The phylum Firmicutes constitutes the most diverse and abundant group of gastrointestinal microbiota which are grouped into four classes, Bacilli, Clostridia, Erysipelotrichia, and Negativicutes. They constitute about 39% of gut bacteria in healthy adults, but may increase to as high as 80% in an imbalanced microbial community.

Gram-negative Bacteroidetes and grampositive Firmicutes are bacterial phyla that dominate the entire human digestive tract, including the mouth, nose, throat, and colon. An abnormal result in one or both of these phylum suggest imbalanced normal microbes in the GI tract. Further, high Firmicutes and low Bacteroidetes (resulting in a high F/B ratio) suggest microbial imbalance which may be related to increased caloric extraction from food, fat deposition and lipogenesis, impaired insulin sensitivity, and increased inflammation.

- High Firmicutes and low Bacteroidetes abundances have been equivocally associated with obesity. A high-fat diet is associated higher abundance of both Firmicutes and Proteobacteria, and lower abundance of Bacteroidetes in mice.

- Increased levels of Firmicutes have been associated with Crohn’s disease and ulcerative colitis.

- Low abundance of Firmicutes and greater abundance of Akkermansia muciniphila have been reported in lean individuals.