The role of the microbiota in pneumonia: friend or foe?

A better understanding of the distribution and composition of airway microbiota could help to protect people against pneumonia.

• Protective microbes
• Dangerous perturbations

What is already known on this topic
The microbes inhabiting the respiratory tract and the gut can both protect against and raise the risk of lung infections, or pneumonia. The normal respiratory tract and gut microbiota protects against pneumonia by inhibiting pathogen colonization and regulating immune responses. But disturbances of these microbiotas compromise their function and predispose to pneumonia.

What this research adds
Researchers summarized the available literature on how the healthy microbiota protects against pneumonia and how alterations of the microbiota composition influence susceptibility to lung infections. They also reviewed treatment approaches, including probiotics, which could help to preserve a healthy microbiota in the respiratory tract.

Conclusion

To refine therapeutic interventions for preserving a healthy microbiota and prevent pneumonia, more work is needed to understand the distribution and composition of the community of bacteria that inhabit the respiratory tract.

The microbes inhabiting the respiratory tract and the gut can both protect against and raise the risk of lung infections, or pneumonia. The normal respiratory tract and gut microbiota protects against pneumonia by inhibiting pathogen colonization and regulating immune responses. But disturbances of these microbiotas compromise their function and predispose to pneumonia.

Now, researchers led by Bastian Opitz at the Charité–Universitätsmedizin Berlin have reviewed the available literature on how the healthy microbiota protects against pneumonia and how alterations of the microbiota composition influence susceptibility to lung infections. 

The work, published in Science Translational Medicine, also discusses treatment approaches, including probiotics, which could help to preserve a healthy microbiota in the respiratory tract.

Protective microbes

“Community-acquired pneumonias are caused by bacteria including Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus or viruses such as the influenza virus,” the researchers say. On the other hand, hospital-acquired pneumonias are caused by opportunistic pathogens such as Pseudomonas aeruginosa, which are frequently resistant to multiple antibiotics. 

The microbes inhabiting the upper respiratory tract include Proteobacteria, Firmicutes, Proteobacteria, and Bacteroidetes. The microbiota of the lower respiratory tract is similar in composition to that of the upper respiratory tract, but the amount of bacterial DNA in fluid from the lungs is lower. 

In healthy people, the gut microbiota is dominated by the Bacteroidetes and Firmicutes families as well as Actinobacteria and Proteobacteria. The development of both the upper respiratory tract microbiota and the gut microbiota are influenced by factors including birth delivery mode and feeding mode, and they change as people age, losing their diversity.

Several studies have shown that commensal bacteria protect against pneumonia by competing for nutrients, directly killing disease-causing bacteria, or inducing localized immune responses that neutralize pathogens. For example, some species of Dolosigranulum and Corynebacterium release anti-pneumococcal metabolites, and Streptococcus salivarius produce antimicrobial molecules called bacteriocins that inhibit the growth of S. pneumoniae

Dangerous perturbations

Antibiotic treatments and critical care interventions can save the life of hospitalized people, but in some cases these approaches disrupt the respiratory tract microbiota, raising the risk of pneumonia. For example, broad-spectrum antibiotics, which are widely used in intensive care units, predispose individuals to lung infections with P. aeruginosa.

“In addition, antibiotics used clinically might severely compromise the patient’s antimicrobial defenses in the distal lung, thereby further enhancing the risk for hospital-acquired and ventilator-associated pneumonia,” the authors say. 

However, scientists have developed strategies to preserve or boost microbiota-dependent defense mechanisms. “Although many of these strategies are currently far from being standard of care, they have the potential to protect large numbers of patients against pneumonia,” the researchers say. 

For instance, probiotics can increase or maintain the microbiota’s functions in individuals at risk of lung infections. Clinical trials have shown that probiotics such as Lactobacillus strains can reduce the incidence of respiratory infections caused by syncytial virus as well as sepsis in preterm infants or newborns. 

However, further work is needed to refine these therapeutic interventions for keeping a healthy microbiota in the clinic, the researchers note. “More research is required to better characterize the spatiotemporal distribution of the microbiota in the respiratory tract in health and disease, the molecular mechanisms of colonization resistance in the upper airways, and the influence of the airway and gut microbiota on pulmonary immunity,” they add.

A better understanding of the distribution and composition of the community of bacteria that inhabit the respiratory tract could help to prevent microbiota alterations and protect people against pneumonia, the authors say.