Specialised functions of two common plasmid mediated toxin-antitoxin systems,ccdABandpemIK, inEnterobacteriaceae

Toxin-antitoxin systems (TAS) are commonly found on bacterial plasmids and are generally involved in plasmid maintenance. In addition to plasmid maintenance, several plasmid-mediated TAS are also involved in bacterial stress response and virulence. Even though the same TAS are present in a variety of plasmid types and bacterial species, differences in their sequences, expression and functions are not well defined. Here, we aimed to identify commonly occurring plasmid TAS inEscherichia coliandKlebsiella pneumoniaeand compare the sequence, expression and plasmid stability function of their variants. 27 putative type II TAS were identified from 1063 plasmids ofKlebsiella pneumoniaein GenBank. Among these,ccdABandpemIKwere found to be most common, also occurring in plasmids ofE.coli. Comparisons ofccdABvariants, taken fromE.coliandK.pneumoniae, revealed sequence differences, whilepemIKvariants from IncF and IncL/M plasmids were almost identical. Similarly, the expression and plasmid stability functions ofccdABvariants varied according to the host strain and species, whereas the expression and functions ofpemIKvariants were consistent among host strains. The specialised functions of some TAS may determine the host specificity and epidemiology of major antibiotic resistance plasmids.