Composition, acquisition, and distribution of the Vi exopolysaccharide-encoding Salmonella enterica pathogenicity island SPI-7. by Pickard, D, Wain, J, Baker, S, Line, A, Chohan, S, Fookes, M, Barron, A, Gaora, P, Chabalgoity, J, Thanky, N, Scholes, C, Thomson, N, Quail, M, Parkhill, J, Dougan, G