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Culture dependent and independent analysis of bacterial communities associated with commercial salad leaf vegetables.

Jackson CR, Randolph KC, Osborn SL, Tyler HL - BMC Microbiol. (2013)

Bottom Line: Ralstonia sequences made up a greater portion of the community in surface sterilized than non-surface sterilized samples, indicating that it was largely endophytic, while Acinetobacter sequences appeared to be primarily associated with the leaf surface.Analysis of molecular variance indicated there were no significant differences in bacterial community composition between organic versus conventionally grown, or surface-sterilized versus non-sterilized leaf vegetables.The presence of a range of bacterial populations as endophytes presents an interesting phenomenon as these microorganisms cannot be removed by washing and are thus ingested during salad consumption.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Biology, The University of Mississippi, University 38677, USA. cjackson@olemiss.edu.

ABSTRACT

Background: Plants harbor a diverse bacterial community, both as epiphytes on the plant surface and as endophytes within plant tissue. While some plant-associated bacteria act as plant pathogens or promote plant growth, others may be human pathogens. The aim of the current study was to determine the bacterial community composition of organic and conventionally grown leafy salad vegetables at the point of consumption using both culture-dependent and culture-independent methods.

Results: Total culturable bacteria on salad vegetables ranged from 8.0 × 10(3) to 5.5 × 10(8) CFU g(-1). The number of culturable endophytic bacteria from surface sterilized plants was significantly lower, ranging from 2.2 × 10(3) to 5.8 × 10(5) CFU g(-1). Cultured isolates belonged to six major bacterial phyla, and included representatives of Pseudomonas, Pantoea, Chryseobacterium, and Flavobacterium. Eleven different phyla and subphyla were identified by culture-independent pyrosequencing, with Gammaproteobacteria, Betaproteobacteria, and Bacteroidetes being the most dominant lineages. Other bacterial lineages identified (e.g. Firmicutes, Alphaproteobacteria, Acidobacteria, and Actinobacteria) typically represented less than 1% of sequences obtained. At the genus level, sequences classified as Pseudomonas were identified in all samples and this was often the most prevalent genus. Ralstonia sequences made up a greater portion of the community in surface sterilized than non-surface sterilized samples, indicating that it was largely endophytic, while Acinetobacter sequences appeared to be primarily associated with the leaf surface. Analysis of molecular variance indicated there were no significant differences in bacterial community composition between organic versus conventionally grown, or surface-sterilized versus non-sterilized leaf vegetables. While culture-independent pyrosequencing identified significantly more bacterial taxa, the dominant taxa from pyrosequence data were also detected by traditional culture-dependent methods.

Conclusions: The use of pyrosequencing allowed for the identification of low abundance bacteria in leaf salad vegetables not detected by culture-dependent methods. The presence of a range of bacterial populations as endophytes presents an interesting phenomenon as these microorganisms cannot be removed by washing and are thus ingested during salad consumption.

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Similarities of bacterial communities associated with leafy salad vegetables as derived from pyrosequencing. Samples are organically (Org) and conventionally grown baby spinach (Spi), romaine lettuce (Rom), red leaf lettuce (Red), iceberg lettuce (Ice), and green leaf lettuce (Gre) and include intact and surface sterilized (S) subsamples. Community similarity is determined from Jaccard similarity scores followed by nonmetric multidimensional scaling (A) or UPGMA dendrogram construction (B). Analyses are run on subsamples of 1507 sequences from each sample, and show the mean outcome of 1000 individual subsampling runs.
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Figure 3: Similarities of bacterial communities associated with leafy salad vegetables as derived from pyrosequencing. Samples are organically (Org) and conventionally grown baby spinach (Spi), romaine lettuce (Rom), red leaf lettuce (Red), iceberg lettuce (Ice), and green leaf lettuce (Gre) and include intact and surface sterilized (S) subsamples. Community similarity is determined from Jaccard similarity scores followed by nonmetric multidimensional scaling (A) or UPGMA dendrogram construction (B). Analyses are run on subsamples of 1507 sequences from each sample, and show the mean outcome of 1000 individual subsampling runs.

