The Dysbiosis Problem
Dysbiosis—abnormal microbiota—is clinically important but scientifically vague. Early definitions focused on imbalance: loss of beneficial bacteria or overgrowth of pathogens. However, "normal" varies enormously between individuals, and many disease states show different dysbiotic patterns. This imprecision hampers research and clinical application.
The Anna Karenina Principle
Borrowing from ecology, Leo Tolstoy's opening—"all happy families are alike; each unhappy family is unhappy in its own way"—applies to microbiomes. Healthy microbiota show consistent functional capacity (fermenting dietary fiber, producing butyrate, resisting pathogens), but dysbiotic states achieve dysfunction through multiple independent pathways. One patient's IBD involves Faecalibacterium loss; another's involves pathobiont bloom (e.g., Enterococcus); a third shows reduced diversity. Single dysbiosis definitions fail to capture this heterogeneity.
Functional Redundancy and Keystones
Ecosystems employ functional redundancy: multiple species perform similar roles. In the gut, Faecalibacterium, Roseburia, and Eubacterium all produce butyrate. Loss of one is often compensated; loss of all is catastrophic. Dysbiosis often reflects keystone species loss—species with disproportionate impact (e.g., Oxalobacter for oxalate metabolism, Ruminococcus bromii for resistant starch). Their absence disrupts function even if overall diversity remains.
Pathobiont Blooms and Resilience
Pathobionts are bacteria normally controlled by competitors and immunity but exploit dysbiotic opportunities. Clostridium difficile blooms after antibiotic-induced loss of colonisation resistance. Pathobiont expansion indicates functional failure, not necessarily a specific microbial change. Resilience (recovery after perturbation) and resistance (capacity to prevent invasion) are ecological parameters worth measuring alongside taxonomy.
Alternative Frameworks
Community typing (enterotypes, copro-types) and functional capacity analysis may better characterize dysbiosis than species-level imbalance. Assessing SCFA production, pathogenic potential, and immune interaction alongside taxonomy provides richer understanding. Rather than treating dysbiosis as a single entity, view it as ecosystem dysfunction with multiple possible causes.