Toxin-antitoxin (TA) systems were initially discovered as plasmid addiction systems on low-copy-number plasmids. Thousands of TA loci have since been identified on chromosomes, plasmids and mobile elements in bacteria and archaea with diverse roles in bacterial physiology and in maintenance of genetic elements. Here, we identified and characterised a plasmid mediated type II TA system in Enterobacteriaceae as a member of the ParDE super family. TA-finder was used to identify TA systems in a conjugative IncI antibiotic resistance plasmid. Blast, MEGA, and PSIPRED software were used to elucidate distribution, identity and secondary structures. Plasmid maintenance, stress tolerance and biofilm production were assessed. This TA system (hereafter, ParDEI) is distributed among IncI and IncF-type antibiotic resistance and virulence plasmids found in avian and human-source Escherichia coli and Salmonella. ParDEI is a plasmid stability and stress response module that increases tolerance of aminoglycoside, quinolone and β-lactam antibiotics in E. coli by ~100-1000-fold, and thus to levels beyond those achievable in the course of antibiotic therapy for human infections. ParDEI also confers a clear survival advantage at 42 °C and expression of the ParEI toxin in trans induces the SOS response, inhibits cell division and promotes biofilm formation. The spread of plasmid-borne high-level antibiotic tolerance is clearly a major threat and likely to be an important factor in the success of the IncI and IncF plasmids which carry it and the important pathogens in which these are resident.