Early-life antibiotics impair immunity to bacterial pneumonia. (A) Fold change (FC) in the number of T cells within the lungs of WT mice 1 wk following retropharyngeal inoculation with F. tularensis LVS, compared to uninfected animals (n = 3/group, rep. of 2 expts.). (B) WT mice received ampicillin (Amp) by daily oral gavage from 2 to 4 wk and were co-housed with untreated age-matched mice for 1 wk. At 8 wk, animals were inoculated retropharyngeally with 400 CFUs of F. tularensis LVS and analyzed 2 wk later. (C) Mice were treated as described in B and weight change was monitored daily (n = 5–6/group, rep. of 2 expts.). (D and E) The number (D) and frequency (E) of MAIT cells in the lungs following the treatment described in B (n = 4–5/group). (F) WT and Mr1^−/− mice received ampicillin (Amp) by daily oral gavage from 2 to 4 wk of age and were then co-housed with untreated age-matched mice for 1 wk. At 8 wk, animals were inoculated retropharyngeally with 800 CFUs of F. tularensis LVS. (G) Survival following treatment described in F (Mr1^−/−n = 10/group; WT n = 9–10/group, rep. of 2 expts.). (H) WT mice received ampicillin (Amp) by daily oral gavage from 2 to 4 wk and were co-housed with untreated age-matched mice for 1 wk. At 7 wk, mice received ∼2.5 × 10⁴ magnetically enriched MAIT cells by retroorbital injection. At 8 wk, animals were inoculated retropharyngeally with 400 CFUs of F. tularensis LVS. (I) Mice were treated as described in H and weight change was monitored daily (n = 10/group, rep. of 2 expts.). Data represent the mean ± SEM, with *P <0.05, **P < 0.01, ***P < 0.001, and ****P < 0.0001, and “ns” or no label indicates comparison was not significant as calculated by ordinary one-way ANOVA with Šidák’s multiple comparison correction (A), multiple unpaired t tests (C–E and I), or Mantel–Cox test (G).