Free-Living Bacteria May Utilize Chromosomal Toxin–Antitoxin Systems to Mediate K Sensing and Control by Continuously Modulating the Ratio of Injury: Repair Throughout the Life Cycle

A recent publication proposed that the main biological function of chromosomal toxin–antitoxin systems (TASs) in free-living bacteria is to optimize fitness by mediating K Sensing and Control via a Nutrient-Responsive Cybernetic System. Viable cell density data were consistent with analog (continuous) regulation of population dynamics and cellular physiology throughout the life cycle; however, exactly how bacteria utilize TASs to regulate this was not explained in that publication. Two different concepts of injury have been proposed in the field of microbiology: (1) injury due to external physical and chemical stresses, which lead to sublethal (reversible) or lethal (irreversible) injury depending on the degree of injury, and (2) injury due to internal, self-inflicted stresses mediated by TA toxins. While self-inflicted injury due to TA toxins has been recognized as playing a role in growth arrest and dormancy, which can be reversed by repair, there is little support for TA toxins causing irreversible programmed cell death under normal physiological conditions. The purpose of the present paper was to explain how merging the above two concepts of injury might reveal how TASs optimize the fitness of free-living bacteria under normal physiological conditions by continuously regulating the ratio of injury: repair throughout the life cycle.