Optimizing enrichment conditions for the isolation of Escherichia coli O157 in soils by immunomagnetic separation

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Aims:To compare a range of enrichment broths and enrichment temperatures for the isolation of Escherichia coli O157 by immunomagnetic separation (IMS) from sandy, loam and clay soils.Methods and Results:Soils were spiked with cocktails of four atoxigenic strains of E. coli O157 and four strains of commensal E. coli. The organisms were stressed by subjecting soils to cycles of freeze/thawing, followed by drying at 20°C for up to 4 days. Nine enrichment broths were trialled based on buffered peptone water, tryptone soya broths and EC broths supplemented with a range of selective additions. Enrichments were incubated for 6 h and assessed by target recovery after IMS on cefixime tellurite sorbitol MacConkey agar (CTSMAC) incubated at 37°C for 24 h. A comparison of enrichment temperatures (37 and 42°C) was also performed. Buffered peptone water (with or without vancomycin) and tryptone soya broth (with or without novobiocin) gave significant increases in recovery of E. coli O157 compared to others tested. In addition, broths incubated at 42°C were superior to those at 37°C for the recovery of E. coli O157.Significance and Impact of the Study:This study showed that sub-lethally damaged E. coli O157 surviving in soil can be sensitive to antimicrobial additions. The choice and concentration of these additions is vitally important to optimize target recovery. Some IMS protocols, established for the isolation of E. coli O157, may be unsuitable for the examination of soil samples.

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