Human intestinal tract - the driving force of evolution of Vibrio parahaemolyticus?

Gastroenteritis obtained from undercooked or contaminated shellfish is a problem worldwide. One of the key pathogens that cause this infection is Vibrio parahaemolyticus. Theethakaew et al. (2013) studied the genetic and evolutionary relationships between isolates obtained in Thailand from clinical, human carriers and environmental sources. Since there is such a high consumption of seafood in Thailand, knowledge concerning this pathogen is of importance to allow the development of effective intervention strategies that prevent risk of infection of the public and economic costs to seafood producers.

Multilocus sequence typing (MLST) has been used in many other studies to analyse population genetics and molecular epidemiology of bacterial pathogens. This study used a modified version of MLST to identify potential sources of infection by analysing the population structure of V. parahaemolyticus. They were unable to find the most likely epidemiological sources of human carrier isolates, but determined that they were not always connected with seafood sources. There were five human carrier isolates that were identical to clinical isolates, indicating a genetic link between the two. Therefore human carriers may potentially transmit pathogens directly or indirectly to others or may contaminate seafood during production methods. However, 10 other human carrier isolates were also found, suggesting the human intestinal tract may act as a reservoir for novel V. parahaemolyticus strains.

A Bayesain analysis was also used. Although this was unable to differentiate between the isolates of the different epidemiological sources, they found the protein recA has an important role in the population structure of V. parahaemolyticus. Two divergent recA alleles were present. Nucelotide Blast analysis demonstrated these alleles to be closely linked to V. cincinnatiensis, a human bacterial pathogen and V. halioticoli, a bacterial pathogen found in the guts of the abalone mollusc, confirming these alleles had been obtained via horizontal gene transfer from other species. Other studies have also recorded a high diversity of recA in a range of Vibrio species, suggesting this is not a suitable marker for evolutionary analysis.

As recA has an intricate mosaic structure, intragenic recombination was also found to be an important aspect in the evolution of V. parahaemolyticus. Other species of Vibrio have likewise demonstrated intragenic recombination of recA, suggesting this may be a common source of recombination in this species. Since the study found human carrier and clinical isolates to contain mosaic recA proteins most frequently it could be suggested recombination occurs in the human intestinal tract more often than in the environment. Other studies investigating human carriers in seafood factories found a low number of carriers, however these individuals were found to harbour multiple strains of V. parahaemolyticus for a prolonged period of time. The existence of concurrent multiple strains supplies evidence for the exchange of DNA both within and between strains when in the intestinal tract, suggesting this may be an important driving force in the evolution of V. parahaemolyticus. Nonetheless, further studies are needed to determine the residence time of this pathogens in humans, the rate of horizontal gene transfer and ultimately the frequency at which new strains are evolving in order to develop successful intervention strategies of this pathogen.

Genetic Relationships of Vibrio parahaemolyticus Isolates from Clinical, Human Carrier, and Environmental Sources in Thailand, Determined by Multilocus Sequence Analysis

Chonchanok Theethakaew^a, Edward J. Feil^c,  Santiago Castillo-Ramírez^c, David M. Aanensen^d,  Orasa Suthienkul^e,  Douglas M. Neil^b and Robert L. Davies^a

http://aem.asm.org/content/79/7/2358.full?sid=46f4d79f-c54e-425e-b474-3221917da9b4