Table 1.

Data extraction and summary of the human studies investigating the relationships between gut microbiota dysbiosis and RA.

Author	Study Design	Study Subjects	Biospecimen	Microbiological Analysis	Findings
Chen et al. (2016) [21]	Cross-sectional	n = 40 RA n = 32 HCs	Fecal samples	16S rDNA sequencing	Decrease in Faecalibacterium and expansion of Eggerthellales and Collinsella.
Scher et al. 2013 [59]	Cross-sectional	n = 44 new onset RA n = 26 chronic RA n = 16 PsA n = 28 HCs	Fecal samples	16S rDNA sequencing	Prevotella copri strongly correlated with disease in new onset RA patients. Increases in Prevotella abundance correlated with a reduction in Bacteroides and a loss of reportedly beneficial microbes in new onset RA subjects.
Pianta et al. (2018) [60]	Observational	n = 127 RA	Fecal samples	16S rDNA sequencing	Prevotella copri antibody responses were rarely found in patients with other rheumatic diseases or in HCs.
Yu et al. (2021) [61]	Cross-sectional	n = 26 RA n = 26 HCs	Fecal samples	16S rDNA sequencing	Klebsiella, Escherichia, Eisenbergiella, and Flavobacterium were more abundant in the RA patients, while Fusicatenibacter, Megamonas, and Enterococcus were more abundant in the HCs.
Kishikawa et al. (2020) [62]	Cross-sectional	n = 82 RA n = 42 HCs	Fecal samples	16S rDNA sequencing	Multiple species belonging to the Prevotella genus increased in the RA gut metagenome.
Alpizar-Rodriguez et al. (2019) [63]	Cross-sectional	n = 50 HCs n = 83 pre-clinical RA	Fecal samples	Culture-independent microbiota analyses	The microbiota of individuals in “pre-clinical RA stages” was significantly altered compared with FDR controls. A significant enrichment of the bacterial family Prevotellaceae, particularly Prevotella spp., in the “pre-clinical RA” group (p = 0.04) was observed.
Chen et al. (2021) [64]	Cross-sectional	n = 29 RA n = 30 HCs	Fecal samples	16s rRNA sequencing	At the genus level, Bacteroides, Faecalibacterium, and some probiotics decreased in the RA group, while 97 genera, including Lactobacillus, Streptococcus, and Akkermansia, increased in the RA group.
Diamanti et al. (2020) [65]	Cross-sectional	n = 30 RA with high adherence to Mediterranean diet, n = 30 RA with low adherence to Mediterranean diet	Fecal samples	16s rRNA sequencing	A healthier gut microbiota composition was observed in the high adherence to the Mediterranean diet group, with a significant decrease in Lactobacillaceae and an almost complete absence of Prevotella copri with respect to the low/moderate adherence group.
Sun et al. (2022) [66]	Cross-sectional	n = 42 RA n = 39 HCs	Fecal samples	MiSeq sequencing	The gut microbiota of RA patients was characterized by a decreased abundance of Pholiota, Scedosporium, and Trichosporon.
Kitamura et al. (2022) [37]	Observational	n = 87 RA	Fecal samples	16s rRNA sequencing	Total bacteria counts were correlated with endotoxin neutralizing capacity (p < 0.001) and inversely correlated with serum lipopolysaccharide (p < 0.001) and anti-Pg-LPS IgA antibody levels (p < 0.001).