The rise of transmissable antibiotic resistance (AbR) in the bacterial family Enterobacteriaceae, particularly Escherichia coli and Klebsiella pneumoniae, is of major concern. In these bacteria, resistance is mainly spread by self-transmissible plasmids, which commonly contain toxin-antitoxin systems (TAS). TAS ensure the stable maintenance of plasmids in bacteria by killing plasmid free cells, and contribute to the spread of AbR in and within species. This study aimed to define the TAS present in plasmids found in K. pneumoniae, and to determine the functions of several common TAS in various Enterobacteriaceae species.
TAS in plasmids found in K. pneumoniae were determined bioinformatically using TA Finder, and the promoter regions of TAS from different species compared. Promoter-GFP reporter systems were used to determine promoter strengths in normal growth conditions, as well as expression changes during environmental stress. Cell survival assays were also used to confirm TAS roles in stress response.
A total of fifteen different TAS were identified among 306 K. pneumoniae plasmids, with two (ccdAB and pemIK) also being common in other Enterobacteriaceae plasmids. The promoters of these two TAS taken from various species and plasmids types showed variation in strength depending on the strain it was present in rather than the species. Neither plasmid borne ccdAB nor pemIK responded to nutrient limitation or antibiotic stress, nor did they confer any survival advantage under antibiotic stress.
The prevalence of common TAS varies between species and/or plasmid types, with some being relatively specific. TAS common to multiple plasmids vary in promoter sequence, and their expression varies between host strains, not species. Plasmid borne ccdAB and pemIK are likely to be specialised plasmid maintenance systems, and do not play a role in bacterial stress response.