What is the microbiome?
The human body is inhabited by trillions of microorganisms, which are collectively known as the human microbiota or microbiome. Included within these microorganisms are thousands of different species of bacteria, viruses, protozoa, and fungi. The human microbiome is involved in key roles in our body including digestion and nutrition, protection from harmful microorganisms, and immunity1.
Microbes inhabit all our body surfaces, including the internal surfaces of our digestive system. The majority are concentrated in the gut – specifically the large intestine. It is believed that our bodies contain 10 times more bacterial cells than human cells! Microbes and humans have co-evolved to ultimately depend on each other for survival.
Early perspectives of microorganisms: the germ theory of disease
In the early 19th century, the germ theory of disease was born. This theory proposes that illness arises from the actions of infectious microorganisms or pathogens. Proposed by Hungarian physician Ignaz Semmelweis (1818-65) and consolidated by French microbiologist Louis Pasteur (1822-95), the germ theory strongly influenced medical opinion and was crucial in identifying many diseases caused by microbes and finding ways to prevent them through vaccination, antibiotics, sanitation, and better living conditions.
A changing perspective
While many people still associate bacteria, viruses, and other microorganisms as our enemy, the reality is that most microorganisms present in our microbiome do not cause illness and actually promote the proper functioning of our body. The microbiome is sometimes referred to as a supporting organ since it plays so many important roles in the healthy daily functioning of the human body.
How the microbiome influences our health
The microbiome is involved in:
- The proper functioning of our immune system;
- Breaking down potentially toxic compounds in our food;
- Synthesis of certain vitamins and nutrients such as the B vitamins and vitamin K;
- Fermentation of indigestible fibres in our intestine to produce short chain fatty acids (SCFAs). SCFAs are used by the body as a nutrient source but also play an important role in muscle function and possibly the prevention of chronic diseases such as cancers and bowel disorders4
- Prevent the overgrowth of harmful microorganisms by competing for nutrients and attachment sites to the lining of the gut, a key site of immune activity and production of antimicrobial proteins.
Autoimmune diseases such as diabetes, rheumatoid arthritis or fibromyalgia are associated with dysfunction in the microbiome. Disease-causing microbes accumulate in the body over time, eventually impacting gene activity and subsequent metabolic processes that result in an abnormal immune response against substances and tissues normally present in the body6.
The microbiome and colorectal cancer
Research has shown that an imbalance or dysbiosis of the gut microbiome is present in patients with colorectal cancer (CRC).
Bacteria may affect CRC directly or indirectly, by:
- Secreting compounds (metabolites) which may affect the cancer development process (carcinogenesis);
- By invading tissues;
- By modulating the host immune response (e.g. bacteria-immune cell interactions).
The Fusobacterium, Peptostreptococcus, Porphyromonas, Prevotella, Parvimonas, Bacteriodes, and Gemella are among the most well-known CRC-associated bacteria.
The complex relationship between CRC and the gut microbiome has become an important area of current CRC research. While many effects of CRC-associated bacteria have been described, the underlying mechanisms that result in cancer are yet to be understood.