The type III secretion (T3S) injectisome is a specialized protein nanomachine that is critical for the pathogenicity of many Gram-negative bacteria, including purveyors of plague, typhoid fever, whooping cough, sexually transmitted infections and major nosocomial infections. This syringe-shaped 3.5-MDa macromolecular assembly spans both bacterial membranes and that of the infected host cell. The internal channel formed by the injectisome allows for the direct delivery of partially unfolded virulence effectors into the host cytoplasm. The structural foundation of the injectisome is the basal body, a molecular lock-nut structure composed predominantly of three proteins that form highly oligomerized concentric rings spanning the inner and outer membranes. Here we present the structure of the prototypical Salmonella enterica serovar Typhimurium pathogenicity island 1 basal body, determined using single-particle cryo-electron microscopy, with the inner-membrane-ring and outer-membrane-ring oligomers defined at 4.3 Å and 3.6 Å resolution, respectively. This work presents the first, to our knowledge, high-resolution structural characterization of the major components of the basal body in the assembled state, including that of the widespread class of outer-membrane portals known as secretins.

The ability of Desulfovibrio vulgaris Hildenborough to reduce, and therefore contain, toxic and radioactive metal waste has made all factors that affect the physiology of this organism of great interest. Increased salinity is an important and frequent fluctuation faced by D. vulgaris in its natural habitat. In liquid culture, exposure to excess salt resulted in striking elongation of D. vulgaris cells. Using data from transcriptomics, proteomics, metabolite assays, phospholipid fatty acid profiling, and electron microscopy, we used a systems approach to explore the effects of excess NaCl on D. vulgaris. In this study we demonstrated that import of osmoprotectants, such as glycine betaine and ectoine, is the primary mechanism used by D. vulgaris to counter hyperionic stress. Several efflux systems were also highly up-regulated, as was the ATP synthesis pathway. Increases in the levels of both RNA and DNA helicases suggested that salt stress affected the stability of nucleic acid base pairing. An overall increase in the level of branched fatty acids indicated that there were changes in cell wall fluidity. The immediate response to salt stress included up-regulation of chemotaxis genes, although flagellar biosynthesis was down-regulated. Other down-regulated systems included lactate uptake permeases and ABC transport systems. The results of an extensive NaCl stress analysis were compared with microarray data from a KCl stress analysis, and unlike many other bacteria, D. vulgaris responded similarly to the two stresses. Integration of data from multiple methods allowed us to develop a conceptual model for the salt stress response in D. vulgaris that can be compared to those in other microorganisms.Originally isolated in 1946 from clay soils in Hildenborough, Kent, United Kingdom, Desulfovibrio vulgaris Hildenborough belongs to the sulfate-reducing class of bacteria that are ubiquitous in nature (23, 45). These anaerobes generate energy by reducing sulfate (42) and play important roles in global sulfur cycling and complete mineralization of organic matter. D. vulgaris has been implicated in biocorrosion of oil and gas pipelines both on land and in the ocean (5, 23, 57). Members of this species have also been found to reduce metals in sediments and soils with high concentrations of NaCl and a milieu of toxic metals (6) and to cope with salt stresses that result from environmental hydration-dehydration cycles. An understanding of the ability of D. vulgaris to survive in the presence of high concentrations of NaCl and osmotic stress is critical for determining the biogeochemistry at metal-contaminated sites for bioremediation and natural attenuation and for predicting the potential for biocorrosion of pipelines and tanks in soils, sediments, and off-shore oil production facilities (8,38,62). The availability of an annotated genomic sequence for D. vulgaris makes this organism ideal for studying the complex physiology of sulfate-reducing bacteria (25).The bacterial response to hyperionic stress includes a range of mechan...

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