The B. abortus aidB mutant is more sensitive to EMS. The sensitivity of B. abortus wild-type, aidB mutant strain, complemented aidB mutant and aidB overexpression strains was scored by counting the c.f.u. recovered after 4 h of incubation 2YT medium at 37°C, in the presence of 0.2, 0.4 or 1% EMS. The results are expressed as the percentage of c.f.u. compared to a control in which EMS was omitted. Bacteria were obtained from cultures stopped during exponential growth phase.