Chitin-Induced Carbotype Conversion in Vibrio vulnificus

Abstract

As an etiological agent of bacterial sepsis and wound infections, Vibrio vulnificus is unique among the Vibrionaceae. The most intensely studied of its virulence factors is the capsular polysaccharide (CPS). Over 100 CPS types have been identified, yet little is known about the genetic mechanisms that drive such diversity. Chitin, the second-most-abundant polysaccharide in nature, is known to induce competence in Vibrio species. Here, we show that the frequency of chitin-induced transformation in V. vulnificus varies by strain and that (GlcNAc) 2 is the shortest chitin-derived polymer capable of inducing competence. Transformation frequencies (TFs) increased 8-fold when mixed-culture biofilms were exposed to a strain-specific lytic phage, suggesting that the lysis of dead cells during lytic infection increased the amount of extracellular DNA within the biofilm that was available for transfer. Furthermore, we show that V. vulnificus can undergo chitin-dependent carbotype conversion following the uptake and recombination of complete cps loci from exogenous genomic DNA (gDNA). The acquisition of a partial locus was also demonstrated when internal regions of homology between the endogenous and exogenous loci existed. This suggested that the same mechanism governing the transfer of complete cps loci also contributed to their evolution by generating novel combinations of CPS biosynthesis genes. Since no evidence that cps loci were preferentially acquired during natural transformation (random transposon-tagged DNA was readily taken up in chitin transformation assays) exists, the phenomenon of chitin-induced transformation likely plays an important but general role in the evolution of this genetically promiscuous genus.After 3.8 billion years of evolution (39), bacteria, whether commensal or pathogenic, have evolved numerous ways to cope with the sophisticated defensive strategies of the host's immune system. A simple but very effective strategy is the production and variation of surface polysaccharides that can alter proper host immune function (13,21). This type of antigenic variation is an important mechanism used by pathogenic microorganisms for escaping the neutralizing activities of antibodies and phagocytes. Antigenic variation may occur during the course of an infection, or an organism can exist in nature as multiple antigenic types (antigenic variety). The colonized host often provides the ideal selective environment for the emergence of new antigenic variants of bacteria, providing that the organism's other virulence determinants remain intact.Vibrio vulnificus is an estuarine bacterium that is pathogenic to humans and animals (35). Its continued environmental persistence and transmission is bolstered by its ability to colonize shellfish and form biofilms on various marine biotic surfaces, such as plankton, algae, fish, and eels (4, 40, 42). Primary septicemia and wound infections can arise following the consumption or handling of contaminated food or water, and the fatality rate of septicemic patients is grea...

Genomic evidence of adaptive evolution in emergent Vibrio parahaemolyticus ecotypes

“…In V. parahaemolyticus, genes within this cluster are involved in the synthesis of capsular polysaccharides (CPS) and play a role in the formation of biofilms (Chen et al, 2010). The cps cluster is known to be highly variable, and previous work has shown that recombination within this locus can be driven by chitin-induced transformation (Neiman et al, 2011). Five CPS proteins, unique to ST36, showed significant homology to putative virulence factors described in other Gram-negative bacteria.…”

Connecting Environment and Genome Plasticity in the Characterization of Transformation-Induced SOS Regulation and Carbon Catabolite Control of the Vibrio cholerae Integron Integrase

“…In this line it is worth noting that most antibiotics have been found to induce the SOS response in V. cholerae and increase the mutation frequency, even when present in subinhibitory concentrations (2), pointing to these as another source of mutability in this species. Moreover, all Vibrio species genomes sequenced so far carry the gene set required for natural transformation (55), and competence has be demonstrated for several of these (34,53,55); thus, the mechanisms described here could likely be applied to them. It would now be interesting to establish whether SOS is induced in the gut and, if so, to what extent it could induce the integrase.…”

“…Interestingly, SOS also regulates some toxin-antitoxin (TA) systems (61); it can thus play a role in integron maintenance as there are 13 cassettes carrying TA systems in the V. cholerae SI (54). Several Vibrio species have been also demonstrated to be naturally competent (34,53,55), and it is very likely that many other Vibrio species, if not all, can also be transformed and, thus, that the SOS response induction we describe in V. cholerae is a common process for these species. It is very likely that the SOS response is highly conserved in all Vibrio species as LexA and LexA binding boxes are found upstream of the SOS regulon common genes (unpublished data), as well as upstream of the intIA gene in all the sequenced Vibrio genomes (13).…”

Interactions ofVibriospp. with Zooplankton

“…TfoX upregulates competence genes (119), which are involved in the production of a type IV pilus complex, four chitinases (including ChiA-1 and ChiA-2), and a chitoporin (127,128). The degradation of chitin (as discussed above) leads to increased concentrations of (GlcNAc) 2 , the smallest chitin polymer that induces competence in V. vulnificus (126) and in V. cholerae (127). Although GlcNAc is present in other sources, such as glycolipids and polysaccharides, the dimer (GlcNAc) 2 is only derived from chitin degradation.…”

“…Successful chitinbased natural transformation of V. vulnificus (120), V. parahaemolyticus (121), and V. fischeri (122) have been reported and whole genome studies give evidence that HGT in V. cholerae is widespread in the natural environment (123). For example, studies have highlighted the importance of chitin-induced transformation and HGT in the conversion of nontoxigenic to toxigenic serogroups of V. cholerae (124, 125) and carbotype conversion of V. vulnificus (126), indicating that chitininduced transformation may play a large role in the acquisition of new genes in the environment.…”