Gut microbe found to worsen sepsis by triggering hyperinflammatory immune responses

Why do some people recover easily from bacterial infections while others rapidly deteriorate into life-threatening sepsis? According to a new study published in Nature Communications, the answer may lie not only in the invading pathogen itself, but also in the microorganisms already living inside the gut.
Sepsis is a severe condition in which the body's immune system overreacts to infection, causing widespread inflammation and organ damage. In many cases, the excessive immune response itself becomes more dangerous than the bacteria causing the infection.

Recent studies have suggested that gut microbiota play an important role in regulating baseline immune status and may influence susceptibility to infectious diseases.
Researchers has now identified a specific gut microbial group that can dramatically worsen sepsis by excessively sensitizing immune cells.
The researchers observed that even genetically identical mice showed strikingly different infection outcomes depending on the composition of their gut microbiota. When exposed to the same amount of pathogenic bacteria, some mice survived with relatively mild symptoms, whereas others rapidly deteriorated and showed significantly lower survival rates due to overwhelming immune activation.

Further analysis revealed that one key factor associated with severe disease was the enrichment of a gut bacterial family known as Muribaculaceae. Among these microbes, a bacterium called Sangeribacter muris KT1-3 was found to produce metabolites that placed immune cells into an excessively hypersensitive state.
As a result, when pathogens invaded the body, the immune system reacted far more aggressively than necessary, leading to uncontrolled inflammation and fatal sepsis.
To confirm that the gut microbiota itself was responsible for these effects, the team also performed fecal microbiota transplantation experiments. When gut microbes associated with severe infection were transferred into otherwise resistant mice, survival rates declined sharply. Conversely, transferring healthier microbial communities improved survival outcomes.

The study further demonstrated that tiny metabolites produced by specific gut microbes can prime immune cells beyond their normal activation threshold. This exaggerated immune sensitivity caused even relatively small external stimuli to trigger explosive inflammatory reactions, ultimately resulting in life-threatening sepsis.
These findings suggest that sepsis severity is determined not only by the virulence of invading pathogens but also by the composition of the gut microbial environment.
This study demonstrates that gut microbiota can fundamentally alter the intensity of immune responses and thereby determine infection outcomes.

Enrichment of specific gut microbes, particularly Muribaculaceae and Sangeribacter muris KT1-3, increases sepsis severity by producing metabolites that excessively sensitize immune cells, leading to hyperinflammatory responses and reduced survival. Fecal microbiota transplantation confirmed that gut microbial composition directly influences susceptibility to severe sepsis.

Seonghan Jang et al, A Muribaculaceae-enriched microbiota exacerbates TLR4-dependent Acinetobacter baumannii-induced hyperinflammatory sepsis, Nature Communications (2026). DOI: 10.1038/s41467-026-72435-3