Contribution of the Bacillus subtilis ytrGABCDEF operon to antibiotic stress adaptation

The Bacillus subtilis ytrGABCDEF operon encodes a putative ABC transporter, its transcriptional repressor YtrA, and YtrG, a small transmembrane protein of unknown function. The operon is induced by inhibitors of the membrane-bound lipid II cycle, cold shock, and during the exponential to stationary phase transition and has been implicated in antibiotic stress adaptation, cell wall synthesis, sporulation, biofilm formation, and competence. Here, we assessed its contribution to antibiotic stress adaptation using a broad range of ytr mutant strains and different susceptibility testing setups. We could not confirm a consistent role of the operon in antibiotic survival. However, we observed a stimulating effect of ABC transporter expression on cell wall synthesis and turnover and could pinpoint this to the membrane-bound lipid II cycle. We conclude that the ytrGABCDEF operon is induced by antibiotics that target lipid-linked cell wall precursors, not because it constitutes a dedicated antibiotic stress response but rather because these stress conditions mimic its natural trigger. We speculate that this could be related to the accumulation of intracellular cell wall precursor compounds, a signature effect of antibiotics that bind to lipid-linked cell wall precursors such as lipid II or bactoprenol phosphate.

The Bacillus subtilis ytrGABCDEF operon is a reliable and specific marker for the inhibition of cell wall biosynthesis by antibiotics. It has therefore advanced as a common reporter in transcriptomic, proteomic, and reporter gene studies aimed at elucidating antibiotic mechanisms of action. Despite this established role, its function is poorly understood, and its contribution to survival under antibiotic exposure is debated. Here, we provide evidence that the function of the operon is not related to antibiotic stress but rather that its induction is a by-product of antibiotic-induced changes in cell wall metabolism.