[1] UNIV PENN, DEPT MICROBIOL, PHILADELPHIA, PA 19104 USA
[2] UNIV PENN, DEPT BIOCHEM, PHILADELPHIA, PA 19104 USA
[3] UNIV PENN, SCH DENT MED, CTR ORAL HLTH RES, PHILADELPHIA, PA 19104 USA
[4] GENENTECH INC, DEPT VACCINE DEV, S SAN FRANCISCO, CA 94080 USA
[5] UNIV CHICAGO, DEPT MICROBIOL, CHICAGO, IL 60637 USA