Firmicutes
Plain-English definitions of microbiome research vocabulary — the terms you'll see across articles, testing services, and probiotic claims.
The largest bacterial phylum in the human gut, encompassing diverse genera including Lactobacillus, Clostridium, Faecalibacterium, and Ruminococcus. Many important SCFA producers belong to this phylum. An elevated Firmicutes-to-Bacteroidetes ratio has been associated with obesity in some studies, though the relationship is complex and inconsistent.
Related terms
Bacteroidetes
One of the two dominant bacterial phyla in the human gut (alongside Firmicutes). Bacteroidetes species are generally skilled at breaking down complex plant polysaccharides and are associated with leaner body composition in some studies. The Firmicutes-to-Bacteroidetes ratio was once considered a key health marker, though its significance is now debated.
Short-Chain Fatty Acids (SCFAs)
Metabolites produced when gut bacteria ferment dietary fibre. The three main SCFAs — butyrate, propionate, and acetate — are critical for colon health, immune regulation, and energy metabolism.