(A) Experimental outline: aged and young rats received subcutaneous M. vaccae injections once per week for three weeks. Five days following the final injection, rats underwent a laparotomy or sham procedure. Three days post-surgery, rats underwent training in a pre-exposure fear-conditioning paradigm. Freezing behavior in the (B) conditioned and (C) novel context of the fear-conditioning paradigm is presented as percent of total time (6 min) freezing (Freezing %). Results were analyzed using a 2 × 2 × 2 ANOVA with age, surgery, and M. vaccae treatment as between-subjects factors (n = 5-6/group). Data are expressed as mean ± SEM. *differs from all other groups, p < 0.05.