The dominant phyla and what they do
The adult gut microbiome is dominated by a few bacterial phyla. Bacteroidetes and Firmicutes together make up the large majority of bacteria, with Actinobacteria (including Bifidobacterium), Proteobacteria, and Verrucomicrobia (including Akkermansia) present in smaller proportions. Within these phyla, hundreds of species ferment fiber, synthesize some vitamins, train the immune system, and produce short-chain fatty acids.
Composition is highly individual and shifts over a lifetime, shaped early by birth mode and feeding and later by diet, medication, and environment.
What shifts the ratios
Diet is a major lever: fiber-rich, plant-diverse eating tends to support a broader range of fermenters, while antibiotics can sharply reduce diversity for a period. The popular idea of an ideal Firmicutes-to-Bacteroidetes ratio as a health metric has not held up well, and simple ratios are now viewed cautiously by researchers.
This is general educational information, not medical advice. There is no single correct microbiome, commercial microbiome tests have limited clinical validity, and gut-health concerns should be discussed with a qualified professional.