Whole-Genome Sequencing Identifies In Vivo Acquisition of a bla CTX-M-27 -Carrying IncFII Transmissible Plasmid as the Cause of Ceftriaxone Treatment Failure for an Invasive Salmonella enterica Serovar Typhimurium Infection

We report a case of ceftriaxone treatment failure for bacteremia caused bySalmonella enterica subsp.enterica serovar Typhimurium, due to thein vivo acquisition of abla CTX-M-27 -encoding IncFII group transmissible plasmid. The original β-lactamase-susceptible isolate ST882S was replaced by the resistant isolate ST931R during ceftriaxone treatment. After relapse, treatment was changed to ciprofloxacin, and the patient recovered. Isolate ST931R could transfer resistance toEscherichia coli at 37°C. We used whole-genome sequencing of ST882S and ST931R, theE. coli transconjugant, and isolated plasmid DNA to unequivocally show that ST882S and ST931R had identical chromosomes, both having 206 identical single-nucleotide polymorphisms (SNPs) versusS . Typhimurium 14028s. We assembled a complete circular genome for ST931R, to which ST882S reads mapped with no SNPs. ST882S and ST931R were isogenic except for the presence of three additional plasmids in ST931R. ST931R and theE. coli transconjugant were ceftriaxone resistant due to the presence of a 60.5-kb IS26 -flanked,bla CTX-M-27 -encoding IncFII plasmid. Compared to 14082s, ST931R has almost identical Gifsy-1, Gifsy-2, and ST64B prophages, lacks Gifsy-3, and instead carries a unique Fels-2 prophage related to that found in LT2. ST882S and ST931R both had a 94-kb virulence plasmid showing >99% identity with pSLT14028s and a cryptic 3,904-bp replicon; ST931R also has cryptic 93-kb IncI1 and 62-kb IncI2 group plasmids. To the best of our knowledge,in vivo acquisition of extended-spectrum β-lactamase resistance byS . Typhimurium andbla CTX-M-27 genes in U.S. isolates ofSalmonella have not previously been reported.