title: Comparison of eight different agars for the recovery of clinically relevant non-O157 Shiga toxin-producing Escherichia coli from baby spinach, cilantro, alfalfa sprouts and raw milk.
abstract: The FDA Bacteriological Analytical Manual (BAM) Chapter 4a recommends several agars for isolating non-O157 Shiga toxin-producing Escherichia coli (STEC); not all have been thoroughly tested for recovering STECs from food. Using E. coli strains representing ten clinically relevant O serogroups (O26, O45, O91, O103, O104, O111, O113, O121, O128, O145) in artificially-contaminated fresh produce--bagged baby spinach, alfalfa sprouts, cilantro, and raw milk--we evaluated the performance of 8 different agars. Performance was highly dependent upon strain used and the presence of inhibitors, but not necessarily dependent on food matrix. Tellurite resistant-negative strains, O91:-, O103:H6, O104:H21, O113:H21, and O128, grew poorly on CHROMagar STEC, Rainbow agar O157, and a modified Rainbow O157 (mRB) agar. Although adding washed sheep's blood to CHROMagar STEC and mRB agars improved overall performance; however, this also reversed the inhibition of non-target bacteria provided by original formulations. Variable colony coloration made selecting colonies from Rainbow agar O157 and mRB agars difficult. Study results support a strategy using inclusive agars (e.g. L-EMB, SHIBAM) in combination with selective agars (R & F E. coli O157:H7, CHROMagar STEC) to allow for recovery of the most STECs while increasing the probability of recovering STEC in high bacterial count matrices.
authors.affiliation: Division of Microbiology, Center for Food Safety and Applied Nutrition, United States Food and Drug Administration, College Park, MD 20740, USA. Electronic address: julie.kase@fda.hhs.gov.
authors.forename: Julie A
authors.initials: JA
authors.lastname: Kase
authors.affiliation: Oak Ridge Research Institute for Science and Education, Oak Ridge, TN 37830, USA. Electronic address: Anna.Laasri@fda.hhs.gov.
authors.forename: Anna
authors.initials: A
authors.lastname: Maounounen-Laasri
authors.affiliation: Division of Microbiology, Center for Food Safety and Applied Nutrition, United States Food and Drug Administration, College Park, MD 20740, USA. Electronic address: Insook.Son@fda.hhs.gov.
authors.forename: Insook
authors.initials: I
authors.lastname: Son
authors.affiliation: Office of Regulatory Affairs-San Francisco Laboratory, United States Food and Drug Administration, Alameda, CA 94502, USA. Electronic address: Andrew.Lin@fda.hhs.gov.
authors.forename: Andrew
authors.initials: A
authors.lastname: Lin
authors.affiliation: Division of Microbiology, Center for Food Safety and Applied Nutrition, United States Food and Drug Administration, College Park, MD 20740, USA. Electronic address: Thomas.Hammack@fda.hhs.gov.
authors.forename: Thomas S
authors.initials: TS
authors.lastname: Hammack
mesh_terms: D000362:Agar / Q000737:chemistry; D000818:Animals; D001424:Bacterial Infections / Q000382:microbiology*; D002417:Cattle; D015169:Colony Count, Microbial / Q000295:instrumentation* / Q000379:methods; D028526:Coriandrum / Q000382:microbiology*; D003470:Culture Media / Q000737:chemistry / Q000378:metabolism; D000455:Medicago sativa / Q000382:microbiology*; D008892:Milk / Q000382:microbiology*; D054323:Shiga-Toxigenic Escherichia coli / Q000254:growth & development* / Q000302:isolation & purification / Q000378:metabolism; D018724:Spinacia oleracea / Q000382:microbiology*
chemical_list: D003470:Culture Media; D000362:Agar
keywords: Bacteriological agar; Fresh produce; Raw milk; STEC