Antibiotic treatment accelerates turnover of circulating phagocytes from the bloodstream. (A) Representative flow cytometry plots from BrdU pulse-chase experiments using mice previously treated with conventional water (CNV, black), broad-spectrum antibiotics (ABX, open), or neomycin (NEO, gray). Events represent CD45⁺CD11b⁺Ly6C⁺ cells stained for intracellular BrdU content and Ly6G (neutrophils: Ly6G⁺; inflammatory monocytes: Ly6G⁻) on days 2 and 4 after BrdU injection, compared with no-BrdU controls. Frequency of BrdU⁺ events among blood neutrophils (PMNs, B) and IMs (C) assessed on days 2 through 5 after BrdU administration. (D) Rate constants (k, day⁻¹) for 1-phase exponential decay quantified from BrdU⁺ frequencies depicted in panels B and C. Steady-state cell frequencies (% of CD45⁺ events) of PMNs and IMs measured from the bone marrow (E), spleens (F), and blood (G) of CNV-, ABX-, and NEO-treated mice. Absolute cell numbers: Bone marrow: CNV PMNs 9.99 ± 0.19, ABX PMNs 8.51 ± 0.39, NEO PMNs 8.98 ± 0.29, CNV IMs 2.05 ± 0.07, ABX IMs 1.60 ± 0.14, NEO IMs 1.61 ± 0.09 (all ×10⁶ cells per femur). Spleen: CNV PMNs 1.01 ± 0.05, ABX PMNs 0.67 ± 0.08, NEO PMNs 0.72 ± 0.08, CNV IMs 3.26 ± 0.20, ABX IMs 2.40 ± 0.46, NEO IMs 2.07 ± 0.11 (all ×10⁶ cells per spleen). Blood: CNV PMNs 882 ± 64, ABX PMNs 472 ± 38, NEO PMNs 530 ± 45, CNV IMs 227 ± 18, ABX IMs 92 ± 22, NEO IMs 154 ± 14 (all cells per μL). Horizontal lines in scatter plots depict median values. All other data presented as mean ± SEM with ≥5 mice per group. Statistical significance was assessed by Student t test for pairwise comparisons and 1-way ANOVA with Newman-Keuls posttest for comparisons of >2 conditions. *P < .05; **P < .01; ***P < .001. NS, not significant.