1. Molecular Genetics Laboratory, Instituto de Tecnologia Quı́mica e Biológica, Universidade Nova de Lisboa, Oeiras, Portugal1;
2. Microbiology Department, School of Medicine, Universidad Complutense,2 and
3. Medical Department, GlaxoSmithKline Pharmaceuticals,3 Madrid, Spain; and
4. Laboratory of Microbiology, The Rockefeller University, New York, New York4