Dysbiosis

Dysbiosis describes a shift in the composition, diversity or metabolic output of the microbiota away from a configuration associated with host health, though it is an operational rather than precisely defined term because no single healthy reference community exists. It is characterised by loss of beneficial taxa such as short-chain fatty acid–producing Firmicutes, expansion of potentially pathobiontic species, reduced alpha-diversity, or altered functional capacity — changes associated with inflammatory bowel disease, metabolic syndrome, type 2 diabetes and colorectal cancer, among others. Causal directionality is difficult to establish in human studies, since most evidence is associative and dysbiosis may be both a driver of and a consequence of host inflammation. As a hallmark of ageing recognised in the 2023 update by López-Otín and colleagues, age-associated dysbiosis is increasingly considered a contributor to inflammaging and frailty.