1. Institute of Odontology and Stomatology, University of Florence, Viale Morgagni 85 50134 Florence, Italy, The University of Texas Health Science Center, Dental Branch, Department of Microbiology, P.O. Box 20068, Houston, Texas 77225
2. The University of Texas Health Science Center, Dental Branch, Department of Microbiology, P.O. Box 20068, Houston, Texas 77225