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High resolution clustering of Salmonella enterica serovar Montevideo strains using a next-generation sequencing approach.

Allard MW, Luo Y, Strain E, Li C, Keys CE, Son I, Stones R, Musser SM, Brown EW - BMC Genomics (2012)

Bottom Line: In no case, however, did variability associated with sequencing methods or sample preparations create inconsistencies with our current phylogenetic results or the subsequent molecular epidemiological evidence gleaned from these data.Implementation of a validated pipeline for NGS data acquisition and analysis provides highly reproducible results that are stable and predictable for molecular epidemiological applications.This reproducibility applies to all levels within and between serovars of Salmonella suggesting that investigators using these methods can have confidence in their conclusions.

View Article: PubMed Central - HTML - PubMed

Affiliation: Office of Regulatory Science, Center for Food Safety & Applied Nutrition, U,S, Food & Drug Administration, 5100 Paint Branch Parkway, College Park, MD 20740, USA. Marc.Allard@fda.hhs.gov

ABSTRACT

Background: Next-Generation Sequencing (NGS) is increasingly being used as a molecular epidemiologic tool for discerning ancestry and traceback of the most complicated, difficult to resolve bacterial pathogens. Making a linkage between possible food sources and clinical isolates requires distinguishing the suspected pathogen from an environmental background and placing the variation observed into the wider context of variation occurring within a serovar and among other closely related foodborne pathogens. Equally important is the need to validate these high resolution molecular tools for use in molecular epidemiologic traceback. Such efforts include the examination of strain cluster stability as well as the cumulative genetic effects of sub-culturing on these clusters. Numerous isolates of S. Montevideo were shot-gun sequenced including diverse lineage representatives as well as numerous replicate clones to determine how much variability is due to bias, sequencing error, and or the culturing of isolates. All new draft genomes were compared to 34 S. Montevideo isolates previously published during an NGS-based molecular epidemiological case study.

Results: Intraserovar lineages of S. Montevideo differ by thousands of SNPs, that are only slightly less than the number of SNPs observed between S. Montevideo and other distinct serovars. Much less variability was discovered within an individual S. Montevideo clade implicated in a recent foodborne outbreak as well as among individual NGS replicates. These findings were similar to previous reports documenting homopolymeric and deletion error rates with the Roche 454 GS Titanium technology. In no case, however, did variability associated with sequencing methods or sample preparations create inconsistencies with our current phylogenetic results or the subsequent molecular epidemiological evidence gleaned from these data.

Conclusions: Implementation of a validated pipeline for NGS data acquisition and analysis provides highly reproducible results that are stable and predictable for molecular epidemiological applications. When draft genomes are collected at 15×-20× coverage and passed through a quality filter as part of a data analysis pipeline, including sub-passaged replicates defined by a few SNPs, they can be accurately placed in a phylogenetic context. This reproducibility applies to all levels within and between serovars of Salmonella suggesting that investigators using these methods can have confidence in their conclusions.

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Phylogenetic GARLI tree from resequencing of matching human-food isolate pairs, individual colonies, and sub-passages of a single strain of S. Montevideo. Terminal names, scale bar, and branch lengths are as in Figure 1. The tree was rooted with two outgroup isolates, both of which were obtained from Pistachio. The laboratory-generated isolates were indistinguishable in a phylogenetic analysis with all replicates clustering together. Some of the biological, laboratory, or technical replicate genomes yielded nucleotide differences and these are seen as longer terminal branches for several isolates on the tree. These few changes did not alter relatedness or inclusivity/exclusivity among the matching food/human isolates.
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Figure 5: Phylogenetic GARLI tree from resequencing of matching human-food isolate pairs, individual colonies, and sub-passages of a single strain of S. Montevideo. Terminal names, scale bar, and branch lengths are as in Figure 1. The tree was rooted with two outgroup isolates, both of which were obtained from Pistachio. The laboratory-generated isolates were indistinguishable in a phylogenetic analysis with all replicates clustering together. Some of the biological, laboratory, or technical replicate genomes yielded nucleotide differences and these are seen as longer terminal branches for several isolates on the tree. These few changes did not alter relatedness or inclusivity/exclusivity among the matching food/human isolates.

Mentions: As expected, these laboratory-generated isolates were indistinguishable in a phylogenetic analysis with the single parsimony informative SNP separating the 237_Lunch_Meat_IA_1 S. Montevideo isolate series from the other Iowa matching clinical-food isolates (Figure 5). Among the replicate genomes, only two sequences, genomes from S. Montevideo isolate 237 from the second and third round passages, retained actual SNP variation that emerged on the tree. That is, save for a single nucleotide difference present in the original 237 sample and two of the four downstream passages (i.e., 237-second round and 237-third round), none of the additional biological, laboratory, or technical replicate genomes yielded nucleotide differences after alignment and quality filtration. It is important to note that these few changes did not alter relatedness or inclusivity/exclusivity among the matching food/human isolates. Rather, the only structural difference in the tree to arise from these three changes was in the form of branch length for the individual isolates affected. Additionally, it is noteworthy that, in the larger outbreak clone tree, the Connecticut and Iowa matching isolates were both phylogenetically inseparable from their sister isolates, and collectively, both strain sets sorted squarely among the spiced-meat food, environmental, and clinical isolates associated with the same contamination event (Figure 3). These findings indicate that when NGS data are quality filtered and inspected carefully using inclusivity/exclusivity criteria, the resultant stable and informative SNP data can be used effectively to phylogenetically partition closely related isolates of S. enterica (i.e., S. Montevideo). Consistent with this find, a bolus of successful applications is now accruing [17-20].


