Increased resistance against tellurite is conferred by a mutation in the promoter region of uncommon tellurite resistance gene in the -negative Shiga toxin-producing O157:H7.

Unlabelled: Resistance to potassium tellurite (PT) is an important indicator in isolating Shiga toxin-producing (STEC) O157:H7 and other major STEC serogroups. Common resistance determinant genes are encoded in the gene cluster. We found an O157:H7 isolate that does not harbor but is resistant to PT. One nonsynonymous mutation was found in another PT resistance gene, , through whole-genome sequence analyses. To elucidate the contribution of this mutation to PT resistance, complementation of and the related gene in isogenic strains and quantitative RT‒PCR were performed. The results indicated that the point mutation not only changed an amino acid of , but also was positioned on a putative internal promoter of and increased PT resistance by elevating mRNA expression. Meanwhile, the amino acid change in had negligible impact on the PT resistance. Comprehensive screening revealed that 2.3% of O157:H7 isolates in Japan did not harbor the gene cluster, but the same mutation in was not found. These results suggested that PT resistance in can be enhanced through one mutational event even in -negative strains.

Importance: Selective agents are important for isolating Shiga toxin-producing (STEC) because the undesirable growth of microflora should be inhibited. Potassium tellurite (PT) is a common selective agent for major STEC serotypes. In this study, we found a novel variant of PT resistance genes, , in STEC O157:H7. Molecular experiments clearly showed that one point mutation in a predicted internal promoter region of upregulated the expression of the gene and consequently led to increased resistance to PT. Because genes are ubiquitous across , these results provide universal insight into PT resistance in this species.