Eubacteria (True Bacteria) represent the most prolific lineage of life, forming a living skin over the entire planet. Current estimates place the total bacterial population between $2 \times 10^{30}$ and $5 \times 10^{30}$ cells. In terms of sheer mass, they hold approximately 70 gigatons of carbon, making them the second-largest biomass group on Earth after plants.

Bacterial density is highest in soil and sediments. A single gram of fertile soil can host up to 10 billion bacterial cells, serving as the chemical engines that decompose organic matter and fix nitrogen for plant growth. Like Archaea, the bulk of the bacterial population (approx. 70%) is locked in the deep subsurface, living in a state of suspended animation.

Closer to home, the human microbiome represents a dense bacterial ecosystem. Contrary to older myths of a 10:1 ratio, modern science confirms a 1:1 ratio: the average human body contains roughly 38 trillion bacterial cells and 30 trillion human cells. While equal in number, these bacteria contribute only about 0.2 kg to total body weight due to their microscopic size.