Enterohemorrhagic Escherichia coli O157 Strains Showing White Aspect on CHROMagar O157

Abstract

One hundred and seventy-five strains of enterohemorrhagic Escherichia coli (EHEC) O157, expressing stx1 and stx2 genes, were isolated from 464 stool specimens collected from patients of foodbone outbreak associated with consumption of Japanese dumpling (dango and kashiwamochi) occurred in Yamagata city, Japan. We found that two strains of them formed white colonies on selective and differential agar, CHROMagar O157. CHROMagar O157 has been expected to contain at least three substrates, X-α-galactoside, X-β-glucoside and X-β-glucuronide, which is utilized to detect α-galactosidase, β-glucosidase and β-glucuronidase, respectively. Many enterobacteriaceae species including non-0157 E. coli, form blue colonies on CHROM agar O157 as the result of hydrolysing the substrates by β-glucosidase and β-glucuronidase, whereas EHEC O157 has been supposed not to form blue colonies but mauve colonies because of its α-galactosidase activity instead of the lack of both β-glucosidase and β-glucuronidase. As expected, the fermentation tests of EHEC O157 (stx1+stx2) having white aspect on CHROMagar O157 showed negative results for the activities ofα-galactosidase, β-glucosidase and β-glucuronidase. The result of PCR, however, showed that they have both of melA and melB genes. Therefore, it has been suggested that EHEC O157 (stx1+stx2) forming white colonies on CHROM agar O157 has the mutations in melA and melB genes, resulting in the suppression of α-galactosidase.