TABLE 1.

Overview of four most common bacterial phyla within the gut microbiome, listed by generally decreasing relative abundances in healthy individuals ^{a , 2} , ⁹ , ¹⁰

Phylum (2021 revised nomenclature)	Brief description	Genus examples
Bacteroidetes (Bacteroidota)	Gram‐negative; typically obligately anaerobic; often abundant	Bacteroides, Prevotella
Firmicutes (Bacillota)	Gram‐positive; typically obligately anaerobic; often abundant; highly diverse phyla	Clostridium, Enterococcus, Eubacterium, Faecalibacterium, Lactobacillus, Roseburia, Ruminococcus, Streptococcus
Actinobacteria (Actinomycetota)	Gram‐positive; typically obligately anaerobic/requires low oxygen levels	Bifidobacterium, Corynebacterium, Eggerthella
Proteobacteria (Pseudomonadota)	Gram‐negative; mostly facultatively anaerobic; includes many pathogenic species	Enterobacter, Escherichia, Klebsiella, Serratia

^a Bacteroidetes and Firmicutes are listed alphabetically. Collectively these two phyla comprise up to 90% of a healthy microbiome. ³ , ⁹