Poster Presentation Australian Society for Microbiology Annual Scientific Meeting 2022

Insertion of blaCTX-M-15 within a phage-inducible chromosomal island in Shigella flexneri serotype 6 (#152)

Eby M Sim 1 2 3 , Andrew N Ginn 1 2 , Elena Martinez 1 2 3 , Peter Howard 4 , Jon Iredell 1 2 5 , Vitali Sintchenko 1 2 3
  1. Sydney Institute for Infectious Diseases, University of Sydney, Westmead, New South Wales, Australia
  2. Microbial Genomics Reference Laboratory, Institute of Clinical Pathology and Medical Research, NSW Health Pathology, Westmead, New South Wales, Australia
  3. Centre of Infectious Disease and Microbiology – Public Health, Westmead Hospital, Westmead, New South Wales, Australia
  4. NSW Enteric Reference Laboratory , Institute of Clinical Pathology and Medical Research, NSW Health Pathology, Westmead, New South Wales, Australia
  5. Centre of Infectious Disease and Microbiology, Westmead Institute of Medical Research, Westmead, New South Wales, Australia

Ceftriaxone is typically used for treatment of Shigellosis but isolates resistant to third generation cephalosporins have been reported globally, primarily conferred by genes encoding extended-spectrum beta-lactamases such as blaCTX-M-15.  The blaCTX-M-15 gene is typically found in a transpositional unit consisting of an upstream ISEcp1 and a downstream partial open reading frame (orf477Δ). Apart from providing a promoter for blaCTX-M-15, ISEcp1 also confers intracellular mobility, with the transpositional unit detected in both plasmids and chromosomes in different Gram-negative bacterial pathogens. In this study, we utilised long read sequencing to examine the chromosomal insertion of a ISEcp1-blaCTX-M-15-orf477Δ transpositional unit within a clinical strain of Shigella flexneri. The transpositional unit was determined within a phage-inducible chromosomal island (PICI) and its insertion disrupted a key gene required for PICI transfer. This PICI was determined to be integrated within the portal gene of a larger prophage. These gene disruptions in both PICI and the prophage can negate direct phage-mediated horizontal gene transfer of blaCTX-M-15. However, the presence of other key prophage genes suggests a series of cascading events that could provide an alternate route of intercellular transfer of blaCTX-M-15. These findings highlight the value of long-read sequencing in investigations of intercellular transfer of antibiotic resistance genes and its contribution to understanding of genomic diversity in pathogens and infection control interventions.