Identification of a Plasmid-Mediated Tellurite-Resistance Gene Operon Conferring High-Level Tellurite Resistance in...

Escherichia albertii is an emerging diarrheagenic pathogen that causes foodborne outbreaks. Several selective media for E. albertii have been developed for outbreak investigation or surveillance of a potential contamination source. Although these selective media contain potassium tellurite (PT) as a selective agent, the genetic determinants for PT resistance in E. albertii are unclear. Here, we analyzed the whole genomes of our 22 E. albertii isolates and found three that harbored the IncHI2-type plasmid encoding the tellurite resistance operon (ter-operon). The minimum inhibitory concentration (MIC) value of PT against ter-positive E. albertii was 640 μg/mL for all three strains, which is higher than those against other E. albertii isolates (5 to 40 μg/mL). The conjugation assay using three ter-positive E. albertii strains found that two of the three ter-positive plasmids transferred into a PT-susceptible E. coli recipient when tested individually, resulting in an increase in the MIC of PT to more than 640 μg/mL. The three ter-positive plasmids had sequence diversity, especially in the region coding antimicrobial resistance genes. The results of a plasmid database (PLSDB) search implied the circulation of ter-positive IncHI2 plasmids among species belonging to several genera within the order Enterobacterales. Phylogenetic analysis with genomes downloaded from a public database identified the ter-operon in 93 of 751 E. albertii genomes (12.4%) belonging to multiple E. albertii lineages. These data suggest that the plasmid-encoded ter-operon plays an important role in the dissemination and acquisition of high PT resistance in E. albertii.