Lower Airway Dysbiosis in NTM-Positive Bronchiectasis Is Linked to NET-Dominant Severe Phenotypes

Background

Bronchiectasis is a chronic lung condition in which the airways become abnormally widened, damaged, and prone to mucus buildup and recurrent infection. Over time, many patients develop persistent cough, frequent flare-ups, shortness of breath, and declining lung function. In recent years, researchers have learned that bronchiectasis is not only an infection-driven disease, but also an inflammatory one, especially in people whose airways are dominated by neutrophils, a type of white blood cell that helps fight infection.

At the same time, the microbial community in the lungs, known as the airway microbiome, has emerged as an important factor in disease severity. In some patients, the lower airways are dominated by Pseudomonas aeruginosa; in others, a mixture of oral bacteria can be detected. Another important and increasingly recognized group of organisms is nontuberculous mycobacteria, or NTM, which are environmental mycobacteria found in water and soil. NTM lung disease is becoming more common worldwide and can be difficult to diagnose and treat.

This study explored how lower airway microbes and neutrophilic inflammation interact in bronchiectasis, especially in patients who are positive for NTM. The researchers were particularly interested in neutrophil extracellular traps, or NETs, which are web-like structures released by activated neutrophils to trap microbes. Although NETs can help defend against infection, excessive NET formation can also damage lung tissue and worsen inflammation.

What the researchers studied

The investigators analyzed bronchoscopy samples from 200 people with bronchiectasis. Of these, 108 were NTM-negative and 92 were NTM-positive. They used 16S rRNA gene sequencing to identify bacterial communities in the lower airways, cell counts to measure inflammatory cells, and immunoassays to quantify NETs in bronchoalveolar lavage fluid.

To better understand cause-and-effect relationships, the team also used a preclinical mouse model. In that model, mice were exposed to oral commensal bacteria through microaspiration, along with NTM infection, to see how these exposures shaped immune responses in the lower airways.

Main findings in human samples

The lower airways of patients with NTM-positive bronchiectasis showed a distinct microbial pattern. As expected, Mycobacterium was enriched in this group. Importantly, the researchers also found increased levels of oral commensals such as Veillonella, Prevotella, and Streptococcus. These bacteria are commonly found in the mouth and upper airway, but they may enter the lungs through microaspiration, especially in people with impaired airway clearance.

This finding suggests that NTM-positive bronchiectasis is not simply associated with one pathogen. Instead, it often involves a broader dysbiosis, meaning a disrupted microbial balance, in which several organisms may coexist and potentially influence one another.

NET levels were significantly higher in bronchoalveolar lavage fluid from NTM-positive patients. In other words, the more severe microbial disturbance was linked to a stronger neutrophil response. Network analyses showed that Mycobacterium and oral commensals tended to co-occur with neutrophils and NETs, supporting the idea that these organisms may be part of a shared inflammatory ecosystem in the lower airway.

The study also found that certain oral bacteria were associated with more severe clinical phenotypes. These included cavitary disease, in which holes or cavities form in the lung tissue, and frequent exacerbations, meaning repeated worsening of symptoms. These are important findings because they suggest that bacteria beyond the main pathogen may help define which patients are at highest risk.

What the mouse model showed

The animal experiments provided a mechanistic clue. When oral commensal bacteria and Mycobacterium were combined in the mouse model, the lungs developed a sustained proinflammatory immune response. This response included increased Th17 cells, γδ T cells, and PD-1-positive T lymphocytes, along with higher NET levels.

These immune cells are associated with ongoing inflammation and host defense, but when overactivated they can contribute to tissue injury. Th17 cells, in particular, are known to promote neutrophil recruitment and are often implicated in chronic airway inflammation. The increase in NETs in this model supports the idea that microbial combinations can amplify neutrophil-driven damage rather than simply triggering a short-lived defense response.

Why NETs matter

NETs are an important feature of neutrophilic lung disease. They are made of DNA and antimicrobial proteins that can immobilize microbes. However, too many NETs can make mucus thicker, obstruct the airways, and injure surrounding tissue. In bronchiectasis, this may worsen the cycle of infection, inflammation, and airway damage.

The present study suggests that NTM-positive bronchiectasis is associated with a NET-predominant inflammatory pattern. This matters because it may help explain why some patients have more aggressive disease, why certain microbiome profiles are linked with worse outcomes, and why treatment may need to address both infection and inflammation.

Clinical implications

These findings have several practical implications. First, they reinforce that clinicians should not focus only on the presence or absence of NTM. The overall airway microbiome may help identify patients at higher risk for severe disease.

Second, the presence of oral commensal bacteria in the lower airways may be a marker of microaspiration or impaired airway protection. In practice, this could prompt a broader evaluation of contributing factors such as reflux, swallowing dysfunction, poor oral hygiene, or ineffective airway clearance.

Third, the association between oral bacteria, Mycobacterium, NETs, and severe clinical features suggests potential future therapeutic strategies. These may include more personalized microbiome-directed approaches, improved mucus clearance, reduction of aspiration risk, and therapies that target neutrophil-driven inflammation. At present, such strategies remain investigational, but they represent an important direction for future research.

For patients with NTM lung disease, treatment remains complex and often requires prolonged multidrug antibiotic regimens, careful monitoring for drug toxicity, and individualized decisions about whether and when to treat. In bronchiectasis, standard care also includes airway clearance therapy, management of underlying causes, vaccination, and treatment of coexisting conditions. This study does not replace current management guidelines, but it adds important biological insight that may improve future care.

What this study adds

This work helps move bronchiectasis research beyond a single-pathogen model. It shows that in NTM-positive disease, the lower airway environment may be shaped by a mixture of mycobacteria and oral bacteria, along with a strong neutrophilic and NET-based immune response. The result is a more severe disease phenotype in some patients, including those with cavities and recurrent exacerbations.

The key message is that airway disease severity may be driven not just by the organism most easily identified in testing, but by the larger microbial and immune context in which that organism exists. That broader view may be especially important in chronic lung infections such as bronchiectasis and NTM lung disease.

Limitations and future directions

As with many microbiome studies, this research shows association rather than definitive causation in the human samples. The bacterial signatures and NET elevations were measured at specific time points, so it is not possible to prove from patient data alone that one caused the other. The mouse model strengthens the biological plausibility, but human disease is more complex.

Future studies will need to determine whether modifying the airway microbiome can reduce NET formation, whether NET levels can serve as useful biomarkers, and whether patients with certain microbial profiles benefit from targeted interventions. Longitudinal studies will also be important to see how these airway patterns change over time and how they relate to treatment response.

Conclusion

In patients with bronchiectasis, especially those with NTM positivity, lower airway dysbiosis appears to be linked with stronger neutrophilic inflammation and more severe clinical disease. Mycobacterium and oral commensal bacteria were both enriched in the lower airways, associated with higher NET levels, and connected to worse phenotypes such as cavitary disease and frequent exacerbations. These findings highlight the importance of looking beyond the primary pathogen and considering the full microbial-immune environment when studying and managing bronchiectasis and NTM lung disease.