Asthma, a chronic respiratory condition, significantly impacts patients' quality of life, affecting their social, professional, and family spheres. Recent studies suggest a link between asthma and the gut microbiome, with commensal bacteria potentially playing a protective role in lung inflammation.
In a groundbreaking human study, researchers discovered a diverse microbial community in the bronchial tree, challenging the notion of its sterility. Subsequent research revealed significant differences in the respiratory and intestinal microbiomes of asthmatics compared to healthy individuals.
The gut microbiome's influence on allergic inflammation may be mediated by bacterial metabolites. However, the factors determining the microbiome composition in asthma patients remain unclear. Most studies have focused on children, but adult asthmatics have also been examined, revealing differences in the gut microbiome based on asthma phenotype, which could impact treatment responses.
This study investigated the effect of diet and medication on the gut microbiome composition in asthma patients and its subsequent impact on treatment efficacy.
Methods and Study Groups:
The study included 49 asthma patients (15 men, 34 women) with an average age of 51 years. Patients were divided into three groups based on asthma control levels over the past four weeks. A control group of 18 healthy volunteers was also included. The study was approved by the relevant ethics committee, and all participants provided informed consent. Exclusion criteria included infections, organ dysfunctions, cancer, pregnancy, and recent probiotic or antibiotic use.
The study consisted of three parts: analyzing symptom questionnaires, the Food Frequency Questionnaire (FFQ), and testing the intestinal microbiota using traditional breeding and 16S rRNA sequencing methods.
Stool Samples and Bacterial Detection:
Stool samples were collected, stored, and processed for quantitative culture and bacterial species detection. The Vitek 2 Compact automatic bacteriological analyzer and MALDI Biotyper apparatus were used for automated biochemical and mass spectrometry methods, respectively.
Sequencing and Microbiome Analysis:
Bacterial DNA was isolated, and V3-V4 amplicons were prepared for sequencing. The Illumina MiSeq platform was used for sequencing at a laboratory in the Republic of Korea. Microbiome bioinformatics were performed using QIIME 2, assessing alpha and beta diversity metrics and assigning taxonomy to features using the q2-feature-classifier plugin.
Results:
There were no significant differences between study groups in terms of age, gender, BMI, food allergies, or hypersensitivities. The FFQ-6 revealed similar dietary habits between asthma patients and the control group. However, asthma patients reported significantly more symptoms related to the skin, respiratory system, and gastrointestinal tract.
Stool culture tests revealed abnormal results in 40 asthma patients, with the most frequent reason being an Escherichia coli titer < 106. Klebsiella pneumoniae and Clostridium perfringens were also detected.
NGS methods showed a significantly lower alpha diversity, assessed using the Simpson index, in asthma patients compared to the control group. Beta diversity analyses revealed significant differences in gut microbial composition between asthma patients and controls, with notable shifts in microbial composition and taxa abundance.
Differential abundance analysis (DAA) showed a significantly lower abundance of Faecalibacterium and a higher abundance of the Christensenellaceae taxa family in the patient group compared to healthy volunteers. DAA also revealed statistically significant depletion of Oscilospirales, Anaerovoracaceea, and Family XIII in patients with uncontrolled asthma compared to controlled asthma. In patients taking glucocorticoids, there was a statistically significant enrichment of Anaerovoracaceae and Christensenellaceae and depletion of Faecalibacterium.
Discussion:
The study demonstrated reduced biodiversity of intestinal microflora in asthma patients. The significant difference in the Simpson index suggests a less evenly distributed microbiota in asthma patients, with certain taxa dominating. Beta diversity analyses further supported significant differences between asthma patients and controls in microbial community structure.
Greater diversity and richness of the gut microbiome are associated with good health. Previous studies have shown conflicting results regarding alpha diversity in asthma patients, which may be due to confounding factors such as diet, geographic region, and environmental contaminants. The high complexity of asthma phenotypes may also impact the interpretation of results.
Adequate asthma control is a significant clinical challenge, and understanding the factors affecting microbial diversity and species involvement could suggest strategies for patients. Next-generation sequencing studies have identified gut microbiome alterations in asthmatic patients, and this study comprehensively assessed dietary habits and correlated them with microbiological data.
The control group's predominant bacteria were Firmicutes, Bacteroidetes, Actinobacteria, Proteobacteria, and Verrrucomicrobia, consistent with studies on healthy individuals. Asthmatic patients had a significantly higher percentage of Proteobacteria, particularly the Gammaproteobacteria class. Previous studies have suggested that a higher level of Proteobacteria is associated with an increased lipopolysaccharide production, intensifying the allergic inflammatory reaction in the respiratory tract and increasing the risk of allergic rhinitis.
Changes in the gut microbiome may also occur in other allergic diseases, such as allergic rhinitis, and a link between the lung and gut microbiome has been discovered in COPD. This study showed a decreased abundance of Faecalibacterium and an increase in Christensenellaceae in the asthma group. Christensenellaceae depletion may have a protective function in the context of asthma, while Faecalibacterium, as commensal bacteria, modulate the intestinal microbiota through SCFA production, exerting an anti-asthmatic effect.
The immunomodulatory properties of probiotic bacteria have been studied for their potential in preventing allergic diseases. Probiotics may have a positive effect on the immune system in asthma patients, with recent research showing beneficial effects on lung function and asthma control. Although there are no current recommendations for probiotic use in asthma, the findings are promising and may lead to clinical interventions using probiotic supplementation as a therapeutic strategy.
The study also showed a statistically significant reduction in Oscilospirales in patients with uncontrolled asthma. Oscillospira, an uncultured bacteria, is capable of producing SCFAs and has been characterized as a next-generation probiotic candidate.
An interesting observation was the presence of Escherichia coli at a titer <106 in most asthma patients. Escherichia coli is non-pathogenic but has beneficial effects on the host, including vitamin synthesis and immunomodulatory properties. Lower abundance of non-pathogenic E. coli strains is associated with allergic diseases, and oral administration of a non-pathogenic E. coli strain has been shown to reduce allergic symptoms in mice.
This study is one of the few to assess the gut microbiota using both NGS and stool culture methods. While NGS is the gold standard, it has limitations, and using traditional stool culture prevents the identification of unculturable microorganisms. The optimal approach is to use both methods, as done in this study.
In summary, significant differences were found in Alpha and Beta diversity in asthma patients compared to controls. Statistically significant differences in DAA were also observed depending on asthma control and glucocorticosteroid intake. Despite similar dietary habits and BMI, asthma patients had lower richness and diversity of intestinal microbiota compared to the control group. Differences in microbiome composition could be linked to disease severity and medication use, suggesting a potential influence of microbial taxa on asthma severity.
These observations provide important evidence of significant changes in the gut microbiome in asthma patients and require confirmation in future studies with larger patient populations. Analyzing the impact of gut microbiome changes on metabolic pathway disruptions in asthma patients may also be valuable.
Conclusions:
Despite similar dietary habits and BMI, asthma patients had lower richness and diversity of intestinal microbiota in the Simpson index compared to the control group. Differences in microbiome composition could be linked to disease severity and medication use, suggesting a potential influence of microbial taxa on asthma severity. These observations require confirmation in larger studies and provide valuable insights into the gut microbiome's role in asthma.
