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# 1.1 Overview of the gut microbiome

The gut microbiome is composed of trillions of bacteria, viruses, archaea and fungi. In fact, it is estimated that there are 1014 bacteria in the gut – that’s 100,000,000,000,000 individual bacteria living in our guts! However, the microbiome is dynamic and many of these bacteria will be dying or lost on a daily basis, while it is thought that 1011 new bacteria enter the gut every day, travelling from the pharynx (back of the throat) to the rest of the gastrointestinal (GI) tract.

## Fun fact: Dealing with very large or very small numbers

You may have noticed that the paragraph above used two ways of writing the same number. One approach involved writing out lots of zeros, while the other approach used the number 10, raised to the power of 14 – an approach known as scientific notation.

Using scientific notation removes the difficulty of having to keep an eye on all those zeros and reduces the potential to accidentally miss one out or include an extra one! Scientific notation can be used to write very large numbers and also for writing very small numbers.

For example:

- (a) A population of 100,000,000 microorganisms could be written as 1 x 108
- (b) The diameter of a particular bacterium might be written as 0.000006 m, or 6 x 10-6 m

- Question 2

  In example (a) above, how many zeros are there in the number written in full, and what is the power of 10 used when the number is written in scientific notation?

- There are 8 zeros in the number written in full, and the power of 10 used in the scientific notation is 8. Essentially, you have moved the decimal point 8 spaces.

- Question 3

  In example (b) above, how many zeros are there in the number written in full, and what is the power of 10 used when the number is written in scientific notation?

- There are 6 zeros in the number written in full, and the power of 10 used in the scientific notation is -6. Essentially, you have moved the decimal point 6 spaces.

This may help you to convert from a number in full, to a number of scientific notation. When converting a very large number into scientific notation, the power of 10 will be a positive value (as in example a), and when converting a very small number into scientific notation, the power of 10 will be a negative value (as in example b). To work out what the power should be, you simply count how many times you moved the decimal point.

The composition of the gut microbiome changes throughout our lives. During the first 3 years of life our gut microbiome is formed and affected by various factors, such as breastfeeding, weaning and bacteria found in the local environment. As we age, two groups of bacterial species become predominant, called Firmicutes and Bacteroidetes, which form about 90% of the gut microbiota (Arumugam, 2011).

Figure 4 shows the distribution of the most common bacterial groups found in microbiomes around the body. You may notice that the different parts of the GI tract contain different proportions of the bacteria.

Figure 4 Distribution of bacteria found on or in different parts of an average healthy body

- Question 4

  What is the most common bacterial group in the oesophagus and in the intestines?

- Firmicutes is the most common group of bacteria in the oesophagus, while Bacteriodetes is the most common bacterial group in the small and large intestines.

The six most common bacterial groups found in the microbiota are Firmicutes, Bacteroidetes, Fusobacteria, Actinobacteria, Cyanobacteria and Proteobacteria. The distribution of bacterial groups or species in an individual forms their enterotype, and this is named after the predominant bacterial type present in that individual. For example, an individual with the highest proportion of Bacteroidetes in their gut microbiome would be referred to as having a Bacteroidetes enterotype. The identification of these enterotypes may have important consequences for health (you’ll learn more about the importance of bacterial diversity and balance later).

The bacteria of the gut microbiome interact with the local gut environment and with each other to play several important roles in the human body. These fall into five main functional categories:

- metabolic
- structural
- protective
- communication
- immune.

You will explore the many functions of the gut microbiome in detail later in the course.

◄Contents►Next: 1.2 Balancing the gut microbiome

OpenLearn - The gut microbiome – balancing the body
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