Phenotypic and genetic markers for serotype-specific detection of Shiga toxin-producing Escherichia coli O26 strains from North America.

Phenotypic and genetic markers of Shiga toxin-producing Escherichia coli (STEC) O26 from North America were used to develop serotype-specific protocols for detection of this pathogen. Carbohydrate fermentation profiles and prevalence of gene sequences associated with STEC O26 (n = 20) were examined. Non-STEC O26 (n = 17), E. coli O157 (n = 20), E. coli O111 (n = 22), and generic E. coli (n = 21) were used as comparison strains. Effects of supplements: cefixime-tellurite, 4-methylumbelliferyl-beta-D-glucuronide (MUG) and chromogenic additives (5-bromo4-chloro-3-indolyl-beta-D-galactopyranoside (X-Gal), 5-bromo-4-chloro-3-indolyl-beta-D-glucuronide (X-GlcA) and o-nitrophenyl-beta-D-galactopyranoside (ONPG), added to isolation agar media were examined. Tests for presence of gene sequences encoding beta intimin (eae beta), Shiga toxin 1 and 2 (stx1 and stx2), H7 flagella (flicCh7), enterohemolysin (ehlyA), O26 somatic antigen (wzx), and high pathogenicity island genes (irp2 and fyuA) were conducted using multiplex polymerase chain reaction. Pulsed-field gel electrophoresis (PFGE) of XbaI restriction endonuclease genomic DNA digests was used to establish clonality among E. coli O26 strains. Of the 26 carbohydrates tested, only rhamnose had diagnostic value. Rhamnose non-fermenters included STEC O26 (100%), non-STEC O26 (40%), generic E. coli (29%), E. coli O111 (23%), and E. coli O157 (0%). Rhamnose non-fermenting colonies growing on Rhamnose-McConkey agar supplemented with X-GlcA, X-Gal, or ONPG, respectively, were blue, white, or faint yellow, whereas rhamnose-fermenters were red. Blue colonies from X-GlcA-containing media were the most well-defined and easiest to pick for further tests. All STEC O26 were MUG-fluorescent, while STEC O157 (n = 18) were non-fluorescent. E. coli O111 and generic E. coli strains were either MUG-positive or-negative. Serotype-specific detection of STEC O26 was achieved by selecting cefixime-tellurite-resistant, MUG-fluorescent, rhamnose-nonfermenting colonies, which carried stx1, eae beta, irp2, and wzx gene sequences. STEC O26 prevalence in dairy farm environmental samples determined using the developed isolation and genetic detection protocols was 4%. PFGE indicated the presence of one major cluster of E. coli O26 with 72-100% DNA fragment-length digest similarity among test strains. The serotype-specific detection methods described herein have potential for routine application in STEC O26 diagnosis.