Pathogenic Characteristics of Shrimp Early Mortality Syndrome (EMS)-Causing Vibrio parahaemolyticus: A Comparative Transcriptomic Study Suggests the Relationship Between Metabolic Fitness and Virulence Gene Expression.

Vibrio parahaemolyticus (VP) is a major bacterial species that causes early mortality syndrome (EMS) in shrimps. EMS can be classified into two groups based on histological signs of hepatopancreatic tissues, i.e., acute hepatopancreatic necrosis disease (AHPND) and non-AHPND. To investigate how toxin-producing AHPND and toxin-lacking non-AHPND VP could lead to EMS, growth characteristics and transcriptomic analyses of the representative strains, 5HP and 2HP, were compared. Non-pathogenic VP represented by strain S02 was also used. Two types of growth media included rich medium represented by tryptic soy broth plus 1.5% NaCl (TSB) and depleted media represented by artificial seawater (SW) and spent seawater (SSW). SSW refers to a sterile used-SW medium from healthy shrimp rearing or shrimp-conditioned SW. Growth characteristics under these media indicated that TSB and SSW supported better growth than SW, suggesting shrimp-conditioned SW is sufficient to support normal VP growth. Transcriptomic analyses revealed that both EMS isolates shared overall expression patterns. Metabolic stress adaptation systems of non-AHPND strain 2HP under SW and SSW were more upregulated than in AHPND strain 5HP. Specific virulence genes (i.e., zot1 [zonula occludens toxin] and vopS [type III secretion effector]) and a general stress response gene (i.e., rpoS [stress response sigma factor]) were upregulated in strain 2HP under both SW and SSW. These expression profiles of strain 2HP suggest higher persistence, which might be useful for cell survival and non-AHPND pathogenesis even without toxin production. We proposed that these genes encode virulence factor 'candidates' in non-AHPND VP.