[1] Univ London Imperial Coll Sci Technol & Med, Ctr Mol Microbiol & Infect, London SW7 2AZ, England
[2] Univ London Imperial Coll Sci Technol & Med, Dept Biochem, Ctr Struct Biol, London SW7 2AZ, England
[3] Royal Free Hosp, Ctr Paediat Gastroenterol, London NW3 2QG, England
[4] Inst Butantan, BR-05503900 Sao Paulo, Brazil
[5] Univ Birmingham, Inst Child Hlth, Birmingham B4 6LH, W Midlands, England