High resolution clustering of Salmonella enterica serovar Montevideo strains using a next-generation sequencing approach.

Allard MW, Luo Y, Strain E, Li C, Keys CE, Son I, Stones R, Musser SM, Brown EW - BMC Genomics (2012)

Phylogenetic GARLI tree from resequencing of matching human-food isolate pairs, individual colonies, and sub-passages of a single strain of S. Montevideo. Terminal names, scale bar, and branch lengths are as in Figure 1. The tree was rooted with two outgroup isolates, both of which were obtained from Pistachio. The laboratory-generated isolates were indistinguishable in a phylogenetic analysis with all replicates clustering together. Some of the biological, laboratory, or technical replicate genomes yielded nucleotide differences and these are seen as longer terminal branches for several isolates on the tree. These few changes did not alter relatedness or inclusivity/exclusivity among the matching food/human isolates.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3368722&req=5

Figure 5: Phylogenetic GARLI tree from resequencing of matching human-food isolate pairs, individual colonies, and sub-passages of a single strain of S. Montevideo. Terminal names, scale bar, and branch lengths are as in Figure 1. The tree was rooted with two outgroup isolates, both of which were obtained from Pistachio. The laboratory-generated isolates were indistinguishable in a phylogenetic analysis with all replicates clustering together. Some of the biological, laboratory, or technical replicate genomes yielded nucleotide differences and these are seen as longer terminal branches for several isolates on the tree. These few changes did not alter relatedness or inclusivity/exclusivity among the matching food/human isolates.
Mentions: As expected, these laboratory-generated isolates were indistinguishable in a phylogenetic analysis with the single parsimony informative SNP separating the 237_Lunch_Meat_IA_1 S. Montevideo isolate series from the other Iowa matching clinical-food isolates (Figure 5). Among the replicate genomes, only two sequences, genomes from S. Montevideo isolate 237 from the second and third round passages, retained actual SNP variation that emerged on the tree. That is, save for a single nucleotide difference present in the original 237 sample and two of the four downstream passages (i.e., 237-second round and 237-third round), none of the additional biological, laboratory, or technical replicate genomes yielded nucleotide differences after alignment and quality filtration. It is important to note that these few changes did not alter relatedness or inclusivity/exclusivity among the matching food/human isolates. Rather, the only structural difference in the tree to arise from these three changes was in the form of branch length for the individual isolates affected. Additionally, it is noteworthy that, in the larger outbreak clone tree, the Connecticut and Iowa matching isolates were both phylogenetically inseparable from their sister isolates, and collectively, both strain sets sorted squarely among the spiced-meat food, environmental, and clinical isolates associated with the same contamination event (Figure 3). These findings indicate that when NGS data are quality filtered and inspected carefully using inclusivity/exclusivity criteria, the resultant stable and informative SNP data can be used effectively to phylogenetically partition closely related isolates of S. enterica (i.e., S. Montevideo). Consistent with this find, a bolus of successful applications is now accruing [17-20].

Bottom Line: In no case, however, did variability associated with sequencing methods or sample preparations create inconsistencies with our current phylogenetic results or the subsequent molecular epidemiological evidence gleaned from these data.Implementation of a validated pipeline for NGS data acquisition and analysis provides highly reproducible results that are stable and predictable for molecular epidemiological applications.This reproducibility applies to all levels within and between serovars of Salmonella suggesting that investigators using these methods can have confidence in their conclusions.

View Article: PubMed Central - HTML - PubMed

Affiliation: Office of Regulatory Science, Center for Food Safety & Applied Nutrition, U,S, Food & Drug Administration, 5100 Paint Branch Parkway, College Park, MD 20740, USA. Marc.Allard@fda.hhs.gov

ABSTRACT

Background: Next-Generation Sequencing (NGS) is increasingly being used as a molecular epidemiologic tool for discerning ancestry and traceback of the most complicated, difficult to resolve bacterial pathogens. Making a linkage between possible food sources and clinical isolates requires distinguishing the suspected pathogen from an environmental background and placing the variation observed into the wider context of variation occurring within a serovar and among other closely related foodborne pathogens. Equally important is the need to validate these high resolution molecular tools for use in molecular epidemiologic traceback. Such efforts include the examination of strain cluster stability as well as the cumulative genetic effects of sub-culturing on these clusters. Numerous isolates of S. Montevideo were shot-gun sequenced including diverse lineage representatives as well as numerous replicate clones to determine how much variability is due to bias, sequencing error, and or the culturing of isolates. All new draft genomes were compared to 34 S. Montevideo isolates previously published during an NGS-based molecular epidemiological case study.

Results: Intraserovar lineages of S. Montevideo differ by thousands of SNPs, that are only slightly less than the number of SNPs observed between S. Montevideo and other distinct serovars. Much less variability was discovered within an individual S. Montevideo clade implicated in a recent foodborne outbreak as well as among individual NGS replicates. These findings were similar to previous reports documenting homopolymeric and deletion error rates with the Roche 454 GS Titanium technology. In no case, however, did variability associated with sequencing methods or sample preparations create inconsistencies with our current phylogenetic results or the subsequent molecular epidemiological evidence gleaned from these data.

Conclusions: Implementation of a validated pipeline for NGS data acquisition and analysis provides highly reproducible results that are stable and predictable for molecular epidemiological applications. When draft genomes are collected at 15×-20× coverage and passed through a quality filter as part of a data analysis pipeline, including sub-passaged replicates defined by a few SNPs, they can be accurately placed in a phylogenetic context. This reproducibility applies to all levels within and between serovars of Salmonella suggesting that investigators using these methods can have confidence in their conclusions.

Show MeSH
Related in: MedlinePlus