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

Background: As 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.

Results: The 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.

Conclusions: Our 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.

Keywords: Food safety; Listeria monocytogenes; Microbial safety and quality; Multi Locus Sequence Typing (MLST); Salads; Smoked salmon.

Fig. 1

Phylogenetic tree using the Neighbour-Joining (NJ) method based on concatenated nucleotide sequences of seven MLST house-keeping genes of 16 Sequence Types (ST) of 64 L. monocytogenes isolates recovered from salad and smoked salmon samples collected between September 2011 and October 2012 (phases I & II). Bootstrap values of >70% are shown on the branches and are calculated from 1,000 replicates. The horizontal scale bar of 0.005 represents nucleotide substitution rate per site. ^*Novel sequence types determined in this study (http://bigsdb.pasteur.fr/listeria/) ^a Isolates were recovered from smoked salmon samples unless specified. Isolates were recovered from ^ba chilled cooked prawn sample, ^ca chilled cooked shrimp sample and ^dpre-packed chicken salad samples. ^dA total of 31 isolates were recovered from 24 samples of Brand A’s pre-packed smoked salmon. Six samples were found positive with two different L. monocytogenes STs. These include the co-detection of ST2 and ST1218 in two samples; the co-detection of ST2 and ST1220, ST2 and ST1221, ST1220 and ST1222, as well as ST87 and ST1223 in one sample respectively

Fig. 2

Standard Plate Count (SPC) of pre-packed salads and salad bar ingredients. NS: The difference in SPC was not significant (p > 0.05). *: The difference in SPC was significant (p < 0.05). **: The difference in SPC was significant (p < 0.005). ***: The difference in SPC was significant (p < 0.001). Outliers. ---: Singapore’s regulatory limit for SPC in ready-to-eat food (<5 log CFU/g) [37]