Differences in bacterial populations in the healthy and diseased human airways have already been recognized as a possible contributing factor in the pathogenesis of diseases of the respiratory tract, however, the bacterial population in the airways of subjects diagnosed with coal worker’s pneumoconiosis (CWP) has not yet been investigated
The lung microbiome during health overlaps mainly with the microbiome of the oral cavity, not with those from nose and gastric tract, which provides evidence for microaspiration as a common way of formation of the lung microbiome in healthy individuals, but it is less species-rich and shows signs of specific elimination of some bacterial species from the upper respiratory tract. The lung microbiome shows great interindividual variability [9, 22].
The «healthy» lung microbiome may be perturbed by pulmonary diseases and some environmental factors, for example, by the diverse microbial exposure during development of childhood asthma .
In the course of our investigation, we have determined certain regularities in the composition of microbial communities in the sputum of coal miners suffering from occupational lung fibrosis.
The data obtained by us do not allow to determine the primary cause of disease: changes in the microbiota under the influence of environmental factors or the pulmonary pathology. At the same time, we suggest that the risk of developing CWP is connected to the composition of the microbial community in the sputum since it is known that certain bacterial taxa are capable of producing pro- or anti-inflammatory lipopolysaccharides. Pro-inflammatory lipopolysaccharides bind the CD14 / TLR4 / MD2 receptor complex in many cell types, but especially in monocytes, dendritic cells, macrophages and B cells, which contributes to the secretion of pro-inflammatory cytokines, nitric oxide and eicosanoids . Changes in the balance of lipopolysaccharide production in the direction of an inflammatory response is likely to influence the immunologic reaction significantly, leading to a pathologic development.
When evaluating the relative abundance of bacterial types in the sputums of CWP patients, we noted a significant decrease in Bacteroidetes, primarily the genus Prevotella. Previously, a decrease in the relative level of Bacteroidetes and especially representatives of the genus Prevotella was reported in patients with bronchial asthma  and COPD . It is known that bacteria from the Bacteroidales order produce anti-inflammatory forms of lipopolysaccharides thereby providing immune escape for the entire microbiota community . In particular, the proportion of anti-inflammatory lipopolysaccharides produced by representatives of Bacteroidetes can reach 79% of all anti-inflammatory lipopolysaccharides present in healthy subjects .
An increased level of pro-inflammatory lipopolysaccharides in blood plasma was previously observed in many inflammatory diseases, including COPD . We noticed an increase in the level of Pasteurellaceae (Proteobacteria) in patients with CWP and an increase in the levels of the potentially pathogenic genus Haemophilus also belonging to this family. Proteobacteria, being the main producers of pro-inflammatory lipopolysaccharides, have been repeatedly associated with various inflammatory and allergic diseases . Pasteurellaceae spp. are commensals of the mucous membranes but can act as opportunistic pathogens following a decrease in immunity due to various factors. They have the ability to synthesize pore-forming toxins (RTX), pro-inflammatory lipopolysaccharides and immunogenic lipoproteins .
In assessing beta diversity, a significant increase in the genus Streptococcus and, in particular, of the Streptococcus agalactiae was observed in individuals with CWP diagnosis.
Streptococcus agalactiae (also known as GBS) is an important opportunistic bacterium that can cause pneumonia, sepsis and meningitis in newborns and in patients with weakened immunity. GBS bacteria effectively attach to pulmonary epithelial cells and are capable of invasion. This is initiated by attachment to extracellular matrix molecules such as agglutinin, fibronectin, fibrinogen and laminin, which facilitates their attachment to host cell surface proteins, such as integrins. Thus, the invasive potential of GBS is influenced by changes in the surface proteome of the host cells, which can be caused by various lung pathologies . The molecular mechanisms of human cytopathology caused by GBS bacteria is under intensive investigation currently [33–34].
In the sputum of men with CWP diagnosis we found, in addition to Streptococcus, a small, but statistically significant increase in the abundance of representatives of the genera Gemella and Bacillus as compared to the control group. Gemella, a genus of Gram-variable motionless asporogenic bacteria, is a commensal of the oral cavity in the healthy population but it can act as a causative agent of lung abscess . Gemella can also cause endocarditis , can play a role in exacerbating pneumonia and act as a biomarker in patients with cystic fibrosis .
In the microbiome of miners with CWP diagnosis, we noted a significant decrease in the abundance of representatives of the genera Selenomonas, Megasphaera, and Dialister compared to the controls. Interestingly, while the bulk of emerging data about these bacterial species concern their association with human pathologies, one study demonstrates that Dialister is locally reduced in tumor biopsies from lung cancer patients. The Dialister population was larger in material taken from an unaffected area from the same patients. Moreover, the level of Dialister was higher in the control group than in any of the samples from cancer patients .
We have evaluated the impact of factors like smoking and age, on the differential representation of the bacterial taxa in the sputum of coal miners with CWP diagnosis. Our correlation analysis showed no significant age-dependent differences in the representation of the Streptococcus, Prevotella (f. Prevotellaceae), Selenomonas and Megasphaera taxa. Thus, we infer that the variation in sputum microbiomes that we observe is not age related, but that exposure to occupational hazards, such as coal dust may be the causative factor for the variation in sputum microbiomes. The impact of smoking was evaluated only in the control group. We confirmed a decrease of Neisseria in the sputum of smokers as previously reported . Additionally, we revealed a significant decrease of Bulleidea and Peptostreptococcus species in the sputums of the smokers.
In sputum samples from active coal miners we detect significantly more of three bacterial taxa: Lachnoanaerobaculum orale, Prevotella Tannarae and Uncultured eubacterium E1-K12, as compared to former miners, all with a CWP diagnosis. This may provide evidence for specific damages to the respiratory tract of the coal miners not coupled to age, smoking or exposure to hazardous occupational factors. Recently, an increased level of Lachnospiraceae and Lachnoclostridium was reported in patients with silicosis, albeit in the gut microbiome .
We describe the differential representation of certain bacterial species in the sputum of coal miners with CWP diagnosis. The causative contribution of this to CWP pathogenesis requires further investigation.