Depletion of fecal microbiota and direct effects of Abxs are key factors in Abx-associated BM B-cell depletion. (A) WBCs and absolute lymphocyte counts in whole blood in SPF and GF mice exposed to Ctrl and Abx conditions for 14 days. (B) BM B220⁺ B cells and (C) B-cell progenitor populations in SPF and GF mice exposed to Ctrl and Abx conditions. (D) Fecal microbial abundance was measured by 16S rRNA gene qPCR for SPF WT Ctrl mice, Abx-treated mice, and mice treated with Abx plus FMT for the indicated number of days. (E) WBCs and (F) absolute lymphocyte cell counts from whole blood were quantified from mice in each of these conditions. (G) Fecal microbial abundance by 16S rRNA qPCR, WBCs, blood lymphocytes, and B220⁺ BM cells were profiled on day 21 from mice in each condition. For panels A-C, the Mann-Whitney U test was used to test statistical significance between groups; and for panel G, the Kruskal-Wallis test was used. Data are displayed as mean ± SEM from 2 independent experiments. ∗P ≤ .05; ∗∗P ≤ .01; ∗∗∗P ≤ .001. rDNA, ribosomal DNA.