In bacterial populations, Type-II toxin-antitoxin (TA) systems, where toxins are proteins, play a crucial role in responding to environmental stressors such as antibiotic treatment. This study employs a synergistic experimental and theoretical approach to investigate the kacAT Type-II TA system, which might play an essential role in the antibiotic persistence of Klebsiella pneumoniae. Antibiotic persistence describes the survival of a small subpopulation of bacterial cells that resist antibiotic treatment despite being genetically identical to the larger susceptible population. We observe a pronounced upregulation of the antitoxin (kacA) and toxin (kacT) genes following antibiotic exposure, attributed to the degradation of KacA and a subsequent shift in the KacA:KacT ratio that alleviates repression on its own promoter region. To capture these nonlinear phenomena, we have developed a quantitative model that successfully replicates the experimentally observed changes in the KacA:KacT ratio and kacAT transcript levels. The model also elucidates the puzzling observation that KacAT overexpression enhances antibiotic stress tolerance, whereas its deletion remains inconsequential. This aligns with recent evidence that undermines the long-standing hypothesis that Type-II TA systems are involved in spontaneous persister cell formation. Additionally, a bioinformatics analysis of kacAT-like systems in sequenced K. pneumoniae genomes reveals a non-random distribution of these loci, casting further doubt on the widely assumed cooperative action of TA systems. Our findings thus challenge prevalent theoretical assumptions, offering new insights into the mechanics of antibiotic persistence and TA systems.
This work is supported by the Science Fund of the Republic of Serbia (grant no. 7750294, q-bioBDS)