Tocilizumab-based GVHD prophylaxis is associated with lower Enterococcus abundance. (A) The alpha diversity, as measured by the Inverse Simpson index, is shown over time relative to transplant day. Both the Toci and No Toci groups exhibited a similar decrease in diversity over the first 2 weeks following infusion. No difference was observed in diversity between groups when compared using the Wilcoxon test for 5-day intervals. (B) The relative abundance of Enterococcus over time was markedly different between the Toci and No Toci cohorts (multivariable linear mixed-effects regression P = .0056). (B) Principal component plots are shown; 4 clusters were identified when using k-means clustering (top left). Toci exposure (top-right), antibacterial exposure within previous 4 days (bottom left), and most prevalent genus in each sample (bottom right) are shown. The Toci vs No Toci contribution to the clusters was significantly different (Kruskal-Wallis; P = 1.3e^-14). Antibiotics to which patients were exposed in the 4 days before each sample collection were classified by their extent of anticipated perturbation of human intestinal microbiome communities as previously described.⁴⁷