Use of long read sequencing to characterise the genomic architecture of mobile genetic elements encoding blaCTX-M-15 in Escherichia coli causing travellers’ diarrhoea

Increasing levels of antimicrobial resistance (AMR) have been documented in Escherichia colicausing travellers’ diarrhoea, particularly to the third-generation cephalosporins. Diarrhoeagenic E. coli (DEC) can act as a reservoir for the exchange of AMR genes between bacteria residing in the human gut, enabling them to survive and flourish through the selective pressures of antibiotic treatments.

Using Oxford Nanopore Technology (ONT), we sequenced eight different sequence types (ST) belonging to five different pathotypes of extended-beta-lactamase-producing DEC harbouring blaCTX-M-15from four patients recently returned to the UK from Pakistan. The aim of the study was to determine whether blaCTX-M-15 was chromosome or plasmid-encoded to better understand the mechanisms of onward transmission of AMR determinants.

In Patient A, blaCTX-M-15was plasmid-encoded in both DEC isolates (ST504/ST3032), whereas in Patient B blaCTX-M-15was located on the chromosome in both DEC isolates (ST227/1283). Patients C and D both had one isolate where blaCTX-M-15 was located on the plasmid and one chromosomally encoded (ST443/182 and ST38/99, respectively). The two plasmids associated with Patient A were different although one exhibited high similarity to the plasmid from Patient C. In the four isolates where blaCTX-M-15 was chromosomally encoded, the site of insertion and the characteristics of the inserted plasmid segment differed.

Analysis of long-read sequencing data enables us to characterise the genomic architecture of mobile genetic elements encoding AMR determinants. These data may contribute to a better understanding of persistence and onward transmission of AMR determinants in MDR E. coli causing gastrointestinal and extra-intestinal infections.