Tn4661-mediated transfer of bla_CTX-M-15 from Klebsiella michiganensis to an outbreak clone of Pseudomonas aeruginosa

Carriage of CTX-M-type extended-spectrum β-lactamase (ESBL) is rare in Pseudomonas aeruginosa. During routine surveillance of an endemic ST-621 P. aeruginosa at a large hospital, isolate MRSN 100690 carrying bla_CTX-M-15 was cultured from a patient (P2). This was the first detection of this ESBL in the endemic ST-621 lineage. All 1 488 bacterial isolates collected from the same facility in the 12 months prior to the incidence of 100 690 were screened for the presence of bla_CTX-M-15. A set of 183 isolates was identified, in which corresponding patient metadata was evaluated for spatiotemporal overlaps with P2. The resulting three iso-lates, along with 100 690, were long-read sequenced using the Oxford Nanopore MinION platform to determine a potential donor of bla_CTX-M-15. The screen revealed a single Klebsiella michiganensis isolate, MRSN 895358, which carried an IncA/C2 plasmid harbouring bla_CTX-M-15. Notably, the patient harbouring 895358, P1, occupied the same hospital room as P2 9 months prior. Genomic alignment revealed that both isolates shared an identical 80.8 kb region containing the IncA/C2 plasmid replicon and bla_CTX-M-15. This region was plasmid bound in 895 358, but chromosomally bound in 100 690 due to Tn4661-mediated transposition. ESBL bla_CTX-M-15 was acquired and subsequently integrated into the chromosome of a ST-621 P. aeruginosa, likely initiated by plasmid transfer from a K. michiganensis strain.