[Regulatory role of SoxR in <i>Citrobacter braakii</i> JPG1 in physiological response to aerobic/anaerobic-menadione stress].

SoxR, one of bacterial transcriptional regulators, plays a crucial role in bacterial responses to oxidative stress induced by unfavorable environmental conditions. So far, the understanding of bacterial responses to oxidative stress mainly stems from a handful model bacteria such as <i>Escherichia coli</i> and the studies on non-model bacterial responses to oxidative stress are limited. In this study, <i>Citrobacter braakii</i> JPG1, a commonly occurring strain of enterobacteria, was used as a model for the first time to explore the role of SoxR in the responses to aerobic/anaerobic-menadione stress. First, we analyzed the phylogenetic relationship of SoxR based on the whole genome and constructed the <i>soxR</i>-deleted strain (Δ<i>soxR</i>). Then, the cell counts of the wild type (WT) and Δ<i>soxR</i> were compared under aerobic/anaerobic-menadione stress. The results showed that the cell count of WT exposed to the aerobic-low concentration menadione (0.1 mmol/L) stress for 24 h increased by 4.2 times compared with that at the time point of 0 h, while that of Δ<i>soxR</i> only increased by 1.3 times. The vast majority of WT and Δ<i>soxR</i> cells died after exposure to the aerobic-high concentration menadione (0.3 mmol/L) stress for 24 h, with the cell counts only 29% and 0.2% of those at the time point of 0 h, respectively. Interestingly, the cell counts of WT showed no significant difference between the anaerobic-menadione stress and the control (<i>P</i> > 0.05), and the same was true for Δ<i>soxR</i>. All these results indicated that SoxR of <i>C. braakii</i> JPG1 only has a regulatory effect on the redox cycling compound menadione under aerobic conditions and enhance the antioxidant capacity. Under anaerobic conditions, menadione failed to activate SoxR. The findings from this study provide new insights into understanding both the physiological responses to menadione stress and the regulatory role of SoxR under different oxygen conditions.