Microbial survey of ready-to-eat salad ingredients sold at retail reveals the occurrence and the persistence of Listeria monocytogenes Sequence Types 2 and 87 in pre-packed smoked salmon

Abstract

BackgroundAs the preparation of salads involves extensive handling and the use of uncooked ingredients, they are particularly vulnerable to microbial contamination. This study aimed to determine the microbial safety and quality of pre-packed salads and salad bar ingredients sold in Singapore, so as to identify public health risks that could arise from consuming salads and to determine areas for improvement in the management of food safety.ResultsThe most frequently encountered organism in pre-packed salad samples was B. cereus, particularly in pasta salads (33.3%, 10/30). The most commonly detected organism in salad bar ingredients was L. monocytogenes, in particular seafood ingredients (44.1%, 15/34), largely due to contaminated smoked salmon. Further investigation showed that 21.6% (37/171) of the pre-packed smoked salmon sold in supermarkets contained L. monocytogenes. Significantly higher prevalence of L. monocytogenes and higher Standard Plate Count were detected in smoked salmon at salad bars compared to pre-packed smoked salmon in supermarkets, which suggested multiplication of the organism as the products move down the supply chain. Further molecular analysis revealed that L. monocytogenes Sequence Type (ST) 2 and ST87 were present in a particular brand of pre-packed salmon products over a 4-year period, implying a potential persistent contamination problem at the manufacturing level.ConclusionsOur findings highlighted a need to improve manufacturing and retail hygiene processes as well as to educate vulnerable populations to avoid consuming food prone to L. monocytogenes contamination.Electronic supplementary materialThe online version of this article (doi:10.1186/s12866-017-0956-z) contains supplementary material, which is available to authorized users.

Estimating Listeria monocytogenes Growth in Ready‐to‐Eat Chicken Salad Using a Challenge Test for Quantitative Microbial Risk Assessment

“…This might be linked to the fact that people over 65 years of age, with high notification rates, are not expected consumers of this type of salads, and by that reason were not considered high‐risk population in the study. Additionally, even the number of cases of listeriosis attributed to high‐risk groups, especially immunocompromised patients, may be overestimated, since most of these patients are often advised to avoid the consumption of these types of products, exactly because of the potential health risk it represents (Chau et al., 2017; United States of America Centers for Disease Control and Prevention [USA‐CDC], 2018).…”

Group B Streptococcus Infections Caused by Improper Sourcing and Handling of Fish for Raw Consumption, Singapore, 2015–2016

“…(47.6%, 10/21) and L. monocytogenes (2.0%, 6/303), whose psychrotrophic nature has posed a challenge to the fish industry. L. monocytogenes is also a concern in chilled RTE food because of its ubiquity and persistence in food-processing environments ( 25 ). …”

Genetic diversity and profiles of genes associated with virulence and stress resistance among isolates from the 2010-2013 interagency Listeria monocytogenes market basket survey

“…Although several studies reported that lineage II isolates were significantly overrepresented in foods [61][62][63][64][65][66][67], other studies reported that lineage I isolates predominate in foods. For example, Sauders et al [68] [35], 60% of a collection of 81 isolates from RTE meat products in processing plants and retail establishments in central Portugal sampled between 2011 and 2013 [70], and 82.8% of a collection of 64 food isolates from retail establishments and supermarkets in Singapore sampled between 2011 and 2012 [71].…”