Urease activity of B. abortus 2308-derived strains. Urease activity was determined in bacterial extracts obtained from the indicated strains and growth conditions, and expressed in μmol of NH3 min-1 mg-1 protein The experiments were performed by triplicate with three technical measures per replica. The data shown correspond to one representative experiment and the error bar indicates the standard deviation. An unpaired t-test was performed to determine if the urease activity of each mutant was significantly different than the corresponding wild type control. A. Protein extracts from cultures of the indicated strains grown in BB. B: Protein extracts from cultures grown in BB supplemented with 0.5 mM NiCl2. * indicates p < 0.05.