A non-autochthonous US strain of Vibrio parahaemolyticus isolated from Chesapeake Bay oysters caused the outbreak in Maryland in 2010
In the summer of 2010, V. parahaemolyticus caused an outbreak in the state of Maryland linked to consumption of oysters. The strains were isolated from both stool and oyster samples, and were indistinguishable by PFGE. However, the oysters also contained other potentially pathogenic V. parahaemolyticus strains, exhibiting different PFGE patterns. In order to assess the identity, genetic makeup, relatedness, and potential pathogenicity of the other V. parahaemolyticus strains, we sequenced 11 such strains (2 clinical strains and 9 oyster strains). We analyzed these genomes by in silico MLST and determined their phylogeny using a whole-genome MLST (wgMLST) analysis. Our in silico MLST identified six different STs (ST8, ST676, ST810, ST811, ST34, and ST768), with both clinical and four of the oyster strains identified as ST8. Using wgMLST, we showed that the ST8 strains from clinical and oyster samples were mostly indistinguishable and belonged to the same outbreak, confirming that local oysters were the source of the infections. The remaining oyster strains were genetically diverse, differing in more than 3000 loci from the MD ST8 strains. eBURST analysis comparing these strains with other STs available at the V. parahaemolyticus MLST website, showed that the MD ST8 strains belonged to a clonal complex endemic in Asia. This indicates that the ST8 isolates from clinical and oyster sources were likely not endemic to MD water and points to a possible source of introduction into this area.