TisB protein is the single molecular determinant underlying multiple downstream effects of ofloxacin in Escherichia coli .

Bactericidal antibiotics can cause metabolic perturbations that contribute to antibiotic-induced lethality. The molecular mechanism underlying these downstream effects remains unknown. Here, we show that ofloxacin, a fluoroquinolone that poisons DNA gyrase, induces a cascade of metabolic changes that are dependent on an active SOS response. We identified the SOS-regulated TisB protein as the unique molecular determinant responsible for cytoplasmic condensation, proton motive force dissipation, loss of pH homeostasis, and H 2 O 2 accumulation in Escherichia coli cells treated with high doses of ofloxacin. However, TisB is not required for high doses of ofloxacin to interfere with the function of DNA gyrase or the resulting rapid inhibition of DNA replication and lethal DNA damage. Overall, the study sheds light on the molecular mechanisms by which ofloxacin affects bacterial cells and highlights the role of the TisB protein in mediating these effects.