Genetic relationships of Vibrio parahaemolyticus isolates from clinical, human carrier, and environmental sources in Thailand, determined by multilocus sequence analysis

Abstract

Vibrio parahaemolyticus is a seafood-borne pathogenic bacterium that is a major cause of gastroenteritis worldwide. We investigated the genetic and evolutionary relationships of 101 V. parahaemolyticus isolates originating from clinical, human carrier, and various environmental and seafood production sources in Thailand using multilocus sequence analysis. The isolates were recovered from clinical samples (n = 15), healthy human carriers (n = 18), various types of fresh seafood (n = 18), frozen shrimp (n = 16), fresh-farmed shrimp tissue (n = 18), and shrimp farm water (n = 16). Phylogenetic analysis revealed a high degree of genetic diversity within the V. parahaemolyticus population, although isolates recovered from clinical samples and from farmed shrimp and water samples represented distinct clusters. The tight clustering of the clinical isolates suggests that disease-causing isolates are not a random sample of the environmental reservoir, although the source of infection remains unclear. Extensive serotypic diversity occurred among isolates representing the same sequence types and recovered from the same source at the same time. These findings suggest that the O- and K-antigen-encoding loci are subject to exceptionally high rates of recombination. There was also strong evidence of interspecies horizontal gene transfer and intragenic recombination involving the recA locus in a large proportion of isolates. As the majority of the intragenic recombinational exchanges involving recA occurred among clinical and carrier isolates, it is possible that the human intestinal tract serves as a potential reservoir of donor and recipient strains that is promoting horizontal DNA transfer, driving evolutionary change, and leading to the emergence of new, potentially pathogenic strains.

Fig 1

eBURST analysis of 63 STs of V. parahaemolyticus. The analysis is based on allelic profiles of MLST data and displays clusters of linked and individual unrelated STs. Single-locus variants (SLVs) are illustrated by linkage lines among the nodes. Color coding represents the source of isolation of each ST: red, clinical sample; purple, human carrier; yellow, seafood; green, shrimp tissue; pink, frozen shrimp; dark blue, shrimp farm water. The frequency of each ST is indicated by the size of its node.

Fig 2

Population snapshot of 87 aaSTs of V. parahaemolyticus that were resolved from 348 STs from the V. parahaemolyticus MLST database (http://pubmlst.org/vparahaemolyticus). Two predicted founder groups, aaST2 and aaST34, were identified, and each was surrounded by a ring of subgroup founders and SLVs. Blue represents isolates recovered from the V. parahaemolyticus MLST database, while other colors represent the source of isolation of the Thai isolates in the present study: red, clinical samples; purple, human carrier; yellow, seafood; green, shrimp tissue; pink, frozen shrimp; dark blue, shrimp farm water. The frequency of each aaST is indicated by the size of its node.

Fig 3

Neighbor-joining tree of 102 concatenated sequences of V. parahaemolyticus from multiple sources in Thailand. The numbers at the nodes represent bootstrap values based on 500 replications.

Fig 4

Distribution of polymorphic nucleotide sites among concatenated sequences of 63 STs. The vertical lines represent polymorphic nucleotide sites with respect to the top sequence, ST241. The demarcation and nucleotide lengths of the seven genes are indicated along the bottom scale.