Plasmid-mediated β-lactam resistance in Neisseria gonorrhoeae

Thesis

Plasmids are diverse extrachromosomal elements that play a pivotal role in the rapid dissemination of antimicrobial resistance (AMR). While broad host range plasmids have been extensively studied, host-restricted plasmids remain underexplored, despite their significant impact on treatment options of clinically relevant bacteria. This thesis investigates plasmid-mediated resistance in the sexually transmitted infection (STI) pathogen Neisseria gonorrhoeae (the gonococcus), a leading cause of pelvic inflammatory disease, infertility and ectopic pregnancy, and a WHO priority pathogen due to escalating resistance to all previously recommended antibiotics. Two narrow host range resistance plasmids are found in the gonococcus: the conjugative plasmid pConj and the β-lactamase plasmid pbla, conferring resistance to tetracycline and penicillin antibiotics, respectively. With the pbla-encoded β-lactamase only differing in a few amino acids from extended-spectrum β-lactamases and current treatment relying on third-generation cephalosporins, and the recent implementation of doxycycline (a tetracycline antibiotic) post-exposure prophylaxis (Doxy-PEP), understanding plasmid-mediated resistance in N. gonorrhoeae is of urgent public health importance.

This thesis focuses on three aspects of plasmid-mediated resistance in N. gonorrhoeae: i) the evolution and epidemiology of pbla in gonococci; ii) the molecular characteristics of pbla variants, including resistance, fitness costs and mobility; and iii) the influence of restriction-modification systems (RMSs) on plasmid transfer, using a combination of bioinformatic and molecular genetics approaches. Phylogenetic and comparative analyses suggest that pbla originated from Haemophilus ducreyi, another cause of STI, and has adapted to the gonococcus through gene loss and changes in the bla_TEM resistance determinant. Development and implementation of a pbla typing scheme revealed that there are three major variants in gonococci, which show distinct global distributions and are associated with specific lineages. Functional assays demonstrated these pbla variants differ in their resistance, fitness costs and mobilisation potential, with interactions with pConj driving the transfer of pbla. Furthermore, characterisation of gonococcal RMS indicated minimal impact on plasmid dissemination within the N. gonorrhoeae population.

Together, this thesis enhances understanding of plasmid-mediated β-lactam resistance in N. gonorrhoeae, with implications for surveillance, treatment, and prevention strategies, including the deployment of Doxy-PEP. Furthermore, my work underscores the value of integrating computational and experimental approaches to uncover novel insights into plasmid biology and the evolution and spread of AMR.

Authors: Elsener, T

• DOI: 10.5287/ora-6gjnkgdxj
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: population genomics; evolution; plasmid; mobilisation; conjugation; plasmid host range; STI; antimicrobial resistance; Doxy-PEP; TEM beta-lactamase
• Subjects: Epidemiology; Microbiology; Genomics; Molecular epidemiology