The human gut microbiome and its dysfunctions through the meta-omics prism.

The microorganisms inhabiting the human gut are abundant (10(14) cells) and diverse (approximately 500 species per individual). It is now acknowledged that the microbiota has coevolved with its host to achieve a symbiotic relationship, leading to physiological homeostasis. The gut microbiota ensures vital functions, such as food digestibility, maturation of the host immune system, and protection against pathogens. Over the last few decades, the gut microbiota has also been associated with numerous diseases, such as inflammatory bowel disease, irritable bowel syndrome, obesity, and metabolic diseases. In most of these pathologies, a microbial dysbiosis has been found, indicating shifts in the taxonomic composition of the gut microbiota and changes in its functionality. Our understanding of the influence of the gut microbiota on human health is still growing. Working with microorganisms residing in the gut is challenging since most of them are anaerobic and a vast majority (approximately 75%) are uncultivable to date. Recently, a wide range of new approaches (meta-omics) has been developed to bypass the uncultivability and reveal the intricate mechanisms that sustain gut microbial homeostasis. After a brief description of these approaches (metagenomics, metatranscriptomics, metaproteomics, and metabolomics), this review will discuss the importance of considering the gut microbiome as a structured ecosystem and the use of meta-omics to decipher dysfunctions of the gut microbiome in diseases.