Microbial colonization in adult mice confers resistance to viral bronchiolitis. (a–c) Nrp-1⁺ T reg cell number in lung (a) and MLN (b) and CD39 expression (c). (d) SCFA levels in feces (left) and serum (right) of neonatal and adult WT mice. (e) Relative abundance of bacterial phyla. (f) SCFA levels in WT and pDC^Δ mice treated with broad-spectrum ABX or vehicle. (g) Weight loss over the course of PVM infection. (h) Airway epithelium sloughing. (i) Neutrophil number in BAL. (j) Eosinophil number in BAL. (k) Viral load in the airway epithelium. (l) Propanoic acid levels in the nasal secretions of human subjects with nonviral (n = 10) or viral (n = 20) bronchiolitis. (m) Acetic acid levels in the nasal secretions. (n) IL-6 expression in the nasal secretions. (o) IL-10 expression in the nasal secretions. (p) Correlations between SCFA propanoic acid and IL-6 (left) and IL-10 (right) in infants with viral bronchiolitis. Data are representative of n = 2 experiments with four to six mice in each group and presented as box-and-whisker plots showing quartiles (boxes) and range (whiskers; a–d, f, h–k) or mean ± SEM (i–o) or as individual subjects (p). Data were analyzed by one-way (a–c and h–k) or two-way (g) ANOVA with Tukey’s post hoc test or Mann–Whitney U test (d and l) or Spearman’s rank-order correlation (p); *, P < 0.05; **, P < 0.01; ***, P < 0.001 compared with neonatal mice, without ABX WT mice, or as indicated. See also Table S1.