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ARS Home » Pacific West Area » Albany, California » Western Regional Research Center » Produce Safety and Microbiology Research » Research » Publications at this Location » Publication #357747

Research Project: Ecology and Detection of Human Pathogens in the Produce Production Continuum

Location: Produce Safety and Microbiology Research

Title: Using nanospray liquid chromatography and mass spectrometry to quantitate Shiga toxin production in environmental Escherichia coli recovered from a major produce production region in California

Author
item Silva, Christopher - Chris
item Lee, Bertram
item Yambao, Jaszemyn
item Erickson-Beltran, Melissa
item Quiñones, Beatriz

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/28/2018
Publication Date: 11/28/2018
Citation: Silva, C.J., Lee, B.G., Yambao, J.C., Erickson-Beltran, M.L., Quiñones, B. 2019. Using nanospray liquid chromatography and mass spectrometry to quantitate Shiga toxin production in environmental Escherichia coli recovered from a major produce production region in California. Journal of Agricultural and Food Chemistry. 67(5):1554-1562. https://doi.org/10.1021/acs.jafc.8b05324.
DOI: https://doi.org/10.1021/acs.jafc.8b05324

Interpretive Summary: The E. coli responsible for severe foodborne outbreaks are referred to as Shiga toxin producing E. coli or STEC. The Shiga toxins these bacteria produce are responsible for the severe gastroenteritis symptoms associated with foodborne outbreaks. We selected a set of 45 STEC strains from three California counties to be analyzed for the amounts of Shiga toxin they produce. The STEC in this set comprised six distinct kinds of E. coli, the O113:H21, O121:H19, O157:H7, O6:H34, O177:H25, or O185:H7 serotype. Each contained the genes to make either Shiga toxin type 2 variant a (Stx2a) or variant c (Stx2c). Stx2a and Stx2c are the Shiga toxins associated with the most severe symptoms of an STEC infection. Six of the seven strains of the O113:H21 serotype contained two distinct Stx2a producing genes. Additionally, eight of the strains of the O157:H7 serotype contained Stx2c genes that were interrupted by bacterial DNA (insertion sequence (IS)). Shiga toxin production was induced by nutrient depletion and analyzed by a mass spectrometer and a cell-based (Vero) bioassay. Shiga toxins were produced by the 37 STEC strains possessing uninterrupted Shiga toxin genes. These strains produced partial Shiga toxins in a fifty-fold range (1.1 ng/mL to 49 ng/mL). The IS-interrupted strains produced partial toxins at low levels (0.5 -1.6 ng/mL), compared to another strain possessing uninterrupted Stx2c genes (without an IS interruption) that produced intact Stx2c (5.7 ng/mL).

Technical Abstract: A set of 45 environmental isolates of Shiga toxin producing E. coli (STEC) from three California counties were analyzed for Shiga toxin production by nanospray liquid chromatography-mass spectrometry and Vero cell bioassay. The STEC in this set comprised six serotypes ((O113:H21, O121:H19, O157:H7, O6:H34, O177:H25, and O185:H7) each containing either the stx2a or stx2c operon. Six of the seven O113:H21 were found to contain two distinct stx2a operons. Eight isolates of O157:H7 possessed a stx2c operon whose A subunit gene was interrupted by an insertion sequence (IS629 = IS1203v). Shiga toxin production was induced by nutrient depletion and quantitated by mass spectrometry. The 37 strains produced Shiga toxins in a fifty-fold range (1.1 ng/mL to 49 ng/mL). The IS-interrupted strains expressed low but measurable amounts of the B subunits (0.5 -1.6 ng/mL). Another strain possessed an identical stx operon without an IS interruption and produced intact Stx2c (5.7 ng/mL).