The interplay between plasmids in Shigella sonnei

Thesis

Shigella sonnei causes bacillary dysentery in higher-income countries, with emerging antimicrobial resistance spread by plasmids. My thesis aimed to investigate the interplay between S. sonnei pINV, which is required for virulence, and resistance plasmids. pRES, a naturally occurring 114 kbp conjugative resistance plasmid, was used as an exemplar to investigate the transfer the 228 kbp virulence plasmid pINV, which was previously considered non-mobilisable.

pRES belongs to incompatibility group IncB/O/K/Z and is stably maintained during laboratory culture. The co-existence of pINV and pRES does not affect their maintenance within a bacterial population. pRES is a conjugative plasmid which can mobilises a 5.1 kbp colicin-E1 encoding plasmid as well as pINV. During pINV transfer, pINV and pRES form a single 334 kbp fusion plasmid, and/or hybrid plasmids, via RecA-dependent and -independent recombination, and insertion sequence transposition. Copies of IS21 on pINV and pRES, and a conserved 199 bp region (199R) downstream of the plasmid replicons were detected at the pINV/pRES junctions in fusion and hybrid molecules.

The 199R is conserved in plasmids from IncFII and IncI-complex groups in S. sonnei which replicate unidrectionally. The sequence includes the single-stranded initiation site for synthesis of the leading strand during plasmid replication. Removal of the 199R increases pINV and pRES loss. The first 99 bp of the 199R (99O) overlaps with a previously described 218 bp (218B) deletion on the R1 plasmid, which involves the terR site for replication termination. Deletion of 218B or 99O also increases plasmid loss. RNAfold prediction suggest that the 99O can form secondary structures when single-stranded. SNPs were introduced into the 99O, disrupting these structures and increasing plasmid loss, corresponding to the degree of disruption. Secondary structures in the 199R might influence plasmid replication and inter-plasmid recombination, contributing to the mosaic architecture of many plasmids.

Authors: Lee, QL

• DOI: 10.5287/ora-44v6voky5
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Microbiology