Modulation of the cytoplasmic pH as an alternative strategy to tackle antibiotic resistance in Escherichia coli

Awareness of the problem of antimicrobial resistance (AMR) has escalated and drug-resistant infections are named among the most urgent problems facing clinicians today. Bacteria can acquire resistance to antibiotics by a variety of mechanisms that, at times, involve changes in their metabolic status, thus altering diverse biochemical reactions, many of them pH-dependent. In this work, we found that modulation of the cytoplasmic pH (pHi) of Escherichia coli strains highly-resistant to norfloxacin (NF) provides a thus far unexplored strategy to support resistance. Thus, the pHi, of the above strain, (EV18), developed in our laboratory, was measured with the ratiometric pHluorin, and found to be lower than that of the wild type strain. Moreover, treatment with sodium bicarbonate that increases the cytoplasmic pH decreases its resistance to NF. Strikingly, we could partially generate a resistant phenotype in the wild type strain by incubation with salicylic (SA) or sorbic acid (SoA). Those acids caused a depression of pHi that was accompanied by an increased resistance to NF. The use of marR mutants and pHi modulators other than salicylic acid provides evidence that supports the notion that the effect on resistance described here is due to changes of the pHi. Such effect was also documented for other antibiotics as nalidixic acid (NA), ampicillin (Amp) and chloramphenicol (Cam). We speculate that the decrease of the cytoplasmic pH provides a signaling mechanism that modifies metabolic pathways and serves to cope with stress and to lower metabolic costs.