The reference strain Leptospira biflexa serovar Patoc was obtained from the American Type Culture Collection (ATCC 23582, Manassas, VA), and is a non-pathogenic species. L. interrogans serovar Canicola (pathogenic, strain ATCC 23606 and strain 11203-32) were obtained from the ATCC and Dr. Richard Zuerner (USDA, Ames, IA), respectively. L. interrogans serovar Copenhageni (pathogenic, strain designated Fiocruz L1-130) was provided by Dr. David Haake (UCLA, Los Angeles, CA). Bacterial strains were maintained in ambient air at 30uC. Bacteria utilized for this study were at low passage from the suppliers (#passage 6) and cultured in EMJH medium [1] supplemented with 100 mg/ml of 5-fluorouracil and 1% rabbit serum (Sigma-Aldrich). For some experiments, the bacteria were radiolabeled by addition of 35 S cysteine plus methionine to the medium as described previously [17] . The bacteria were enumerated using a Petroff-Hausser counting chamber and dark field microscopy.