ABSTRACT
Trillions of microorganisms coexist within various parts of the human body, such as the gastrointestinal tract, lungs, mouth, skin and uterus. Although the exact number of these microorganisms is still unknown, they are incredibly diverse. The human-associated microbial communities consist of bacteria, viruses, archaea and eukaryotes. The entire collection of these microbes is called ‘the human microbiota’. Conversely, ‘microbiome’ refers to the collection of genomes of all these microorganisms and their surrounding environment (Marchesi and Ravel 2015). The human genome comprises approximately 23,000 genes, while a bacterial genome comprises 1,000 genes on an average, depending on the microorganism. There are several distinct genotypes (subspecies) within the estimated 500–1,000 bacterial varieties present in humans, suggesting that we host bacterial genes are several hundred times greater than the total number of our human genes (Vyas and Ranganathan 2012, Bull and Plummer 2014, Valdes et al. 2018). Individual humans have host genomes that are 99.9% indistinguishable from one another (Wheeler et al. 2008). However, their gut or hand microbiomes might differ by 80–90%, promoting genetic variation (Fierer et al. 2008, Turnbaugh et al. 2009a). Furthermore, the microbiome constantly changes due to many factors such as age, diet, location, probiotics and antibiotics usage, and environmental conditions (Figure 1). Thus, targeting microbiota instead of the largely stable host DNA is more adaptable and beneficial. It is crucial to understand how the diversity in microbiota impacts our health and triggers disorders. In recent years, an increased interest has been observed in the importance of the gut microbiota in the development as well as initiation of several diseases, including inflammatory bowel disorder (IBD) (Abraham and Cho 2009, Round and Mazmanian 2009), ischemic brain injury (Benakis et al. 2016), hepatic fibrosis (Bajaj et al. 2014) and obesity (Turnbaugh et al. 2009b). Our understanding of the role of the microbiome in disease etiology is continually evolving as the research continues to connect the gut microbiota to an increasing number of diseases, including cancer.
