The Δ27735 mutant was attenuated at the early stage of infection in a mouse model. The bacteria were recovered from spleens infected by the WT strain and the Δ27735 mutant (A). Splenomegaly was assessed by weighing the infected spleen (B). Statistical significance was determined using two-way ANOVA followed by Holm-Sidak’s multiple test. *p < 0.05; **p < 0.01; ***p < 0.001; ns: not significant.