[1] Johns Hopkins Bloomberg Sch Publ Hlth, W Harry Feinstone Dept Mol Microbiol & Immunol, Baltimore, MD 21218 USA
[2] Heidelberg Univ, ZMBH, D-6900 Heidelberg, Germany
[3] Univ London Imperial Coll Sci Technol & Med, Dept Biol Sci, London, England
[4] Inst Food Res, Inst Anim Hlth, Compton Lab, Newbury RG16 0NN, Berks, England