Diverse physiological roles of the MqsR/MqsA toxin/antitoxin system

There is tremendous interest in the use of bacteriophages (phages) to combat multi-drug resistant bacteria. However, to implement successfully phage therapy, host defense systems must be understood. Toxin/antitoxins (TAs) are the most prevalent phage defense system, and the MqsR/MqsA TA system is one of the best-studied systems. This phage-defense system was discovered in a whole-cell, population-averaged, transcriptome study designed to elucidate the biofilm-related genes of Escherichia coli in 2004. Biofilms are cells cemented to themselves or to surfaces. Since its characterization (as of April 2024), MqsR/MqsA has been utilized in over 1200 manuscripts, although its role in cell physiology has been contested. Here, we summarize the important physiological roles of this TA system, including its role in (i) the general stress response via repression of rpoS, (ii) biofilm formation via repression of csgA, (iii) combating bile acid stress in the gastrointestinal tract by inhibiting uptake of the bile salt deoxycholate, (iv) oxidative stress based on single-cell transcriptome studies, and (v) phage defense leading to the persister state.