Association Between Toxin-antitoxin Systems on Plasmids and Persister Formation in CTX-15-producing Klebsiella pneumoniae ST11 Isolates

We investigated the effect of toxin-antitoxin (TA) systems in bla CTX-M-15 -bearing plasmids of Klebsiella pneumoniae on persister formation. The persister formation rate was notably high in transconjugants in plasmids bearing TA system than the transconjugants in plasmids bearing no TA systems. Activation of relA and spoT expression was higher in transconjugants with plasmids bearing TA systems. Thus, TA systems in plasmids may contribute to the maintenance of bla CTX-M-15 -bearing plasmids and host survival via persister formation. Plasmids were obtained from four CTX-M-15-producing K. pneumoniae ST11 clinical isolates that were collected from patients in Hong Kong, Malaysia, India, and Thailand (HK02-026, M16-13, IN03-01, and TH02-34, respectively). Their whole plasmid sequences were determined previously (GenBank accession numbers, KY751926, KY751925, KY499796, and KY499797) (14). While TA genes, such as pemIK, mok-hok, and vagCD, were identified on the bla CTX-M-15 -bearing plasmids of HK02-026 and M16-13 (TA+), the bla CTX-M-15 -bearing plasmids of IN03-01 and TH02-34 had no TA genes (TA – ). Then, recovered plasmids were transferred to E. coli J53 cells, as described previously (6). In vitro antimicrobial susceptibility testing was performed by the broth microdilution method according to Clinical and Laboratory Standards Institute (CLSI) guidelines (15), and susceptibility was defined according to CLSI breakpoints. E. coli ATCC 25922 and Pseudomonas aeruginosa ATCC 27853 were used as control strains.