The Microbiological Quality of Ready-to-Eat Salad and Salad Components Collected from Retail and Catering Premises in England and Northern Ireland

In response to observations of Shiga toxin-producing Escherichia coli (STEC) cases reporting an epidemiological association with salad consumption, a survey was carried out from April 2023 to March 2024 in England and Northern Ireland. The aim of this study was to examine salad to detect foodborne pathogens, including Salmonella spp., STEC, Listeria monocytogenes, Bacillus cereus, and coagulase-positive staphylococci and to determine levels of indicator bacteria, including generic E. coli and Listeria species. Samples of ready-to-eat salads and salad components were collected by local authorities and transported to the laboratory for microbiological examination. During the survey, 2,495 samples were tested. S. Typhimurium was detected in one sample (0.04%), but with no link to human cases. In 26 samples, stx DNA (genes that code for Shiga toxin) was detected by PCR and STEC was culture-confirmed from 3/2,495 samples (0.1%) with no link to human cases. L. monocytogenes was not detected in any samples. Coagulase-positive staphylococci were detected at between 20 and 4.0 log CFU/g levels in 31 (1%) samples, with none at a level of greater than 4.0 log CFU/g. Levels of greater than 5.0 log CFU/g presumptive B. cereus were found in 11 (0.4%), and levels of greater than 2.0 log CFU/g of E. coli were found in 33 (1%) samples. Overall, 2% of all samples were considered to be of unsatisfactory microbiological quality. These results demonstrate the importance of ongoing monitoring of salad products to ensure food safety, along with clear labeling specifying whether the product is RTE or requires further processing and/or washing prior to consumption. However, interpreting results for action is complicated by the challenges in culturing STEC, which limits the usefulness of current monitoring methods.