Rapid Increase of CTX-M-Producing Shigella sonnei Isolates in Switzerland Due to Spread of Common Plasmids and International Clones

The Swiss Centre for Antibiotic Resistance (ANRESIS) has recently noted an increase of extended-spectrum cephalosporin-resistant (ESC-R) Shigella sonnei isolates nationwide (3.8% in 2016 versus 37.5% in 2019). To understand this phenomenon, we analyzed 25 representative isolates (of which 14 were ESC-R) collected in Switzerland during 2016 to 2019. Whole-genome sequencing was achieved using both the Illumina and the Nanopore platforms. Both ESC-R and extended-spectrum cephalosporin-susceptible isolates belonged to sequence type 152 (ST152). The ESC-R isolates carried bla(CTX-M-3) in IncI1-pST57 (n = 5), bla(CTX-M-15) in IncFII (F2:A-:B-) (n = 5), bla(CTX-M-15) in IncI1-pST16, and bla(CTX-M-27), bla(CTX-M-55), or bla(CTX-M-134) in other IncFII plasmids (n = 1 each). Plasmids having the same bla and Inc group exhibited high degrees of genetic identity to each other but also to plasmids previously reported in other Enterobacterales. Core-genome analysis showed that there were 4 main clusters, each of which included strains that differed by <58 single nucleotide variants (SNVs) and that consisted of both bla(CTX-M)-positive and bla(CTX-M)-negative isolates. Moreover, most isolates belonging to the same cluster shared an identical core-genome sequence type (cgST). For instance, cluster 1 included 4 isolates of cgST113036, of which only 3 harbored the IncI1-pST57 bla(CTX-M-3)-positive plasmid. The 25 S. sonnei isolates were also subjected to phylogenetic comparison with deposited international strains. As a result, matching isolates (isolates that had the same cgST and that differed by <8 SNVs) have been reported in the United Kingdom, the United States, France, and the Netherlands. Overall, our results suggest that some common S. sonnei clusters can spread between continents and can be imported into other nations after international trips. Such clusters include, in part, isolates that do not possess bla(ESBL)-harboring plasmids, indicating their tendency to acquire them from other Enterobacterales.