for Biomolecular and Cellular Engineering St Anne's University Hospital Brno Pekarska 53 656 91 Brno Czech Republic

National Centre for Biomolecular Research Masaryk University Kamenice 5 A4 625 00 Brno Czech Republic

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