The respiratory tract microbiome and lung inflammation: a two-way street

Abstract

The lungs are not sterile or free from bacteria; rather, they harbor a distinct microbiome whose composition is driven by different ecological rules than for the gastrointestinal tract. During disease, there is often a shift in community composition towards Gammaproteobacteria, the bacterial class that contains many common lung-associated gram-negative “pathogens.” Numerous byproducts of host inflammation are growth factors for these bacteria. The extracellular nutrient supply for bacteria in the lungs, which is severely limited during health, markedly increases due to the presence of mucus and vascular permeability. While Gammaproteobacteria benefit from airway inflammation, they also encode molecular components that promote inflammation, potentially creating a cyclical inflammatory mechanism. In contrast, Prevotella species that are routinely acquired via microaspiration from the oral cavity may participate in immunologic homeostasis of the airways. v Areas of future research include determining for specific lung diseases (1) whether an altered lung microbiome initiates disease pathogenesis, promotes chronic inflammation, or is merely a marker of injury and inflammation, (2) whether the lung microbiome can be manipulated therapeutically to change disease progression, (3) what molecules (metabolites) generated during an inflammatory response promote cross-kingdom signaling, and (4) how the lung “ecosystem” collapses during pneumonia, to be dominated by a single pathogen.

Worms: Pernicious parasites or allies against allergies?

“…The host genome and the total diversity of the microbiota (the “microbiome”) are important in mediating or reflecting health and disease in the intestine, and in other barrier sites such as the lung and skin. Changes in the gut and lung microbiomes are seen in allergic diseases such as asthma, reflecting the immune axis between these mucosal sites or transfer of bacterial populations through processes such as inhalation of airborne bacteria, bacterial migration along mucosal surfaces and microaspiration of gastric contents …”

Incorporating the airway microbiome into asthma phenotyping: Moving toward personalized medicine for noneosinophilic asthma

“…These include reactive oxygen and nitrogen species produced by neutrophils and other cell types that can be converted by members of the Gammaproteobacteria to support their anaerobic respiration in inflamed obstructed airways. 8 This reflects the versatility of microbes to adapt and even thrive under a variety of conditions, invoking mechanisms like those demonstrated in Gammaproteobacteria, such as Pseudomonas, Moraxella , 9 and Haemophilus 10 species.…”

Microbiome in Chronic Lung Diseases

“…It is likely that the healthy human lung microbiome represents transitory presence of micro-aspirated upper airway microbial flora that is in the process of being cleared by normal lung defence mechanisms 24 . The acronym TBNR, which represents 'transient but not resident' appears to be a fairly accurate characterization of the healthy lung microbiome 25 .…”