In vitro and in vivo characterization of smooth small colony variants of Brucella abortus S19

Brucella abortus is known to produce chronic infections in both humans and a variety of animal species. However, the mechanisms underlying the persistence of the bacteria in the presence of an ongoing immune response are still unknown. In this respect we made use of the observation that in vitro grown B. abortus S19 exhibits heterogenicity in colony size when plated onto TS agar, while experimental infection of mice uniformly results in the in vivo selection of the small colony variant. We demonstrate that the spontaneous smooth small colony variant is characterized not only by a slower growth rate in vitro but also by an increased tolerance to hyperosmotic medium and, most importantly, a less effective clearance from spleens and livers of experimentally infected mice. On a molecular level, a gene with homology to a formerly described galactoside transport ATP binding protein (mglA) was differentially expressed in small versus large colonies of B. abortus S19.