Genetic Markers of Pandemic Vibrio parahaemolyticus : Are They Truly Unique?

V ibrio parahaemolyticus is a gram-negative, halophilic bacterium that occurs in estuarine environments worldwide (DePaola et al., 1990). The thermostable direct hemolysin and thermostable direct hemolysin-related hemolysin encoded by tdh and trh genes, respectively, are considered major virulence factors for this organism (Nishibuchi and Kaper, 1995). With the emergence of pandemic clone of V. parahaemolyticus, this organism has assumed significance in recent years (Matsumoto et al., 2000). A group-specific polymerase chain reaction (GS-PCR) based on the sequence variation in the toxRS operon was developed to differentiate pandemic and nonpandemic strains (Matsumoto et al., 2000), but several researchers reported the occurrence of GS-PCR⁺ V. parahaemolyticus strains that did not appear to belong to the pandemic clone (Osawa et al., 2002; Vongxay et al., 2008). Okura and colleagues have developed a multiplex PCR that distinguishes pandemic clone from other V. parahaemolyticus by combining tdh gene PCR and GS-PCR (Okura et al., 2003). However, this multiplex PCR is not suitable for direct detection of pandemic clone in samples because seafood or water contaminated with both tdh ⁺ and GS-PCR⁺ V. parahaemolyticus may give false-positive results. The eighth open reading frame (ORF8) of a filamentous phage (f237) is unique to pandemic clone and considered as a genetic marker for this clone (Nasu et al., 2000). However, several recent studies have reported the absence of ORF8 in many GS-PCR-positive strains (Bhuiyan et al., 2002; Raghunath et al., 2008). Recently, Vongxay et al. (2008) reported two seafood isolates that were negative for tdh, trh, and GS-PCR, but positive for orf8. Several PCR-based methods have been developed for the detection of pandemic clone of V. parahaemolyticus (Okura et al., 2004; Williams et al., 2004). However, previous reports suggest that these PCR-based methods are unreliable for identification of pandemic clone of V. parahaemolyticus (Parvathi et al., 2006; Raghunath et al., 2008). Hurley et al. (2006) identified seven genomic islands (VPaI-1 to VPaI-7) that were unique to pandemic clone. More recently, it has been reported that only VPaI-1, VPaI-4, and VPaI-5 are specific for pandemic clone (Chao et al., 2009, 2010). However, more number of V. parahaemolyticus isolates from different regions should be screened for these three genomic islands before considering unique for pandemic clone.