Inhibition of polymerase chain reaction for the detection of Escherichia coli O157:H7 and Salmonella enterica on walnut kernels.

The aim of this study was to determine whether Escherichia coli O157:H7 can be reliably detected and isolated from walnut kernels using standard methods of analysis. The limit of detection approached 1 cell per analytical unit (25 g) for E. coli O157:H7 on walnut kernels enriched in modified tryptic soy broth with 20 μg/ml novobiocin and plating onto selective agar media. The presence of PCR inhibitors in walnut kernels was indicated by the failure to detect E. coli O157:H7 from culture positive enrichment broths analysed by PCR, with two separate polymerase and reagent compositions (Dupont BAX E. coli O157:H7 MP system, Promega GoTaq Green for stx) and three methods of template preparation (DuPont BAX, Qiagen DNeasy, Bio-Rad InstaGene). PCR inhibition was overcome by 1:100 dilution in TE buffer of the DNeasy or InstaGene template. PCR inhibition was not relieved by dilution of the BAX template. Similar results were observed for walnut kernels inoculated with Salmonella enterica and analysed for invA, indicating that PCR inhibition is not specific to the organism or primer/template. These results indicate that analysis of walnut kernels for pathogens should be with culture based methods or use protocols for DNA template preparation modified to remove or dilute inhibitors and the need for internal amplification controls in PCR methods.