Dissecting an Antibiotic Resistance Network in Enterococcus faecalis

Abstract

The emergence of antibiotic resistance is one of the most serious global threats to public health faced in the 21st century. At the forefront of these infections lie multidrug-resistant bacteria which can render antimicrobial treatment ineffective. Enterococcus faecalis has become recognised as important nosocomial pathogen due to a high level of intrinsic resistance to antimicrobials - however, our knowledge of these resistance pathways remains incomplete. In accordance with this requirement, the aims of this thesis were to deepen our knowledge of the E. faecalis cell-envelope stress response, to gain an understanding of the unique and complex setup of this antibiotic resistance network. Our previous work has identified the existence of a bacitracin resistance module in E. faecalis, comprised of the ABC transporters, SapAB and RapAB, and the two-component system (TCS) SapRS. Recently, components of this network have been implicated in resistance to daptomycin, a role usually fulfilled by a second TCS LiaFSR. By unravelling the interplay between these two regulatory pathways, we demonstrate the existence of a logic ‘AND’ gate, whereby both antibiotic-induced cellular damage (LiaFSR) and the presence of a substrate drug for the network’s sensory transporters (SapAB) are required to trigger expression of the SapR regulon. To add to the complexity of this network, the Lia and Sap systems are just two of many TCS’s involved in monitoring cell envelope integrity. The CroRS system is unique to the enterococci and is the main determinant of intrinsic β-lactam resistance. However, the genes within the CroR regulon remain unknown. Through experimental evolution, we were able to identify potential processes influenced by CroRS regulation. Firstly, upon evolving the croRS deletion for improved growth rate, we identified a potential role for CroRS in cell-envelope biosynthesis. Secondly, upon adaptation of the croRS deletion to increasing ampicillin concentration, we identified a potential role for CroRS in c-di-AMP metabolism. As we gain a detailed understanding of the cell-envelope stress response within E. faecalis, this may ultimately lead to identifying an Achilles’ heel within this network, enabling the repurposing of existing antimicrobials and the discovery of new therapeutic targets.

Date of Award	22 Feb 2023
Original language	English
Awarding Institution	University of Bath
Sponsors	GW4
Supervisor	Susanne Gebhard (Supervisor), Tim Rogers (Supervisor) & Andrew Preston (Supervisor)