Survival of B. abortus urease mutants to acid exposure. Log n° of bacteria surviving an acid shock of 30 minutes at pH 2.0 in the presence of different amounts of urea. The arithmetic media from three separate experiments was plotted with standard deviations. An unpaired t-test was performed to determine if survival of each strain was significantly different than the corresponding wild type control. * indicates p < 0.05, ** p < 0.01.