prevalence and molecular characteristics of vibrio species in pre ...

27
Vibrio spp. display similar biochemical characteristics and it limits the proper
identification and characterization of Vibrio spp. (Tamrakar et al., 2006). Scientists
have developed PCR based techniques for the detection of toxigenic strains based on
the cholera toxin (ctx) gene, and the toxin coregulated (tcpA) gene (Keasler and Hall,
1993, Rivera et al., 2003).
V. parahaemolyticus has been recognized as major pathogenic agent of
gastroenteritis associated with the consumption of seafood (Nishibuchi and Kaper,
1995). But not all V. parahaemolyticus strains are carrying the virulence capacity
(Raghunath et al., 2008). Clinical strains are categorized into two groups according to
their ability of haemolysing Wagatsuma blood agar. This is named as Kanagawa
phenomenon (KP). Most of the clinical strains of V. parahaemolyticus are haemolytic
(KP + ). This haemolysin was identified as thermo stable direct haemolysin (TDH),
because it was not inactivated by heating at 100°C for 10 min. Kanagawa
phenomenon negative strains were isolated and named as TDH-related haemolysin
(TRH) and its activity is lost when heated at 60°C or higher temperature for 10 min.
(Honda and Iida, 1993). Clinical strains of V. parahaemolyticus most often produce
TDH or TRH haemolysin and carry the gene for tdh and trh respectively (Nishibuchi
and Kaper, 1995, Raghunath et al., 2008). Both TDH and TRH strains have similar
biological, immunological and physiochemical characteristics and they are composed
of 165 amino acids and their amino acid sequence homology is about 67% (Honda
and Iida, 1993). These data suggested that trh and tdh genes probably evolved from a
common ancestor. Researchers identified the tdh gene in some strains of V. mimicus,
V. cholerae non-O1 strains from the marine environment and occasionally from
diarrhoea samples while all the strains of V. hollisae were positive for tdh gene
(Honda and Iida, 1993, Nishibuchi and Kaper, 1995).
Identification of virulent strains of V. parahaemolyticus has been done with
the conventional methods, such as detection of haemolysin in Wagatsuma agar
containing washed human or rabbit erythrocytes, immunological methods (reverse
passive haemagglutination, ELISA) (Honda and Iida, 1993). But for the detection of
TRH there are no available commercial methods. Therefore identification of virulence