Investigating the role of the cryptic AdpS protein in Staphylococcus epidermidis physiology

Staphylococcus epidermidis is a ubiquitous coloniser of human skin, and a frequent cause of opportunistic infections in vulnerable individuals. This thesis investigates the physiological role of the cryptic AdpS protein, the genetic basis of high-level methicillin resistance, and metabolic adaptations to phosphate limitation in S. epidermidis. Infections caused by methicillin resistant S. epidermidis (MRSE) complicates management of affected patients. However research into methicillin resistance in S. epidermidis is relatively neglected compared to S. aureus. The genetic basis of homogeneous oxacillin resistance (HoR) was explored using whole-genome sequencing of resistant mutants derived from strain RP62A. Mutations in the phosphate transporter gene pitA and the RNA polymerase beta subunit gene rpoB were identified as likely contributors to high-level resistance. Initial work on the cryptic AdpS protein focused on biofilm, a key virulence factor in chronic infections. Insertion/excision events involving the mobile genetic element IS256 can inactivate genes involved in biofilm. Previous work implicated an IS256 insertion in a small antibiotic biosynthesis monooxygenase gene – found only in coagulase negative staphylococci – with decreased biofilm production in strain RP62A. During this project and contrary to initial hypotheses, deletion and overexpression studies revealed no impact of this gene, referred to as adpS, on biofilm formation in strains 1457 or CSF41498. This misidentification was shown to stem from co-incidental sequence homology with IS256. However, further bacterial two-hybrid assays identified potential AdpS interaction partners, suggesting alternative physiological roles. During experiments to analyse the impact of the adpS mutation on growth under different culture conditions, we discovered that the ability of S. epidermidis strains to grow in low phosphate chemically defined medium (LP-CDMG) was variable. Strain CSF41498 adapted by acquiring mutations in pyrP, encoding a uracil permease, which enhanced growth and altered resistance to the toxic uracil analogue, 5-fluorouracil (5-FU). These data suggest that PyrP mutations may reduce the uptake of uracil and 5-FU, potentially enhancing the transporter’s specificity for an alternative substrate that supports growth under phosphate limited growth conditions. This work highlights the genetic and physiological diversity of S. epidermidis, challenging assumptions about conserved virulence pathways and resistance mechanisms. Future studies should explore the interaction networks of AdpS and the clinical implications of strain-specific metabolic adaptations, advancing strategies for infection control and treatment.

Funder

Irish Research Council

Publisher

University of Galway

Rights

Attribution-NonCommercial-NoDerivatives 4.0 International

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Collections

University of Galway Theses (PhD Theses)