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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
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Three shifting immune states in sepsis could explain why treatments miss their window
Researchers have mapped how the immune response in people with sepsis changes over time. The work could help pave the way for treatments that target the specific parts of the immune system that are altered over the time course (trajectory) of sepsis illness in adults.Sepsis is a life-threatening condition that occurs when the immune response to infection misfires. In sepsis, vital organs fail, and the condition can be fatal even when treated quickly. It's estimated that there are over 160 million cases and about 21 million deaths from sepsis worldwide each year.
Current treatments for sepsis focus on treating the underlying infection with antimicrobials and providing supportive care for failing vital organs. Despite efforts to treat the misfiring immune system, none have successfully improved outcomes for patients.
In the new study, published this week in Immunity, researchers sought a more detailed understanding of the immune response mechanisms that change over time in patients with sepsis.
They analyzed blood samples collected at four different time points (between admission to and discharge from critical care) from critically ill patients with sepsis . To build a detailed picture of the immune response, called an "immune profile," the researchers examined multiple layers of immune response information in the blood samples, including data on immune cells, gene expression and changing protein expression.
Using machine learning approaches, they then combined these layers of information to generate a more comprehensive immune profile for the first time in patients with sepsis.
The analyses revealed an immune trajectory with three distinct temporal immune states (referred to as STImS) between admission and recovery, with each state involving different immune cell activity and immune response programs.Importantly, these sepsis immune states didn't match the clinical stage of sepsis. For example, the "early" immune state (STImS1) was not the same as the early clinical stage of sepsis, which is often the day a clinician diagnoses sepsis.
The researchers say these findings could have important implications for determining how best to treat patients with sepsis—specifically, which treatments to use and when.
The main aim of this work was to build a profile of sepsis immune responses over time. When people are admitted to a hospital with sepsis, they are usually classed as having 'early' sepsis—but the new findings show that this isn't necessarily the case—their immune system may already be at later stages of the immune response. Knowing exactly what is happening to a patient's immune system during sepsis could identify which treatments are likely to work best.
This research shows the importance of looking at the changing architecture of the immune system in sepsis over time, rather than just taking a snapshot view. We need to find better ways to treat the misfiring immune system. Only by understanding the intricacies of the immune system in all its component parts—by integrating and dynamically mapping cell and molecular immunobiology to determine why the very system designed to protect us from infections is misfiring in sepsis—can we begin to improve outcomes for patients by treating the misfiring immune system, say the researchers.The researchers say the next step for this work is to understand what causes changes in the immune system between the onset of infection and the development of sepsis. Understanding these changes could help identify new treatment targets and approaches to reduce either progression to sepsis or the severity of sepsis, with the potential to improve outcomes and make a difference in the lives of millions affected by serious infections worldwide.
Temporal Analyses of Immune Responses in Sepsis Reveal Asynchrony Between Clinical stage of illness and Immune State, Immunity (2026). DOI: 10.1016/j.immuni.2026.08.003. www.cell.com/immunity/fulltext … 1074-7613(26)00325-0
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