Contains the full lesson along with a supporting toolkit, including teachers’ notes.
The number of bacteria in the normal human gut is very large (1014) ― about ten times more than the total number of cells in the human body (1013 or ten million million). They make up 30% to 60% of the dry mass of faeces. Archaea, fungi and protozoa may also be present but, apart from pathogenic forms, they have not been studied as much as bacteria.
‘Normal’ gut bacteria
The normal human gut contains billions of bacteria and other micro-organisms (collectively called ‘microbiota’); they typically outnumber the cells in the body by a factor or ten (1014 bacteria compared to 1013 body cells). This normal population is also referred to as ‘indigenous microbiota’. Bacteria may make up half the mass of faeces. Gut bacteria are not all the same; in fact there may be hundreds of different species present at any given time and each species thrives on different substrates – sugars (and polysaccharides, especially glucans), proteins, fats, fibre or mixtures. Some can be pathogenic and may produce toxins but most are beneficial and may even produce essential vitamins or help prevent allergies. Normal gut bacteria support the body’s immune system to a significant extent.
Main types of food
In order to live and grow we need food that contains carbohydrates, fat (lipid), protein and fibre, as well as vitamins and other micronutrients.
Carbohydrate, typically in the form of starch, is our main energy source. Fat is an essential component of cell membranes; it acts as a thermal insulator and energy store. Protein is broken down into its constituent amino acids; these are the building blocks for the body’s own protein – including enzymes.
In recent years, there has been growing evidence of the importance of fibre in the diet. Far from being just a bulking agent, it might more correctly be regarded as food for the gut bacteria. Many micro-organisms can break down otherwise indigestible carbohydrates, such as cellulose, into smaller soluble components.
Certain minerals and vitamins (collectively called ‘micronutrients’) are also essential and their absence can lead to serious deficiency diseases such as pellagra and scurvy.