G repeats-in-toxin (RtxA) have been reported to be virulence mechanisms exploited by some strains [3]. Another such accessory virulence factor is the type VI secretion PS-1145 site system (T6SS), which confers cytotoxic effects against both prokaryotic and eukaryotic cells [4?]. Bacteria have developed numerous mechanisms to export proteins, including toxins, acrosstheir cell walls into the surrounding environment or into host cells. To date, six distinctive pathways, collectively called secretion systems and classified into type I to type VI (T1SS ?T6SS), have been identified in Gram-negative bacteria [7]. The T6SS of V. 15755315 cholerae mediates cytotoxicity towards eukaryotic hosts, including murine macrophages [5,8,9] and the amoeba Dictyostelium discoideum [4]. The V. cholerae T6SS is encoded by three gene clusters on two separate chromosomes: one large cluster (VCA0107 ?VCA0124) [10] and two small auxiliary clusters (VCA0017 ?VCA0021 and VC1415 ?VC1421). Bioinformatic analyses and a series of experimental approaches have elucidated the functions of several genes belonging to the V. cholerae T6SS clusters. For example, the Hcp protein [11], secreted by bacteria with a functional T6SS, forms a nanotube structure with an internalCompetition Mechanisms of V. choleraediameter of 4 nm [12]. Three VgrG proteins were shown to interact with each other to form a trimeric complex that structurally resembles a T4-bacteriophage gp5-gp27 tail spike complex [9], but unlike their phage counterparts lack an internal channel [13]. The current working model of the T6SS is based on these observations and the finding that Hcp and VgrG are codependent for secretion. The model proposes that the Hcp nanotube, decorated with a VgrG trimer at its top, is pushed through the bacterial envelope of the buy Terlipressin predator cell and into the prokaryotic or eukaryotic target cell. It is suggested that cytoplasmic VipA and VipB (VCA0107 and VCA0108) form a contractile sheath around the Hcp tube similar to the T4 phage outer sheath; contraction of the VipAB sheath ejects the Hcp tube from the predator cell [14]. The VgrG cap might mediate 
toxicity via the C-terminal extensions of evolved VgrGs upon delivery into the target cell [5]. Alternatively, the cap might dissociate from the Hcp nanotube to allow delivery of soluble toxin(s) or effector molecule(s) through the Hcp conduit [13]. VasH (VCA0117) acts as a sigma-54 activator protein and controls transcription of T6SS genes including hcp and vgrG. We recently reported that the V. cholerae T6SS also exerts contact-dependent killing properties against other Gram-negative bacteria such as Escherichia coli [6]. This finding suggests that V. cholerae may employ the T6SS to compete with commensal bacteria in the human intestine and/or environmental reservoirs. The environmental reservoirs of V. cholerae (river deltas with brackish waters, oceans, and deep seas [15]) are as diverse as the genomic content of this bacterium. The V. cholerae pangenome is estimated to consist of ,6,500 genes [16]. Because all V. cholerae genomes sequenced so far contain the three gene clusters encoding the T6SS, we conclude that the T6SS belongs to the 1,500-gene core genome. Although the T6SS appears to be conserved in V. cholerae, the system is regulated differently between strains. While Table 1. Bacterial strains and plasmids.the O37 serotype V52 strain expresses T6SS genes constitutively, the O1 El Tor strain C6706 represses its T6SS under laboratory conditions. Mutat.G repeats-in-toxin (RtxA) have been reported to be virulence mechanisms exploited by some strains [3]. Another such accessory virulence factor is the type VI secretion system (T6SS), which confers cytotoxic effects against both prokaryotic and eukaryotic cells [4?]. Bacteria have developed numerous mechanisms to export proteins, including toxins, acrosstheir cell walls into the surrounding environment or into host cells. To date, six distinctive pathways, collectively called secretion systems and classified into type I to type VI (T1SS ?T6SS), have been identified in Gram-negative bacteria [7]. The T6SS of V. 15755315 cholerae mediates cytotoxicity towards eukaryotic hosts, including murine macrophages [5,8,9] and the amoeba Dictyostelium discoideum [4]. The V. cholerae T6SS is encoded by three gene clusters on two separate chromosomes: one large cluster (VCA0107 ?VCA0124) [10] and two small auxiliary clusters (VCA0017 ?VCA0021 and VC1415 ?VC1421). Bioinformatic analyses and a series of experimental approaches have elucidated the functions of several genes belonging to the V. cholerae T6SS clusters. For example, the Hcp protein [11], secreted by bacteria with a functional T6SS, forms a nanotube structure with an internalCompetition Mechanisms of V. choleraediameter of 4 nm [12]. Three VgrG proteins were shown to interact with each other to form a trimeric complex that structurally resembles a T4-bacteriophage gp5-gp27 tail spike complex [9], but unlike their phage counterparts lack an internal channel [13]. The current working model of the T6SS is based on these observations and the finding that Hcp and VgrG are codependent for secretion. The model proposes that the Hcp nanotube, decorated with a VgrG trimer at its top, is pushed through the bacterial envelope of the predator cell and into the prokaryotic or eukaryotic target cell. It is suggested that cytoplasmic VipA and VipB (VCA0107 and VCA0108) form a contractile sheath around the Hcp tube similar to the T4 phage outer sheath; contraction of the VipAB sheath ejects the Hcp tube from the predator cell [14]. The VgrG cap might mediate toxicity via the C-terminal extensions of evolved VgrGs upon delivery into the target cell [5]. Alternatively, the cap might dissociate from the Hcp nanotube to allow delivery of soluble toxin(s) or effector molecule(s) through the Hcp conduit [13]. VasH (VCA0117) acts as a sigma-54 activator protein and controls transcription of T6SS genes including hcp and vgrG. We recently reported that the V. cholerae T6SS also exerts contact-dependent killing properties against other Gram-negative 
bacteria such as Escherichia coli [6]. This finding suggests that V. cholerae may employ the T6SS to compete with commensal bacteria in the human intestine and/or environmental reservoirs. The environmental reservoirs of V. cholerae (river deltas with brackish waters, oceans, and deep seas [15]) are as diverse as the genomic content of this bacterium. The V. cholerae pangenome is estimated to consist of ,6,500 genes [16]. Because all V. cholerae genomes sequenced so far contain the three gene clusters encoding the T6SS, we conclude that the T6SS belongs to the 1,500-gene core genome. Although the T6SS appears to be conserved in V. cholerae, the system is regulated differently between strains. While Table 1. Bacterial strains and plasmids.the O37 serotype V52 strain expresses T6SS genes constitutively, the O1 El Tor strain C6706 represses its T6SS under laboratory conditions. Mutat.
