2.5 Protection against pathogens

Another important function of the gut microbiome for human health is protection against invading pathogens (Figure 12). Pathogens can be harmful to the human host in two ways. Firstly, by penetrating through the gut wall and causing an infection and/or inflammation, and secondly by replacing some of the resident symbiotic microorganisms within the microbiome through colonisation, which then causes dysbiosis and alters the normal functioning of the microbiome.

Figure 12 The six groups of pathogens illustrated with their size compared to other cells and objects

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A schematic illustration shows examples of six pathogens with their size compared to other cells or substances. The size of the following is depicted in the illustration: Tapeworm (red tubular structure): 3 to 10 metres long. Human hair (a hair strand): 50 to 180 micrometres thick. Grain of salt (powder salt): 60 micrometres. Protozoa (oval-shaped) (for example, malaria): 1 to 20 micrometres. Fungi (green branched filaments with multiple nuclei) (for example, Candida albicans - thrush): 2.6 to 3.4 micrometres. Bacteria (for example, Escherichia coli): 1 to 3 micrometres. Virus (a red sphere) (for example, SARS-CoV-2): 0.1 to 0.5 micrometres. Prion (protein molecule): 0.002 to 0.010 micrometres.

Figure 12 The six groups of pathogens illustrated with their size compared to other cells and objects

Until recently, it was unclear how the microbiome prevented colonisation by pathogens. However, recent research from the University of Oxford has identified that the beneficial bacteria of the microbiome consume the nutrients that pathogenic bacteria need to grow and replicate, so the pathogenic bacteria cannot thrive and instead die out (Spragge et al., 2023).

The researchers tried to identify if there were particular species or groups of microbiota bacteria that are important for protection against the pathogenic bacteria. While they identified that certain bacterial species were better for consuming the nutrients that the pathogens needed to survive, individual bacterial species were very poor at preventing pathogenic colonisation. Instead, the bacterial species needed to be in communities of at least 50 different types of bacterial species to fully prevent the pathogens from multiplying. This again highlights the ways in which the different bacterial species within the microbiome interact and function collectively, and the importance of a balanced microbiome.