The emergence of chromosomally located blaCTX-M subtypes in Salmonella enterica serotype Kentucky ST198 isolated from diarrhea patients, food, and environmental sources in Henan, China

Among 1,574 <i>Salmonella</i> isolates, 68 <i>S.</i> Kentucky ST198 isolates were identified, all of which exhibited multi-drug resistance (MDR). Each strain carried between 5 and 19 antimicrobial resistance genes. Phylogenetic analysis revealed that isolates identified in China were clustered into two clades (ST198-1 and ST198-2), characterized by a specific point mutation in the <i>gyrA</i> gene and closely related to European isolates. Comparative genomics showed that acquisition of MDR region in clade ST198-2 conferred resistance to azithromycin, fosfomycin, cefotaxime, rifamycin, chloramphenicol, florfenicol, trimethoprim, and ampicillin. Most <i>bla</i> _CTX-M genes were chromosomally located except one strain carrying <i>bla</i> _CTX-M-27 on plasmid p0111. In clade ST198-2, <i>bla</i> _CTX-M-55 or <i>bla</i> _CTX-M-64 genes were detected within the MDR region, while in clade ST198-1, <i>bla</i> _CTX-M-14b was inserted into the coding gene clusters of the T6SS, located chromosomally. Considering the extensively drug-resistant nature of the isolates, continuous surveillance and effective measures to control the transmission of <i>Salmonella</i> Kentucky ST198 are urgently needed.