Mentions: NMDS was used to ordinate each sample in order to evaluate community similarity, i.e. to determine if similar endophytic or overall bacterial populations were associated with the different leaf vegetables or sampling treatments. Two dimensional NMDS based on theta dissimilarity scores was sufficient to account for community differences (stress = 0.19, r2 = 0.81), but yielded few consistent patterns in regards to vegetable type, surface sterilization, and organic or conventional production (Figure 3A). AMOVA confirmed this, with there being no statistically significant differences between samples based on groupings of organic versus conventional (p = 0.17), or surface sterilized versus non-sterilized (p = 0.23). Date of sample purchase was likewise not related to community composition (p = 0.38). Vegetable type did result in significantly different groupings of samples (p = 0.006), however no individual comparisons between pairs of salad vegetable types were significant following the Bonferroni correction (p > 0.005 for all). This pattern based on salad vegetable type was largely driven by the bacterial community associated with the samples of romaine lettuce, which while not statistically significantly different from that on any other individual lettuce type, had a low probability of occurring by chance (p = 0.016-0.049 for the various comparisons). The dendrogram of community similarity (Figure 3B) also showed no consistent separation of endophyte (surface sterilized) assemblages from overall plant associated bacterial communities, a finding that was confirmed by the UniFrac analysis (D = 0.69, p = 0.516). The UniFrac metric did suggest a marginally significant difference between organic and conventionally grown samples (D = 0.79, p = 0.04), but no overall effect of lettuce type (pairwise D scores 0.70-0.84, p > 0.10 for all). A survey of native plants on a prairie reserve found that host plant species did have a significant effect on the leaf endophyte community[28], although that study examined five quite different plant species, rather than the five similar varieties of salad vegetables sampled in this study. Different types of produce ranging from mushrooms to apples have been found to have distinct bacterial communities on their surface, although certain produce types (e.g. spinach, lettuce, sprouts) may have more similar phyllosphere communities[19], as reported here.


Culture dependent and independent analysis of bacterial communities associated with commercial salad leaf vegetables.

Jackson CR, Randolph KC, Osborn SL, Tyler HL - BMC Microbiol. (2013)

Similarities of bacterial communities associated with leafy salad vegetables as derived from pyrosequencing. Samples are organically (Org) and conventionally grown baby spinach (Spi), romaine lettuce (Rom), red leaf lettuce (Red), iceberg lettuce (Ice), and green leaf lettuce (Gre) and include intact and surface sterilized (S) subsamples. Community similarity is determined from Jaccard similarity scores followed by nonmetric multidimensional scaling (A) or UPGMA dendrogram construction (B). Analyses are run on subsamples of 1507 sequences from each sample, and show the mean outcome of 1000 individual subsampling runs.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4219373&req=5

