title: Evaluation of eight agar media for the isolation of shiga toxin-Producing Escherichia coli.
abstract: The growth characteristics of 96 shiga toxin-producing Escherichia coli (STEC) strains representing 36 different O-types (including priority O types O26, O45, O103, O111, O121, O145 and O157) on commercial and in-house agar media were studied. The ability of the strains to grow on agar media with varying selective supplement formulations was evaluated using MacConkey Agar (MAC); Rainbow® Agar O157 (RBA); Rainbow® Agar O157 with manufacturer-recommended selective supplements (RBA-NT); Rainbow® Agar O157 with USDA-recommended selective supplements (RBA-USDA); CHROMagar STEC™ (CH STEC); Tryptone Bile agar containing cefixime and tellurite (TBA-CT); Tryptone Bile agar containing cefixime, tellurite, eosin and methylene blue (TBA-EM); and VTEC agar. All of the strains were able to grow on MAC, RBA and VTEC agar, whereas a number of strains (including some non-O157 priority O types) were unable to grow on the highly selective media CH STEC, RBA-NT, RBA-USDA, TBA-EM and TBA-CT. Only RBA-NT and CH STEC exhibited significant inhibition of background flora from ground beef enrichment. Significant inhibition of background flora from beef trim enrichment was observed with RBA-NT, RBA-USDA, CH STEC, TBA-EM and VTEC agar. With exception of E. coli O157, several different colony morphologies were observed on the differential plating media among strains of the same O type, indicating that this colony morphology is not a reliable means of identifying target STEC. These results suggest that an approach to maximize the recovery of target STEC from beef enrichment cultures is dual plating on lesser (RBA, MAC, VTEC agar) and more highly (RBA-NT, CH STEC) selective agars.
authors.affiliation: Bureau of Microbial Hazards, Health Canada, Frederick Banting Building, Tunney's Pasture, Ottawa, Ontario K1A 0K9, Canada. Electronic address: alex.gill@hc-sc.gc.ca.
authors.forename: Alexander
authors.initials: A
authors.lastname: Gill
authors.affiliation: Ontario Laboratory Network, Canadian Food Inspection Agency, 960 Carling Avenue, Bldg. 22, C.E.F., Ottawa, Ontario K1A 0C6, Canada.
authors.forename: George
authors.initials: G
authors.lastname: Huszczynski
authors.affiliation: Ontario Laboratory Network, Canadian Food Inspection Agency, 960 Carling Avenue, Bldg. 22, C.E.F., Ottawa, Ontario K1A 0C6, Canada.
authors.forename: Martine
authors.initials: M
authors.lastname: Gauthier
authors.affiliation: Ontario Laboratory Network, Canadian Food Inspection Agency, 960 Carling Avenue, Bldg. 22, C.E.F., Ottawa, Ontario K1A 0C6, Canada.
authors.forename: Burton
authors.initials: B
authors.lastname: Blais
mesh_terms: D000362:Agar; D001431:Bacteriological Techniques / Q000379:methods*; D003470:Culture Media / Q000737:chemistry*; D012641:Selection, Genetic; D054323:Shiga-Toxigenic Escherichia coli / Q000254:growth & development / Q000302:isolation & purification*
chemical_list: D003470:Culture Media; D000362:Agar
keywords: Agar media; Escherichia coli; Growth; Selective supplements; Shiga toxin/verotoxin