Characterization of IS26-bracketed bla_CTX-M-65 resistance module on IncI1 and IncX1 plasmids in Escherichia coli...

Area of Science:

• Microbiology
• Genetics
• Public Health

Background:

• Antimicrobial resistance (AMR) is a major global health concern, driven by bacteria like Enterobacteriaceae developing resistance to key antibiotics.
• Extended-spectrum β-lactamases (ESBLs) and 16S rRNA methylases are primary mechanisms conferring resistance, particularly in poultry-associated *Escherichia coli* (E. coli).

Purpose of the Study:

• To characterize a multidrug-resistant (MDR) E. coli strain (HN257) isolated from chicken, focusing on its phylogenetic placement and plasmid-mediated resistance determinants.
• To investigate the genetic context and transmission dynamics of the blaCTX-M-65 gene within this strain.

Main Methods:

• Phylogenetic analysis using Single Nucleotide Polymorphisms (SNPs) to determine the evolutionary relationships of the E. coli strain.
• Plasmid profiling and sequencing to identify resistance genes and their locations.
• Conjugation experiments to assess plasmid transfer and cointegrate formation.

Main Results:

• Strain HN257, belonging to ST224 and serotype O88:H23, was closely related to other poultry-derived E. coli strains.
• The strain harbored four plasmids encoding 16 resistance determinants, including two blaCTX-M-65 genes located on different plasmids within IS26-bracketed modules.
• A non-conjugative IncX1 plasmid (pHN257-4) carrying blaCTX-M-65 and fosA3 was identified, and its fusion with a conjugative IncI1 plasmid (pHN257-F) via IS26 was demonstrated.
• Transconjugants exhibited minimal growth impairment with single or fused plasmids.

Conclusions:

• This study reports the first identification of a non-conjugative IncX1 plasmid carrying blaCTX-M-65 and fosA3 in an MDR E. coli ST224 from poultry.
• The findings provide critical insights into the presence and transmission mechanisms of blaCTX-M-65 in poultry-associated E. coli.
• The minimal impact of these plasmids on bacterial growth suggests efficient dissemination potential.