Table 1.

Number of microbial cells contained in environmental/human samples such as soil, sewage and human intestine.

Habitat	Number of Cells per g	Number of Cells per mL	Species Diversity
Soil	107–109 [22]	1010 [23]	4 × 103–5 × 104 species per g soil [22]
Sewage		109 [24,25]	25 per mL [24]
Marine water		105–106 [18]
Air		1 (=106 cells/m³) [17]
Human gut	1012 [26]		4 × 102 species per g feces [27]
	1011 [28]		5 × 103 species per g feces [18]
Colon (large intestine)		1011 [29] 1011–1012 [30]
Ileum (lower small intestine)		108 [29]
Duodenum and jejunum (upper small intestine)		103–104 [29]
Human mouth (saliva)		108 [18]