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

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.​

Effects of an Eco-Friendly Sanitizing Wash on Spinach Leaf Bacterial Community Structure and Diversity

“…In agreement with previous observations, the phylum Proteobacteria showed the highest total abundance on day 0, followed by phyla Bacteroidetes, Firmicutes, and Actinobacteria [3,6,26,34]. The basal bacterial microbiome of RTE spinach leaves is therefore very similar to that of other minimally processed fruits and vegetables [3,6,27,34,60].…”

“…This study investigated the microbiome profiles of RTE spinach leaves washed with different sanitizers (ECAS, PAA) and compared with leaves washed with tap water and not washed at all (control), at three time points over 10 days (day 0, day 5, and day 10). Although a higher proportion of ASVs was found compared to previous studies [26,27], their richness and evenness (alpha diversity) did not significantly differ among the types of sanitizer and the sampling points. We also found that the types of sanitizing washing, apart from a reduced heterogeneity over time, did not significantly influence the community structure of the bacteria (beta diversity).…”

“…The number of ASVs identified (>2000) in the present study was much higher than that observed in the spinach leaf microbiome profiling studies by Gu et al [26] and Söderqvist et al [27], who identified 673 and 190 operational taxonomic units respectively. In addition, 12 phyla were identified in this study compared to the four phyla observed by Söderqvist et al [27] and 14 phyla detected by Gu et al [26]; however, the number of predominant phyla (n = 4) and their relative proportions are similar in all three studies. In agreement with previous observations, the phylum Proteobacteria showed the highest total abundance on day 0, followed by phyla Bacteroidetes, Firmicutes, and Actinobacteria [3,6,26,34].…”

“…The above reads were assigned to 12 distinct phyla, with the majority identified as Proteobacteria (2949 distinct ASVs), followed by Bacteroidetes (1876 ASVs), Actinobacteria (756 ASVs) and Firmicutes (396 ASVs). All other phyla had ≤8 ASVs (Table 1) and were excluded from further analysis [27] as the percentage abundance of these phyla were approximately 0.1% which would not affect the biological interpretation. All ASVs were assigned to one of 65 bacterial family identified and 84% of reads were further assigned to different bacterial genus with 158 genera identified.…”

“…This method, in addition to providing information on the community structure, provides insights into the association of bacterial phyllosphere diversity with environmental factors [6,23], use of biocidal agents [6,23], and pesticides [6,25]. It also provides the interaction dynamics of the composition of the bacterial community with the various stages of plant growth, post-harvest, during processing and storage [3,23,26,27].…”