Contribution of the Chromosomal Operon to Bacterial Drug Tolerance.

One of the first identified and best-studied toxin-antitoxin (TA) systems in is the F-plasmid-based CcdAB system. This system is involved in plasmid maintenance through postsegregational killing. More recently, homologs have been found on the chromosome, including in pathogenic strains of and other bacteria. However, the functional role of chromosomal genes, if any, has remained unclear. We show that both the native operon of the O157 strain () and the operon from the F plasmid (), when inserted on the chromosome, lead to protection from cell death under multiple antibiotic stress conditions through formation of persisters, with the O157 operon showing higher protection. While the plasmid-encoded CcdB toxin is a potent gyrase inhibitor and leads to bacterial cell death even under fully repressed conditions, the chromosomally encoded toxin leads to growth inhibition, except at high expression levels, where some cell death is seen. This was further confirmed by transiently activating the chromosomal operon through overexpression of an active-site inactive mutant of F-plasmid-encoded CcdB. Both the and operons may share common mechanisms for activation under stress conditions, eventually leading to multidrug-tolerant persister cells. This study clearly demonstrates an important role for chromosomal systems in bacterial persistence. A large number of free-living and pathogenic bacteria are known to harbor multiple toxin-antitoxin systems, on plasmids as well as on chromosomes. The F-plasmid CcdAB system has been extensively studied and is known to be involved in plasmid maintenance. However, little is known about the function of its chromosomal counterpart, found in several pathogenic strains. We show that the native chromosomal operon of the O157 strain is involved in drug tolerance and confers protection from cell death under multiple antibiotic stress conditions. This has implications for generation of potential therapeutics that target these TA systems and has clinical significance because the presence of persisters in an antibiotic-treated population can lead to resuscitation of chronic infection and may contribute to failure of antibiotic treatment.