Evaluation of DNA extraction methods for the detection of Shiga toxin producing Escherichia coli in food by polymerase chain reaction.

Reported food-borne outbreaks of Shiga toxin producing Escherichia coli (STEC) have involved a very diverse range of foods. Contemporary analytical methods for the detection of STEC in foods typically include PCR screening of enrichment media. However, PCR inhibitors present in food enrichments can produce false negative results when screening for DNA sequences associated with the pathogen. To avoid false negative results in enrichment screening, it is advantageous to have DNA extraction methods that are effective at removing PCR inhibitors from a wide range of foods. The standard Canadian STEC method MFLP-52 uses Bio-Rad Instagene Matrix for DNA extraction. In this study, three DNA extraction protocols using commercial kits (Instagene Matrix with Beckman Coulter Ampure XP Beads; Qiagen Gentra Puregene Yeast/Bact. Kit; Qiagen DNeasy Blood & Tissue) were assessed as alternative DNA extraction methods for the detection of the Shiga toxin gene by PCR in enrichments from sixteen different foods inoculated with STEC O157. The inoculated foods were bean sprouts, blackberries, blue cheese, cilantro, cocoa powder, coleslaw, cream of mushroom dried soup mix, cream of vegetable dried soup mix, flaxseed, guacamole, peanut butter, soft cheese, soy butter, spinach, walnut, and wheat flour. Two of the protocols, Instagene Matrix with Ampure XP Beads, and Gentra Puregene Yeast/Bact, produced no false-negative or false positive results in the analysis of triplicate enrichment samples from sixteen inoculated foods.