165 Differences in bacterial composition between serum-incubated IgG-coated and original untreated fecal samples The most common bacterial family observed in fecal samples was Lachnospiraceae (IBD: 40%; HC: 38%), followed by Ruminococcaceae (IBD: 14%; HC: 26%) and Bifidobacteriaceae (IBD: 6.7%; HC: 6.8%). The bacterial family of Bacteroidaceae accounted for a lower fraction of the microbial composition (IBD: 0.8%; HC: 2.0%). More variation in fecal microbial composition was observed on the genus level while comparing mean relative abundances (%) between IgG-coated vs. untreated fecal samples from patients with IBD and from HC (Figure 3, Table 2). Anti-IgG-based flow cytometry of fecal samples of patients with IBD revealed that between 13-77% (mean: 38%) of bacteria showed IgG-coating after incubation with autologous serum, which further increased to a range of 21-75% (mean: 45%) of enriched IgG-bound bacteria after MACS-sorting occurring in ~84% of samples (Supplementary Table S2). After the MACS procedure, several bacterial genera were significantly enriched in fecal samples from both patients with IBD and HC, including Streptococcus, Coprococcus, Dorea, Ruminococcus gnavus-like bacteria, Lactobacillus, Dialister, Veillonella, and Turicibacter (all P<0.05 in both groups). In total, 10 out of 21 (48%) bacterial genera demonstrated significant changes in abundance in IgG-coated vs. untreated fecal samples in both patients with IBD c. The bacterial genera Clostridium, Enterococcus, and Collinsella were only significantly enriched in samples from HC, whereas Lactococcus was only significantly enriched in patients with IBD (Figure 4). In contrast to these enrichments, relative abundances of the bacterial genera Faecalibacterium, Roseburia, and Bacteroides were significantly decreased after IgG-coating in samples from both patients with IBD and HC (all P<0.05). In addition, the relative abundances of Blautia, Eubacterium, and Enterobacteriaceae-like bacteria were solely decreased after IgG-coating in patients with IBD (P<0.01). Unpaired analysis between samples from patients with IBD and HC revealed that the bacterial genera Streptococcus, Lactobacillus and Lactococcus were more abundant in IBD (P<0.05). Antimicrobial IgG immune responses in IBD
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