Emergence of ESBL-producing Salmonella Typhimurium in Australia due to a new clade harbouring blaCTX-M-9

We report the emergence of third-generation cephalosporin resistance in Australian Salmonella Typhimurium for the first time. Phenotypically resistant isolates of human and animal origin were characterized by whole genome sequencing. All were closely related and carried blaCTX-M-9. Dairy cattle were the suspected source based on geographical clustering of animal isolates which were predominantly bovine in origin, raising concerns regarding off-label use of ceftiofur.