Genetic Analysis of Anti-Amoebae and Anti-Bacterial Activities of the Type VI Secretion System in Vibrio cholerae

A type VI secretion system (T6SS) was recently shown to be required for full virulence of Vibrio cholerae O37 serogroup strain V52. In this study, we systematically mutagenized each individual gene in T6SS locus and characterized their functions based on expression and secretion of the hemolysin co-regulated protein (Hcp), virulence towards amoebae of Dictyostelium discoideum and killing of Escherichia coli bacterial cells. We group the 17 proteins characterized in the T6SS locus into four categories: twelve (VipA, VipB, VCA0109–VCA0115, ClpV, VCA0119, and VasK) are essential for Hcp secretion and bacterial virulence, and thus likely function as structural components of the apparatus; two (VasH and VCA0122) are regulators that are required for T6SS gene expression and virulence; another two, VCA0121 and valine-glycine repeat protein G 3 (VgrG-3), are not essential for Hcp expression, secretion or bacterial virulence, and their functions are unknown; the last group is represented by VCA0118, which is not required for Hcp expression or secretion but still plays a role in both amoebae and bacterial killing and may therefore be an effector protein. We also showed that the clpV gene product is required for Dictyostelium virulence but is less important for killing E. coli. In addition, one vgrG gene (vgrG-2) outside of the T6SS gene cluster was required for bacterial killing but another (vgrG-1) was not. However, a bacterial killing defect was observed when vgrG-1 and vgrG-3 were both deleted. Several genes encoded in the same putative operon as vgrG-1 and vgrG-2 also contribute to virulence toward Dictyostelium but have a smaller effect on bacterial killing. Our results provide new insights into the functional requirements of V. cholerae's T6SS in the context of secretion as well as killing of bacterial and eukaryotic phagocytic cells

TssA forms a gp6-like ring attached to the type VI secretion sheath

The type VI secretion system (T6SS) is a supra-molecular bacterial complex that resembles phage tails. It is a killing machine which fires toxins into target cells upon contraction of its TssBC sheath. Here, we show that TssA1 is a T6SS component forming dodecameric ring structures whose dimensions match those of the TssBC sheath and which can accommodate the inner Hcp tube. The TssA1 ring complex binds the T6SS sheath and impacts its behaviour in vivo. In the phage, the first disc of the gp18 sheath sits on a baseplate wherein gp6 is a dodecameric ring. We found remarkable sequence and structural similarities between TssA1 and gp6 C-termini, and propose that TssA1 could be a baseplate component of the T6SS. Furthermore, we identified similarities between TssK1 and gp8, the former interacting with TssA1 while the latter is found in the outer radius of the gp6 ring. These observations, combined with similarities between TssF and gp6N-terminus or TssG and gp53, lead us to propose a comparative model between the phage baseplate and the T6SS

A model study of tidal distributions in the Celtic and Irish Sea regions determined with finite volume and finite element models

An unstructured mesh model of the west coast of Britain, covering the same domain and using topography and open boundary forcing that are identical to a previous validated uniform grid finite difference model of the region, is used to compare the performance of a finite volume (FV) and a finite element (FE) model of the area in determining tide–surge interaction in the region. Initial calculations show that although qualitatively both models give comparable tidal solutions in the region, comparison with observations shows that the FV model tends to under-estimate tidal amplitudes and hence background tidal friction in the eastern Irish Sea. Storm surge elevations in the eastern Irish Sea due to westerly, northerly and southerly uniform wind stresses computed with the FV model tend to be slightly higher than those computed with the FE model, due to differences in background tidal friction. However, both models showed comparable non-linear tide–surge interaction effects for all wind directions, suggesting that they can reproduce the extensive tide–surge interaction processes that occur in the eastern Irish Sea. Following on from this model comparison study, the physical processes contributing to surge generation and tide–surge interaction in the region are examined. Calculations are performed with uniform wind stresses from a range of directions, and the balance of various terms in the hydrodynamic equations is examined. A detailed comparison of the spatial variability of time series of non-linear bottom friction and non-linear momentum advection terms at six adjacent nodes at two locations in water depths of 20 and 6 m showed some spatial variability from one node to another. This suggests that even in the near coastal region, where water depths are of the order of 6 m and the mesh is fine (of order 0.5 km), there is significant spatial variability in the non-linear terms. In addition, distributions of maximum bed stress due to tides and wind forcing in nearshore regions show appreciable spatial variability. This suggests that intensive measurement campaigns and very high-resolution mesh models are required to validate and reproduce the non-linear processes that occur in these regions and to predict extreme bed stresses that can give rise to sediment movement. High-resolution meshes will also be required in pollution transport problem

Comparisons between surface, barotropic and abyssal flows during the passage of a warm-core ring

A comparison is made of results for barotropic, surface and abyssal flows during the formation and passage of a warm-core ring in the East Australian Current. The barotropic velocities are estimated from sea-floor measurements of the horizontal electric field, which is induced by water motion. Values for the surface and near-bottom velocities are obtained generally by more traditional methods. A strong similarity is observed between the directions of the barotropic and surface flows. At a site close to the foot of the continental slope, the barotropic and near-bottom velocities are also similar, both in direction and magnitude. A possible explanation for this effect is that proximity to the coast constrains flow directions and causes the streamlines at depth to converge on the western (or near-shore) side of the warm-core ring. The determination of barotropic velocities enables barotropic volume transports to be estimated and compared with traditional geostrophic volume transports calculated for water motion between the surface and a depth of 1300 m. The barotropic transports are found to be greater than the geostrophic transports by a factor of approximately 1.6, indicating the significance of deep-water flow in the East Australian Current

Comparisons between surface, barotropic and abyssal flows during the passage of a warm-core ring

A comparison is made of results for barotropic, surface and abyssal flows during the formation and passage of a warm-core ring in the East Australian Current. The barotropic velocities are estimated from sea-floor measurements of the horizontal electric field, which is induced by water motion. Values for the surface and near-bottom velocities are obtained generally by more traditional methods. A strong similarity is observed between the directions of the barotropic and surface flows. At a site close to the foot of the continental slope, the barotropic and near-bottom velocities are also similar, both in direction and magnitude. A possible explanation for this effect is that proximity to the coast constrains flow directions and causes the streamlines at depth to converge on the western (or near-shore) side of the warm-core ring. The determination of barotropic velocities enables barotropic volume transports to be estimated and compared with traditional geostrophic volume transports calculated for water motion between the surface and a depth of 1300 m. The barotropic transports are found to be greater than the geostrophic transports by a factor of approximately 1.6, indicating the significance of deep-water flow in the East Australian Current