Oral Bacteria in Feces as a Signature of Gut Microbiota Depletion with Implications for Human Health

A high proportion of oral bacteria in feces has been linked with intestinal diseases and digestive disorders. However, the association based on compositional data has two possible explanations: either oral bacteria translocate to the intestine and proliferate (the Expansion hypothesis), or the population of gut-resident bacteria declines, resulting in a relative increase in oral bacteria (the Marker hypothesis). Distinguishing between the two hypotheses can profoundly impact the interpretation of microbiome-disease associations. By analyzing quantitative microbiome data from various patient cohorts and experiments with mice, we provide strong evidence to support the Marker hypothesis. Specifically, treatments such as antibiotics or diseases may cause collateral damage to the gut microbiome, leading to a higher proportion of oral bacteria in feces without increasing their absolute numbers. The increased representation of oral bacteria in the intestine has significant implications for human health, including reduced overall survival rates in allogeneic hematopoietic cell transplantation recipients.