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.

ABSTRACT

A set of 45 environmental strains of Shiga toxin producing Escherichia 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 ((O113H21, O121H19, O157H7, O6H34, O177H25, and O185H7) each containing either the stx2a or stx2c operon. Six of the seven O113H21 were found to contain two distinct stx2a operons. Eight strains of O157H7 possessed a stx2c operon whose A subunit gene was interrupted by an insertion sequence (IS1203v). Shiga toxin production was induced by nutrient depletion and quantitated by mass spectrometry. The 37 strains produced Shiga toxins in a near 50-fold range (1.4-49 ng/mL). The IS-interrupted strains expressed low but measurable amounts of the B subunits (0.5-1.9 ng/mL). Another strain possessed an identical stx operon without an IS interruption and produced intact Stx2c (5.7 ng/mL).