**3. Pathogenicity of** *Vibrio*

*Vibrio* is usually reported being related to food-borne diseases and aquaculture diseases. Diseases or even death of aquatic animals caused by *Vibrio* infection have been reported worldwide and are showing an increasing trend [7, 11, 18, 67]. Therefore, Vibriosis has caused significant impacts on human health and the development of aquaculture [68–70]. The pathogenicity of *Vibrio* is determined by multiple virulence factors encoded by its virulence genes [71–74]. *Vibrio* infects and destroys the host through a series of processes, including adhesion, invasion, immune escape, in vivo proliferation and production of toxins [46]. *Vibrio* mainly includes adhesion factor, the capsule and polysaccharide, cytotoxin and other virulence factors [75]. Therefore, infection and pathogenesis of *Vibrio* are not completed by a single virulence factor, but the result of the combined action of multiple virulence factors.

#### **3.1 Adhesion factor**

Adhesion is the prerequisite for pathogenic bacteria to cause disease to the body infection, and it is of great significance in invading the host and effectively exerting the virulence [76]. Adhesion is mainly achieved by adhesion factors that specifically recognize and bind to host cells, the ability of *Vibrio* to adhere to and form biofilm on the surfaces of organisms and non-organisms can enhance the virulence and pathogenicity of *Vibrio* [77]. Moreover, adhesion is strongly related to the biofilm formation ability, movement ability and quorum sensing of bacterial [78].

Adhesion factor is a kind of macromolecular substance that can make pathogenic bacteria adhere to the surface of eukaryotic cells, and it plays an important role in the host infection process of *Vibrio*. *Vibrio* has a variety of adhesin, such as fimbriae, cilia, outer membrane protein (OMP), lipopolysaccharide (LPS), extracellular polysaccharide, etc. Among them, fimbriae and cilia belong to fimbriae adhesin, while OMP, LPS, and extracellular polysaccharides belong to non-pilin adhesin [79]. The exposure of OMP to the bacterial surface is a unique and important component of the outer membrane of Gram-negative bacteria, which plays an important role in maintaining the outer membrane structure, ensuring the transport of substances, and stimulating the body to produce antibodies and cytokines. It has been found that the outer membrane protein is an important pathogenic factor closely related to the process of bacterial adhesion and iron uptake [80]. LPS is a lipopolysaccharide substance located in the outermost layer of the cell wall of Gram-negative bacteria. It is not only the main component of the cell wall of Gramnegative bacteria, but also the material basis for the endotoxin of the virulence factor of Gram-negative pathogens [81]. In *Vibrio* pathogens (such as *V. anguillarum*, *V.*  *Community Change and Pathogenicity of* Vibrio *DOI: http://dx.doi.org/10.5772/intechopen.96515*

*vulnificus, V. cholerae*, *V. mimicus*, *V. parahaemolyticus*, etc.), LPS has been proven to be an important pathogenic factor [82].

Although *Vibrio* has a variety of adhesion factors, previous studies on the adhesion of *Vibrio* generally focused on flagellum, and some researchers proposed that flagellum plays an irreplaceable role in the adhesion process of bacteria-infected host [83]. According to the location of the flagellum, the flagellum can be divided into two types: terminal flagellum and peri-flagellum. Belas investigated the differences in the adsorption characteristics of *Vibrio* with different types of flagellum on chitin, and found that peri-flagellum had a stronger affinity for chitin than terminal flagellum [84]. The adhesion of *V. alginolyticus* to the epidermal mucus of *Sparus macrocephalus* was studied by Bordas, which also proved this viewpoint [85]. In addition, studies have found that pili seems to have a strong correlation with the pathogenicity of *Vibrio*. Wright first found in 1989, most clinical isolates of *V. vulnificus* have pili, while environmental isolates lack pili [86]. Analysis of pili protein gene expression during infection indicated that the pili protein expression of the strong strain was higher than that of the weak strain [87]. Moreover, the mobility of the strain was also one of the main factors affecting the adhesion. Kogure found that under the condition of having flagella at the same time, strains with mobility showed faster and stronger adhesion than the strains without mobility [88].

#### **3.2 Capsular and polysaccharide**

After entering the host, bacteria usually activate the host immune system to cause a series of immune responses to eliminate pathogens [89]. In order to survive and reproduce in the host, bacteria must adopt a series of strategies to improve their viability and virulence in the host as well as their resistance to phagocytosis and antibiotics.

The correlation between the capsular and polysaccharide and virulence has been confirmed [90]. The capsule encapsulated on the surface of bacteria is a dense, high molecular weight capsule that plays a major role in evading the host's immune defense. The encapsulated pathogen shows strong resistance to phagocytosis and complement-mediated lethality. Studies have shown that organisms with capsular polysaccharides are more likely to survive in serum, that isolates expressing opaque colonies are more resistant to serum than translucent isolates, and there are differences in colony characteristics between seafood isolates and clinical isolates. Clinical isolates were more resistant to serum complement proteins than environmental isolates, and the clinical genotype had a consistent survival advantage when exposed to serum [91–93]. In addition, the formation of biofilm will also promote the adhesion of pathogens to the host, coordinate the quorum sensing between bacteria, and improve the resistance of pathogens to antibiotics, playing a major role in the escape of pathogens from host immunity.

