Emerging microbiota during cold storage and temperature abuse of ready-to-eat salad​

Karin Söderqvist,Omneya Ahmed Osman,Cecilia Wolff,Stefan Bertilsson,Ivar Vågsholm,Sofia Boqvist

doi:10.1080/20008686.2017.1328963

Emerging microbiota during cold storage and temperature abuse of ready-to-eat salad

Karin Söderqvist, Omneya Ahmed Osman + Show 4 more

Open Access

https://doi.org/10.1080/20008686.2017.1328963

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Abstract

ABSTRACT Introduction: Ready-to-eat (RTE) leafy vegetables have a natural leaf microbiota that changes during different processing and handling steps from farm to fork. The objectives of this study were (i) to compare the microbiota of RTE baby spinach and mixed-ingredient salad before and after seven days of storage at 8°C or 15°C; (ii) to explore associations between bacterial communities and the foodborne pathogens Listeria monocytogenes, pathogenic Yersinia enterocolitica, and pathogen model organism Escherichia coli O157:H7 gfp+ when experimentally inoculated into the salads before storage; and (iii) to investigate if bacterial pathogens may be detected in the 16S rRNA amplicon dataset. Material and methods: The microbiota was studied by means of Illumina 16S rRNA amplicon sequencing. Subsets of samples were inoculated with low numbers (50–100 CFU g−1) of E. coli O157:H7 gfp+, pathogenic Y. enterocolitica or L. monocytogenes before storage. Results and discussion: The composition of bacterial communities changed during storage of RTE baby spinach and mixed-ingredient salad, with Pseudomonadales as the most abundant order across all samples. Although pathogens were present at high viable counts in some samples, they were only detected in the community-wide dataset in samples where they represented approximately 10% of total viable counts. Positive correlations were identified between viable counts of inoculated strains and the abundance of Lactobacillales, Enterobacteriales, and Bacillales, pointing to positive interactions or similar environmental driver variables that may make it feasible to use such bacterial lineages as indicators of microbial health hazards in leafy vegetables. The data from this study contribute to a better understanding of the bacteria present in RTE salads and may help when developing new types of biocontrol agents.

Highlights

Ready-to-eat (RTE) leafy vegetables have a natural leaf microbiota that changes during different processing and handling steps from farm to fork
Relative abundances of bacterial phyla associated with control samples of baby spinach and mixed-ingredient salad are shown in Supplementary Table 1
Flavobacteriales was the second most prevalent order in samples before storage, but their relative abundance decreased during storage at both temperatures and a similar decline was observed for Burkholderiales

Summary

Introduction

Ready-to-eat (RTE) leafy vegetables have a natural leaf microbiota that changes during different processing and handling steps from farm to fork. Raw leafy vegetables host high numbers of naturally occurring microorganisms on the leaf surface (the phyllosphere), with bacteria as the most numerous colonists, reaching up to 108 CFU g−1.[3] The leaf microbiota may be altered by various inputs of contaminants in the field environment or during subsequent processing and storage.[4] Microorganisms can be found within plants as endophytic bacteria originating from soils surrounding the plant roots or from leaf surfaces, that enter the host through the root system or via stomata or wounds because of mechanical damage.[5] Most microorganisms associated with leafy vegetables are considered harmless for humans, but human pathogens may occasionally occur as the result of exposure to contaminated irrigation water or manure, cross-contamination via animals, dirty equipment or human handling.[6] During processing, e.g. washing or cutting, the natural protective barriers of leafy vegetable cells are damaged and the intracellular nutrients they release may further enhance bacterial growth.[7] Storage conditions may further influence the resident microflora.[8] Leafy vegetables and other food products containing this ingredient (e.g. mixed-ingredient salads), critically depend on good hygiene practices during production and processing, appropriate washing and a controlled cool chain to control microbial risks. Washing steps merely reduce the microbial load with 90–99% [6] and is unable to eliminate potential internalized

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