Microevolution of Vibrio parahaemolyticus Isolated from Clinical, Acute Hepatopancreatic Necrosis Disease Infecting Shrimps, and Aquatic Production in China.

Vibrio parahaemolyticus is the leading cause of bacteria-associated foodborne diarrheal diseases and specifically causes early mortality syndrome (EMS), which is technically known as acute hepatopancreatic necrosis disease (AHPND), a serious threat to shrimp aquaculture. To investigate the genetic and evolutionary relationships of V. parahaemolyticus in China, 184 isolates from clinical samples (VPC, n=40), AHPND-infected shrimp (VPE, n=10), and various aquatic production sources (VPF, n=134) were collected and evaluated by a multilocus sequence analysis (MLST). Furthermore, the presence of potential virulence factors (tlh, tdh, and trh) and single nucleotide polymorphisms (SNPs) in V. parahaemolyticus isolates was assessed using genomic sequencing. Analyses of virulence factors revealed that the majority of VPC isolates (97.5%) possessed the tdh and/or trh genes, while most of the VPF isolates (83.58%) did not encode hemolysin genes. Therefore, we hypothesized that the environment is a potential reservoir that promotes horizontal DNA transfer, which drives evolutionary change that, in turn, leads to the emergence of novel, potentially pathogenic strains. Phylogenetic analyses identified VPF-112 as a non-pathogenic maternal strain isolated from aquatic products and showed that it had a relatively high evolutionary status. All VPE strains and some VPC strains were grouped into several small subgroups and evenly distributed on phylogenetic trees. Anthropogenic activities and environmental selective pressure may be important factors influencing the process of transforming strains from non-pathogenic to pathogenic bacteria.