Abstract
Listeriosis is almost entirely transmitted through foods contaminated with Listeria monocytogenes. Ready-to-eat foods present a particular challenge due to their long refrigerated shelf-life, not requiring any heat treatment before consumption. In this work, a shelf-life assessment of an industrially produced ready-to-eat salad was performed using conventional culture-based and molecular methods. L. monocytogenes isolates were confirmed and serogrouped using multiplex PCR, and genetic subtyping was performed by pulsed-field gel electrophoresis (PFGE). PMAxx-qPCR was used as an alternative method for L. monocytogenes quantification in foods. Salad samples were kept at 4 °C, 12 °C, and 16 °C for eight days and analysed. At 4 °C, acceptable results were obtained considering hygiene indicators, i.e., Enterobacteriaceae (ranging from 3.55 ± 0.15 log cfu/g to 5.39 ± 0.21 log cfu/g) and aerobic mesophilic colony counts (5.91 ± 0.90 log cfu/g to 9.41 ± 0.58 log cfu/g) throughout the study, but the same did not happen at 12 °C and 16 °C. L. monocytogenes culture-based quantification exhibited low numbers (<1 log cfu/g) for all temperatures. From 30 presumptive isolates, 10 (33.3%) were confirmed as L. monocytogenes with the majority belonging to serogroup IVb. PFGE subtyping showed that 7 of the 10 L. monocytogenes isolates had 100% of pulsotype similarity, suggesting a possible common contamination source. PMAxx-qPCR revealed a statistically higher L. monocytogenes quantification (>3 log cfu/g) when compared to the conventional culture-based method, suggesting viable but non-culturable forms. Taken together, results underline the need to combine conventional methods with more sensitive, specific, and rapid ones for L. monocytogenes assessment in ready-to-eat foods shelf-life studies to reduce the potential risk for consumers.
Highlights
Contemporary lifestyles have a major influence on food consumption patterns, and one of the major trends is the growing preference for convenience foods to which ready-toeat (RTE) salads are well associated [1,2]
The aw and Potential of Hydrogen (pH) values measured in RTE salad samples at 4 ◦ C, 12 ◦ C, and 16 ◦ C
The obtained results confirmed that the studied salad is a food able to support L. monocytogenes growth, according to Regulation (EC) No 2073/2005 of 15 November 2005, amendments on microbiological criteria for foodstuffs, and the European Reference Laboratory Technical guidance for shelf-life studies on L. monocytogenes in foods [10,43]
Summary
Contemporary lifestyles have a major influence on food consumption patterns, and one of the major trends is the growing preference for convenience foods to which ready-toeat (RTE) salads are well associated [1,2]. RTE salads have long refrigerated shelf-lives that allow the multiplication of psychrotrophic Listeria monocytogenes and do not require a heating step before consumption. These foods should be regarded as potential vehicles of transmission of human listeriosis [6,7]. European member states have reported thousands of confirmed human listeriosis cases per year, with high fatality rates [8]. This trend has been reported by the Centers for Disease Control and Prevention (CDC) in the United
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