Modular determinants of plasmid specialisation

Abstract

Antimicrobial resistance (AMR) is one of the leading public health threats globally today. The carriage of antibiotic resistance genes on mobile genetic elements such as plasmids plays a crucial role in the spread of AMR in Enterobacteriaceae. Once mobilised on a plasmid, resistance genes can be stably maintained in a population even in the absence of selection due to the presence of toxin-antitoxin (TA) systems on the plasmids. Furthermore, bacterial conjugation allows these resistance plasmids to be easily spread across species.

This thesis begins with a review into the diverse functions of type II TA systems (Chapter 2). The mechanisms of the plasmid maintenance functions of TA systems are explored, as well as the increasing number of other functions of this ever-growing group that have recently been discovered. An investigation into the common TA systems carried on plasmids found in Klebsiella pneumoniae, as well as their variation and functions, follows (Chapter 3). pemIK and ccdAB are shown to be the most common TA systems in both K. pneumoniae and Escherichia coli, however ccdAB displays a far greater degree of specialisation than pemIK.

In Chapter 4, the host strain-plasmid relationship is found to have a significant impact on conjugation frequencies across six clinically relevant plasmid incompatibility (Inc) types. This was both in vitro, in solid and liquid mating, as well as in vivo, in an infant mouse (mammalian) gut model. One potential conjugation regulator, Tir, is investigated in Chapter 5. Originally described as a transfer inhibitor in IncL plasmids, out results suggest that this is not the case, with the reported transfer inhibition caused by Tir instead being due to growth inhibition.