Vibrio parahaemolyticus (V. parahaemolyticus) is a Gram-negative, halophilic asporogenous rod. This bacterium is found in temperate and tropical marine and coastal waters globally[1]. Human infections are most commonly associated with the consumption of raw, undercooked, or contaminated shellfish[2]. After infection, it usually causes acute and severe gastroenteritis and gastrointestinal bleeding, characterized by fever, vomiting, diarrhea, and blood in the stool[3]. It is also the major cause of acute hepatopancreatic necrosis disease in shrimp[4].
The serotyping scheme of V. parahaemolyticus is based on the combination of O and K antigens, and includes over 10 different O antigens and more than 70 different K types. The most common clone of V. parahaemolyticus among infections is serotype O3:K6 and its serovariants, leading to an epidemic worldwide since 1996[5]. Additionally, the V. parahaemolyticus serotype O4:K12 has caused gastroenteritis outbreaks in the Pacific Northwest region in the USA around 1997, and on the Atlantic coasts of the USA and Spain in 2012. In China, the V. parahaemolyticus serotype O4:K12 caused gastroenteritis outbreaks in 2006, 2010, 2011 and 2014 in Shanghai[6].
There are four geographically related populations of V. parahaemolyticus, VppUS1, VppUS2, VppX and VppAsia, identified previously, with diverse clones descending from each population[7]. The first two are largely restricted to the US and Northern Europe, while the others are found worldwide, with VppAsia making up the great majority of isolates in the seas around Asia. Patterns of diversity within and between the populations are consistent with them having arisen by progressive divergence via genetic drift during geographical isolation[7].
The life cycle of V. parahaemolyticus is facilitated by a vast array of virulence factors. In addition to the well-known virulence genes of thermostable direct haemolysin (tdh) and/or tdh-related haemolysin (trh), genome sequencing revealed that two type III secretion systems (T3SS1 and T3SS2) were also related to the pathogenicity of V. parahaemolyticus[8]. T3SS1 genes were ubiquitous in V. parahaemolyticus and were associated with the cytotoxic activity; while T3SS2 genes were mainly found in clinical isolates and were associated with the enterotoxicity. There are two versions of T3SS2, including T3SS2α and T3SS2β. The tdh gene and T3SS2α encoding genes are adjacent to each other, both locating on the confirmed pathogenicity island VPaI-7; while the trh gene and T3SS2β genes are adjacent, both locating on the homologous genome island of VPaI-7[9]. Two types VI secretion systems, including T6SS1 and T6SS2, were identified in V. parahaemolyticus. T6SS1 was more frequently found in clinical isolates than environmental isolates, contributing to the adhesion to host cells[10]. Comparative genomic analysis of pandemic and non-pandemic isolates revealed seven genomic islands (GIs), including VPaI-1–7. VPaI-1, VPaI-4, VPaI-5, and VPaI-6 are only found in pandemic isolates[11]. These GIs range from 10 to 81 kb in size, and their average GC content is lower than that of the overall genomes, indicating that they were acquired by horizontal gene transfer.
With the exception of penicillin-like antibiotics, such as penicillin, ampicillin, and amoxicillin, V. parahaemolyticus is usually sensitive to most antibiotics of veterinary and human significance, such as chloramphenicol, tetracyclines, and quinolone[12]. Although medical treatment is not necessary in mild infections, antibiotics are sometimes used in severe or prolonged illnesses. V. parahaemolyticus has been reported to show multidrug resistance during aquaculture production[13].
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