Evaluation of whole and core genome multilocus sequence typing allele schemes for Salmonella enterica outbreak detection in a national surveillance network, PulseNet USA.

Salmonella enterica is a leading cause of bacterial foodborne and zoonotic illnesses in the United States. For this study, we applied four different whole genome sequencing (WGS)-based subtyping methods: high quality single-nucleotide polymorphism (hqSNP) analysis, whole genome multilocus sequence typing using either all loci [wgMLST (all loci)] and only chromosome-associated loci [wgMLST (chrom)], and core genome multilocus sequence typing (cgMLST) to a dataset of isolate sequences from 9 well-characterized Salmonella outbreaks. For each outbreak, we evaluated the genomic and epidemiologic concordance between hqSNP and allele-based methods. We first compared pairwise genomic differences using all four methods. We observed discrepancies in allele difference ranges when using wgMLST (all loci), likely caused by inflated genetic variation due to loci found on plasmids and/or other mobile genetic elements in the accessory genome. Therefore, we excluded wgMLST (all loci) results from any further comparisons in the study. Then, we created linear regression models and phylogenetic tanglegrams using the remaining three methods. K-means analysis using the silhouette method was applied to compare the ability of the three methods to partition outbreak and sporadic isolate sequences. Our results showed that pairwise hqSNP differences had high concordance with cgMLST and wgMLST (chrom) allele differences. The slopes of the regressions for hqSNP vs. allele pairwise differences were 0.58 (cgMLST) and 0.74 [wgMLST (chrom)], and the slope of the regression was 0.77 for cgMLST vs. wgMLST (chrom) pairwise differences. Tanglegrams showed high clustering concordance between methods using two statistical measures, the Baker's gamma index (BGI) and cophenetic correlation coefficient (CCC), where 9/9 (100%) of outbreaks yielded BGI values ≥ 0.60 and CCCs were ≥ 0.97 across all nine outbreaks and all three methods. K-means analysis showed separation of outbreak and sporadic isolate groups with average silhouette widths ≥ 0.87 for outbreak groups and ≥ 0.16 for sporadic groups. This study demonstrates that Salmonella isolates clustered in concordance with epidemiologic data using three WGS-based subtyping methods and supports using cgMLST as the primary method for national surveillance of Salmonella outbreak clusters.

Multidrug-Resistant Salmonella Serotype Anatum in Travelers and Seafood from Asia, United States.

A multidrug-resistant Salmonella enterica serotype Anatum strain reported in Taiwan was isolated in the United States from patients and from seafood imported from Asia. Isolates harbored 11 resistance determinants, including quinolone and inducible cephalosporin resistance genes. Most patients had traveled to Asia. These findings underscore the need for global One Health resistance surveillance.

Sequencing and Characterization of Five Extensively Drug-Resistant Salmonella enterica Serotype Typhi Isolates Implicated in Human Infections from Punjab, Pakistan.

A large outbreak of extensively drug-resistant (XDR) Salmonella enterica serotype Typhi infections is ongoing in Pakistan, predominantly in Sindh Province. Here, we report the sequencing and characterization of five XDR Salmonella Typhi isolates from the Punjab province of Pakistan that are closely related to the outbreak strain and carry the same IncY plasmid.

Pulsed-field gel electrophoresis subtypes of shiga toxin-producing Escherichia coli O157 isolated from ground beef and humans, United States, 2001-2006.

Pulsed-field gel electrophoresis XbaI patterns of Shiga toxin-producing Escherichia coli O157 (STEC O157) isolates (n = 156) found in ground beef sampled from U.S. processing plants and retail stores during 2001 to 2006 were summarized and compared with XbaI patterns from human STEC O157 isolates (n = 14,591) in the national PulseNet E. coli database. Four ground beef samples contained more than one pulsed-field gel electrophoresis subtype of STEC O157. Of the 117 unique patterns found in ground beef, 100 (85%) appeared only once, and 17 (15%) were found in more than one isolate. The six patterns that appeared most frequently in human isolates were also found among the eight most common ground beef patterns. The yearly proportion of human isolates with the two most common patterns changed inversely, such that these patterns traded dominance over the study period. Human isolates with patterns that were first detected in both ground beef and humans contemporaneously were clustered in a 6-month window around the time of the respective ground beef sample. Of the 156 ground beef isolates, 82 (53%) were indistinguishable from at least one human isolate in this 6-month window. The yearly proportions of human STEC O157 isolates that were indistinguishable from ground beef isolates decreased significantly from 2002 to 2003 (12.3-0.8%), and then increased significantly from 2003 to 2006 (overall 0.8-12.6%). This increase in the numbers of human isolates that matched a ground beef isolate occurred during a period of relatively consistent rates of ground beef contamination with STEC O157. Pattern similarity of STEC O157 isolates derived from ground beef and clinical cases may serve as a good predictor of human incidence trends.