#### **3.3 Cytotoxins**

Cytotoxic is the main killer factor of pathogens in the process of attacking the host. Toxins secreted by *Vibrio* can be divided into endotoxin and exotoxin. Endotoxin is the lipid part of lipopolysaccharide released after cell death, and the exotoxin is secreted out of cells to cause damage to the host. At present, the toxins produced by *Vibrio* have been studied in depth, and the expression of many virulence factors is related to the pathogenicity of *Vibrio*, including: Thermostable direct hemolysin (TDH), TDH-related hemolysin (TRH), *V. vulnificus* cytolytic toxin (VVC), cholera toxin (CT), and zonulaoccludens toxin (Zot).

*V. parahaemolyticus* is one of the main bacterial isolates of food poisoning caused by seafood contamination and is usually associated with outbreaks of foodborne diseases [11]. The thermostable direct hemolysin (TDH) and TDH-related hemolysin (TRH) encoded by the *tdh* and *trh* genes are considered to be the main virulence factors of *V. parahaemolyticus*. TDH has a variety of biological activities, such as hemolytic activity, cytotoxicity, cardiotoxicity and enterictoxicity. TDH is a perforated toxin, and its toxic mechanism is to create pores with a diameter of 0 ∼ 2 nm on the erythrocyte membrane, among which the larger pores can make the water and ions in the cells flow out of the cell membrane, and changes in these ion fluxes in the intestine are also the main cause of diarrhea [94, 95]. TRH is a thermolabile toxin, which is similar to TDH in immunology, and can also activate chloride channels and cause changes in ion flux [96]. TDH and TRH share approximately 70% homology [97], and these two genes are considered to be the most important virulence markers of *V. parahaemolyticus* and are commonly used for virulence testing of *V. parahaemolyticus* [98].

*V. vulnificus* is a conditionally pathogenic human pathogen, which can cause severe wound infection, acute gastroenteritis and life-threatening septicemia. In susceptible or immunocompromised individuals, the mortality rate exceeds 50% [99]. VVC encoded by *VvhA* gene of *V. vulnificus* is the key virulence factor of *V. vulnificus,* which mainly plays a role through two mechanisms: cytolysis and apoptosis induction [100]. VVC has species specificity in *V. vulnificus*, which is the only exotoxin that can be secreted out of cells, and belongs to cholesterol-dependent cytolysin of pore-forming protein family [101]. The cytotoxic mechanism of cytotoxin is that it combines with non-esterified cholesterol on cell membrane and aggregates on the cell surface, which makes the cell membrane form a channel and leads to the outflow of intracellular potassium ions, which leads to the rupture of colloid permeable cells. The main mechanism of apoptosis induced by cytotoxin is related to mitochondria. Cytotoxin can lead to the production of mitochondrial reactive oxygen species (ROS) in intestinal epithelial cells, and then lead to cell necrosis and apoptosis [102].

*V. cholerae* has caused several epidemics in history. Cholera toxin (CT) is the main pathogenic factor of O1/O139 *V. cholerae*, which can cause serious damage to intestinal cell function and lead to cholera watery secretory diarrhea [103]. CT is encoded by *ctxA* and *ctxB*. These genes are encoded by CTXΦ of lysogenic filamentous phage and can be transferred between virulent and non-virulent strains [104]. *zot* is also encoded by lysogenic filamentous bacteriophage CTXΦ [105], and its encoded zona-linked toxin (ZOT) is the second virulence index of *V. cholerae*, which can make mucosal cells adhere together, maintain the tight connection structure of mucosal integrity and increase the permeability of intestinal mucosa [106]. Except as a cytotoxin, Zot seems to be related to the assembly of CTX phage in structure and function. *Zot* gene has sequence homology with the coat protein gene, which is probably the coat protein of CTX Φ. This indicates that it may have dual functions [107].

#### **3.4 Other virulence factors**

In addition to secreting toxins, some pathogenic bacteria can secrete a variety of extracellular products, which are also the main factors causing host diseases. For example, Balebona and Morinigo discovered in 1995 that the extracellular products of *V. alginolyticu*s have various enzyme activities such as caseinase, gelatinase, amylase, phospholipase, collagenase, etc., and these extracellular products have strong toxicity to fish cells, which can dissolve fish cells and cause fish death [108]. Balebona et al. infected fish by intramuscular injection with extracellular protease.

*Community Change and Pathogenicity of* Vibrio *DOI: http://dx.doi.org/10.5772/intechopen.96515*

After 6 h, it was observed that the injected extracellular products were lysed, which could lead to fish death in 24 ~ 72 h [109]. Lee et al. found that alkaline serine protease produced by *V.* alginolyticus can reduce thrombin in prawn plasma and prevent hemolymph from agglutinating, which is one of the main lethal factors secreted by *V. alginolyticus. V. vulnificus* metalloproteinases (VVP) is a kind of zinc ion-dependent protease with hemolysis. Miyoshi et al. studies have proved that VVP can enhance vascular permeability, destroy the basement membrane, cause bleeding reaction, cause cell and tissue damage, and eventually develop into sepsis [110].

Iron, as an indispensable trace element, is also an important component of various cellular enzymes, and plays an important role in the growth, reproduction, pathogenicity and cellular metabolism of pathogenic bacteria [111]. The iron carrier of *Vibrio* is an important pathogenic factor, and the iron uptake system mediated by it plays a very important role in bacterial growth and host colonization. Under the condition of lack of iron, the strain will produce a chelation agent iron carrier with high affinity for heme iron ions and low molecular weight, and the iron absorbed from transferrin and lactoferrin will be transported to bacterial cells for its own use through the receptor [112].
