What's in a Stool?

Under normal circumstances, a person defecates about 100 to 250 grams of faeces daily. Some individuals defecate two or three times per day, while others go every other day, however it is generally considered healthy to pass stools at least three times a week. The large bowel is about 1.5 m long and contains about 220 g (range: 58-908 g) of contents (Cummings1991) at any given time.

A normal stool (types 3 and 4 of the Bristol Stool Form Scale) is made up of 75 percent water and 25 percent solid matter (Wenzl1995, Table 1). Very watery stools can contain >90% water, while dry stools can approach 50% moisture content (Nishimuta2006).

Table 1. The seven types of stool forms generally used clinically. Based on the Bristol Stool Form Guide, Lewis1997.

Stool Shape and Consistency	Bristol Type	Hydration	Description
	1	50-<70%	Separate hard lumps. Hard to pass.	Very constipated
	2		Lumpy and sausage-like. More difficult to pass.	Mildly constipated
	3	72-75%	A sausage shape with cracks in the surface. Moisture loss: 100-150 g/day.	Normal
	4	72-75%	Like a smooth, soft sausage or snake. Moisture loss: 100-150 g/day.	Normal
	5		Soft blobs with clear edges. Easy to pass.	Lacking fibre
	6	Semiformed: 82%; soft: 86%	Mushy consistency with ragged edges. Easy to pass. Moisture loss: 350 g/day.	Inflammation; mild diarrhoea
	7	Runny: 91%	Liquid consistency with no solid pieces. Easily passed. Moisture loss: 540-956 g/day.	Inflammation and severe diarrhoea

Of the solid material, approximately 25-54% consists of microbes, both living and dead (Cummings1991). The remainder of the solids comprises ~25% of indigestible food matter such as cellulose and other fibre; 2-15% is cholesterol and fats (likely from dead intestinal cells, dead bacteria and bile salt metabolites); several percent is insoluble inorganic substances such as calcium phosphate; and 2-25% is protein and other nitrogenous material. Normal stools are coated with a thin layer of mucus, which aids in the process of defecation.

Other matter commonly seen in faeces include: dead cells from the gastrointestinal tract; indigestible solids from the diet, such as mushroom, tomato peel and seeds from various fruits; liver waste products (e.g., bilirubin) and unabsorbed bile salts.

Traces of volatile organic compounds are excreted by gut bacteria and give faeces its characteristic repellant odour. Several small heterocyclic compounds, such as indole and skatole derived from the breakdown of tryptophan, are potent contributors to the heady mix. Smelly mercaptans, amines and branched short chain fatty acids may be present to varying degrees, depending on the diet and bowel retention time. While the most abundant gaseous byproducts of microbial digestion (CO₂ and methane) have no odour, other minor volatile byproducts, such as hydrogen sulfide and ammonia, certainly do.

Proportion of Bacteria in Stools

There are nearly 100 billion bacteria (and archaea) per gram of wet stool. This amounts to 1/4 to 1/2 of the solid material, or 12.5-25 g of bacterial biomass!

It is estimated around 50% of the microbes in faecal matter are alive when an individual defecates (BenAmor2005). Since the majority of cells are obligate anaerobes, and are killed when exposed to oxygen, many Bifidobacteria and Clostridium species will eventually die (Chu2017). Some bacteria are more active in the proximal section of the large intestine (e.g., saccharolytic Bacteroides species) and are probably dead soon after the digesta is concentrated in the left colon and rectum. Other microbes are more active in the distal gut (e.g., Dorea longicatena and Anaerobutyricum hallii) and are likely to be still viable when defecation occurs (BenAmor2005).