**1. Introduction**

The genus *Vibrio* is comprised of facultative, anaerobic, Gram-negative, curvedrod bacteria that are widely found in natural aquatic environments such as marine, estuarine, and freshwater [1]. More than 100 species have been currently described in this genus, and at least 12 species represented by *V. cholerae*, *V. parahaemolyticus*, and *V. vulnificus* cause a variety of clinical symptoms in human (Table) [1–5]. In addition, species such as *V. metoecus* and *V. navarrensis* are among the newly isolated species from human, and it is strongly suggested that they are human pathogens of *Vibrio* spp. [6, 7]. On the other hand, the major pathogenic *Vibrio* for aquatic vertebrates or invertebrates are *V. anguillarum*, *V. harveyi*, *V. ordalii* etc., responsible for fatal hemorrhagic septicemic disease called vibriosis in marine animals [5, 8–10]. *Vibrio* spp. prefer warm water temperature (15–35°C), so they are likely to flourish more with rising environmental water temperature due to global warming and thus the probability of infections caused by them.

Human diseases caused by pathogenic *Vibrio* spp. can be divided into two major types based on symptoms: intestinal infection and non-intestinal infection [1, 3]. The intestinal infection includes gastroenteritis and cholera, whereas nonintestinal infection includes septicemia and wound infection (**Table 1**). Cholera is caused by ingestion of food and drinking water contaminated with *V. cholerae* O1/O139 that produces cholera toxin (CT) as a major virulence determinant and characterized by severe diarrhea that rapidly leads to dehydration [11, 12]. Till date it remains a major public health disease with estimated 2.9 million cases and 95,000 deaths annually worldwide [13]. There are many clinical cases of gastroenteritis by *V. parahaemolyticus* due to ingestion of raw fish and shellfish [14, 15].


#### **Table 1.**

*Pathogenic Vibrio species for human and hemolysins produced by them [1, 3].*

The other species such as *V. cholerae* non-O1/non-O139, *V. mimicus*, and *V. fluvialis* are known as agents of foodborne illness [3, 16–19]. On the other hand, *V. vulnificus* is the most studied among *Vibrio* spp. as a causative bacterium of wound infections, though the clinical cases by *V. damsela* and *V. alginolyticus* are also reported [3, 20–24]. *V. vulnificus* is an opportunistic pathogen and poses a threat to individuals with compromised immunity because it can also cause septicemia, which leads to high lethal rates [24–26].

These pathogenic *Vibrio* have been reported to produce various virulence factors, including enterotoxin such as CT produced by *V. cholerae* O1/O139 [12, 27], hemolysin, and Type III secretion system (T3SS) in *V. parahaemolyticus* [28, 29] and extracellular protease in *V. vulnificus* [30]. This chapter has mainly summarized how hemolysins play an important role in the pathogenicity of Vibrio spp. based on studies till date.
