title: Isolation of Shiga toxin-producing Escherichia coli serogroups O26, O45, O103, O111, O121, and O145 from ground beef using modified rainbow agar and post-immunomagnetic separation acid treatment.
abstract: It is estimated that at least 70% of human illnesses due to non-O157 Shiga toxin-producing Escherichia coli (STEC) in the United States are caused by strains from the top six serogroups (O26, O45, O103, O111, O121, and O145). Procedures for isolating STEC from food products  often use plating media that include antimicrobial supplements at concentrations that inhibit background microflora growth but can also inhibit target STEC growth. In this study, an agar medium with lower supplement concentrations, modified Rainbow agar (mRBA), was evaluated for recovery of  STEC serogroups O26, O45, O103, O111, O121, and O145 from ground beef enrichments. A post-immunomagnetic separation (IMS) acid treatment step was additionally used to reduce background microflora and increase recovery of target STEC strains. Ground beef samples (325 g) were artificially  contaminated with STEC and confounding organisms and enriched for 15 h. Recovery of the target STEC was attempted on the enrichments using IMS and plating onto mRBA and Rainbow agar (RBA). Additionally, acid treatment was performed on the post-IMS eluate followed by plating onto mRBA. Using  the combination of mRBA and acid treatment, target STEC were isolated from 103 (85.8%) of 120 of the low-inoculated samples (1 to 5 CFU/325-g sample) compared with 68 (56.7%) of 120 using no acid treatment and plating onto RBA with higher levels of novobiocin and potassium tellurite. The combination  of acid treatment and mRBA provides a significant improvement over the use of RBA for isolation of STEC serogroups O26, O45, O103, O111, O121, and O145 from raw ground beef.
authors.affiliation: US Department of Agriculture, Food Safety and Inspection Service, Eastern Laboratory Outbreaks Section, Athens, Georgia 30605, USA. glenn.tillman@fsis.usda.gov
authors.forename: Glenn E
authors.initials: GE
authors.lastname: Tillman
authors.forename: Jamie L
authors.initials: JL
authors.lastname: Wasilenko
authors.forename: Mustafa
authors.initials: M
authors.lastname: Simmons
authors.forename: Todd A
authors.initials: TA
authors.lastname: Lauze
authors.forename: Joseph
authors.initials: J
authors.lastname: Minicozzi
authors.forename: Brian B
authors.initials: BB
authors.lastname: Oakley
authors.forename: Neelam
authors.initials: N
authors.lastname: Narang
authors.forename: Pina
authors.initials: P
authors.lastname: Fratamico
authors.forename: Ailliam C
authors.initials: AC
authors.lastname: Cray
mesh_terms: D000362:Agar; D000818:Animals; D002417:Cattle; D015169:Colony Count, Microbial / Q000295:instrumentation / Q000379:methods; D003257:Consumer Product Safety; D005506:Food Contamination / Q000032:analysis*; D005516:Food Microbiology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D018189:Immunomagnetic Separation / Q000379:methods*; D008461:Meat Products / Q000382:microbiology*; D012703:Serotyping; D054323:Shiga-Toxigenic Escherichia coli / Q000302:isolation & purification*
chemical_list: D000362:Agar