Defensin Susceptibility and Colonization in the Mouse Model of AJ100, a Polymyxin B-Resistant, Brucella abortus RB51 Isolate

Intracellular pathogens selected for increased susceptibility to polycations are commonly attenuated, yet the effect of decreased susceptibility to polycations on pathogenicity has not been researched. The polymyxin-resistant mutant Brucella abortus AJ100 was characterized by comparing its susceptibility to the polycationic antibiotic polymyxin B, defensins, and lactoferricin, and its colonization and clearance in the mouse model to the parent strain RB51. MIC (minimum inhibitory concentration) values determined by Etest for AJ100 and RB51 were 1.5 and 0.25 μg/ml, respectively. Though AJ100 is less susceptible to polymyxin B than RB51, it was more susceptible than its parent strain to the cationic defensins melittin, magainin 2, and cecropin P1. In the mouse model, initial colonization of the spleen was lower for AJ100 than RB51, and the rate of clearance from the spleen was faster for AJ100 than RB51. However, initial colonization and clearance rates of AJ100 from the liver were indistinguishable from those of RB51. This study suggests that the susceptibility profile of Brucella to polycationic defensins rather than polymyxin B may be indicative of differential survival in the spleen and liver in the mouse and is indicative of spleen and liver residential macrophages’ differing ability to inactivate Brucella.