Figure 3: Similarities of bacterial communities associated with leafy salad vegetables as derived from pyrosequencing. Samples are organically (Org) and conventionally grown baby spinach (Spi), romaine lettuce (Rom), red leaf lettuce (Red), iceberg lettuce (Ice), and green leaf lettuce (Gre) and include intact and surface sterilized (S) subsamples. Community similarity is determined from Jaccard similarity scores followed by nonmetric multidimensional scaling (A) or UPGMA dendrogram construction (B). Analyses are run on subsamples of 1507 sequences from each sample, and show the mean outcome of 1000 individual subsampling runs.
Mentions: NMDS was used to ordinate each sample in order to evaluate community similarity, i.e. to determine if similar endophytic or overall bacterial populations were associated with the different leaf vegetables or sampling treatments. Two dimensional NMDS based on theta dissimilarity scores was sufficient to account for community differences (stress = 0.19, r2 = 0.81), but yielded few consistent patterns in regards to vegetable type, surface sterilization, and organic or conventional production (Figure 3A). AMOVA confirmed this, with there being no statistically significant differences between samples based on groupings of organic versus conventional (p = 0.17), or surface sterilized versus non-sterilized (p = 0.23). Date of sample purchase was likewise not related to community composition (p = 0.38). Vegetable type did result in significantly different groupings of samples (p = 0.006), however no individual comparisons between pairs of salad vegetable types were significant following the Bonferroni correction (p > 0.005 for all). This pattern based on salad vegetable type was largely driven by the bacterial community associated with the samples of romaine lettuce, which while not statistically significantly different from that on any other individual lettuce type, had a low probability of occurring by chance (p = 0.016-0.049 for the various comparisons). The dendrogram of community similarity (Figure 3B) also showed no consistent separation of endophyte (surface sterilized) assemblages from overall plant associated bacterial communities, a finding that was confirmed by the UniFrac analysis (D = 0.69, p = 0.516). The UniFrac metric did suggest a marginally significant difference between organic and conventionally grown samples (D = 0.79, p = 0.04), but no overall effect of lettuce type (pairwise D scores 0.70-0.84, p > 0.10 for all). A survey of native plants on a prairie reserve found that host plant species did have a significant effect on the leaf endophyte community[28], although that study examined five quite different plant species, rather than the five similar varieties of salad vegetables sampled in this study. Different types of produce ranging from mushrooms to apples have been found to have distinct bacterial communities on their surface, although certain produce types (e.g. spinach, lettuce, sprouts) may have more similar phyllosphere communities[19], as reported here.

Bottom Line: Ralstonia sequences made up a greater portion of the community in surface sterilized than non-surface sterilized samples, indicating that it was largely endophytic, while Acinetobacter sequences appeared to be primarily associated with the leaf surface.Analysis of molecular variance indicated there were no significant differences in bacterial community composition between organic versus conventionally grown, or surface-sterilized versus non-sterilized leaf vegetables.The presence of a range of bacterial populations as endophytes presents an interesting phenomenon as these microorganisms cannot be removed by washing and are thus ingested during salad consumption.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Biology, The University of Mississippi, University 38677, USA. cjackson@olemiss.edu.

ABSTRACT

Background: Plants harbor a diverse bacterial community, both as epiphytes on the plant surface and as endophytes within plant tissue. While some plant-associated bacteria act as plant pathogens or promote plant growth, others may be human pathogens. The aim of the current study was to determine the bacterial community composition of organic and conventionally grown leafy salad vegetables at the point of consumption using both culture-dependent and culture-independent methods.

Results: Total culturable bacteria on salad vegetables ranged from 8.0 × 10(3) to 5.5 × 10(8) CFU g(-1). The number of culturable endophytic bacteria from surface sterilized plants was significantly lower, ranging from 2.2 × 10(3) to 5.8 × 10(5) CFU g(-1). Cultured isolates belonged to six major bacterial phyla, and included representatives of Pseudomonas, Pantoea, Chryseobacterium, and Flavobacterium. Eleven different phyla and subphyla were identified by culture-independent pyrosequencing, with Gammaproteobacteria, Betaproteobacteria, and Bacteroidetes being the most dominant lineages. Other bacterial lineages identified (e.g. Firmicutes, Alphaproteobacteria, Acidobacteria, and Actinobacteria) typically represented less than 1% of sequences obtained. At the genus level, sequences classified as Pseudomonas were identified in all samples and this was often the most prevalent genus. Ralstonia sequences made up a greater portion of the community in surface sterilized than non-surface sterilized samples, indicating that it was largely endophytic, while Acinetobacter sequences appeared to be primarily associated with the leaf surface. Analysis of molecular variance indicated there were no significant differences in bacterial community composition between organic versus conventionally grown, or surface-sterilized versus non-sterilized leaf vegetables. While culture-independent pyrosequencing identified significantly more bacterial taxa, the dominant taxa from pyrosequence data were also detected by traditional culture-dependent methods.

Conclusions: The use of pyrosequencing allowed for the identification of low abundance bacteria in leaf salad vegetables not detected by culture-dependent methods. The presence of a range of bacterial populations as endophytes presents an interesting phenomenon as these microorganisms cannot be removed by washing and are thus ingested during salad consumption.

Show MeSH
Related in: MedlinePlus