The Harveyi clade or core group of the Vibrio genus includes both pathogenic and non-pathogenic bacteria. Some of the species belonging to the clade, such as Vibrio harveyi, Vibrio alginolyticus, Vibrio campbellii and Vibrio parahaemolyticus, include serious pathogens of aquatic organisms. Importantly, within these species, some strains are pathogenic, whereas others are not. Other members of the clade, such as Vibrio natriegens and Vibrio mytili are reported to be non-pathogenic. As bacteria belonging to the Harveyi clade show a high diversity with respect to virulence and as the virulent members cause high mortalities in the aquaculture sector, it is important to understand which gene products are responsible for the pathogenicity of the bacteria and how expression of these virulence factors is regulated. This knowledge will ultimately allow for the development of biocontrol measures that aim to reduce the severe losses that are currently faced in the aquaculture sector as a result of vibriosis. In this review, we discuss the pathogenesis of bacteria belonging to the Harveyi clade, their impact on the aquaculture sector, the virulence factors that are involved in pathogenicity and their regulation

Aims: The study was aimed at investigating the presence of typical and atypical virulence genes in isolates belonging to the Harveyi clade (Vibrio harveyi and Vibrio campbellii). Methods and Results: Forty-eight vibrio isolates belonging to the Harveyi clade were screened for the presence of virulence genes that are typical for these bacteria and those found in human pathogenic vibrios such as Vibrio cholerae, Vibrio parahaemolyticus and Vibrio vulnificus and aquatic pathogenic Vibrio anguillarum. The virulence genes were amplified by PCR with specific primers, and the presence further confirmed by dot blot hybridization. The virulence genes vhh, chiA, vhpA, toxR(Vh), luxR and serine protease, typical of Harveyi clade were detected in all the isolates. The haemolysin gene hlyA and the virulence regulator gene toxR(Vc) specific to V. cholerae and the V. anguillarum-specific flagellum gene (flaC) were present in some of the isolates. Challenge tests with gnotobiotic Artemia nauplii did not show any correlation between the presence of the virulence genes and virulence of the isolates. Conclusion: From our results, there appears a remote possibility that vibrios belonging to the Harveyi clade might acquire virulence genes from other vibrios in the aquatic environment through horizontal gene transfer. Significance and Impact of the Study: Vibrios belonging to the Harveyi clade may be an important reservoir of virulence genes of other (human pathogenic) Vibrio species in the aquatic environment. The acquisition of virulence genes by horizontal transfer might increase the ability of Harveyi clade vibrios to infect aquatic organisms by increasing their virulence to a specific host by broadening their host range. The detection of such genes may forewarn the hatchery operators about a potentially virulent pathogen and thus help to develop management measures to handle the problem of vibriosis

Vibrios belonging to the Harveyi clade are pathogenic marine bacteria affecting both vertebrates and invertebrates, thereby causing a severe threat to the aquaculture industry. In this study, the expression of haemolysin, metalloprotease, serine protease, the quorum sensing master regulator LuxR and the virulence regulator ToxR in different Harveyi clade isolates was measured with reverse transcriptase real-time PCR with specific primers. There was relatively low variation in the in vitro expression levels of the quorum sensing master regulator luxR (sevenfold), whereas for the other genes, the difference in expression between the isolates showing lowest and highest expression levels was over 25-fold. Furthermore, there was a significant correlation between expression levels of toxR and luxR and between the expression levels of these regulators and the protease genes. The expression levels of luxR, toxR and haemolysin were negatively correlated with the survival of brine shrimp larvae challenged with the isolates. Finally, a non-virulent, a moderately virulent and a strongly virulent isolate were selected to study in vivo expression of the virulence genes during infection of gnotobiotic brine shrimp larvae. The in vivo gene expression study showed a clear difference in virulence gene expression between both virulent isolates and the non-virulent isolate.

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The type III secretion system encoded by the Salmonella pathogenicity island 2 (SPI-2) has a central role in the pathogenesis of systemic infections by Salmonella. Sixteen genes (ssaU, ssaB, ssaR, ssaQ, ssaO, ssaS, ssaP, ssaT, sscB, sseF, sseG, sseE, sseD, sseC, ssaD and sscA) of SPI-2 were targeted for PCR amplification in 57 seafood-associated serovars of Salmonella. The sseC gene of SPI-2 was found to be absent in two isolates of Salmonella enterica serovar Weltevreden, SW13 and SW39. Absence of sseC was confirmed by sequencing using flanking primers. SW13 had only 66 bp sequence of the sseC gene and SW39 had 58 bp sequence of this gene. A clinical isolate, S. Weltevreden -SW3, 10 : r : z6 -was used to construct a deletion mutant for the sseC gene. Significant reduction in the survival of SW3, 10 : r : z6 DsseC and natural mutants SW13 and SW39 in HeLa cells suggests that sseC has a crucial role in the intracellular survival of S. Weltevreden. Expression of sseC was upregulated during the intracellular phase of both S. enterica serovar Typhimurium and clinical isolate S. Weltevreden SW3, 10 : r : z6, suggesting a crucial role for this gene in the survival of S. Weltevreden inside host cells.