A BRIEF RETROSPECT OF THE GUT MICROBIOTA
The human micro-ecological environment is an incredibly intricate “super bacterial body ” that coexists in organs such as the skin, lungs, and intestines, with specific bacterial colony patterns in different regions. The gut microbiota includes all the bacteria, archaea, viruses, eukaryotes, protozoans, and surroundings in the gastrointestinal tract4. About 100 trillion microbes live in the gastrointestinal tract, and the total number of cells and genomes of these bacteria are 10 times and more than 100 times greater respectively compared to humans5. Surprisingly, such massive colonization still maintains a long-term, mutually beneficial and deep relationship with the host.
The importance of the gut microbiota to the host lies, on the one hand, in the fact that it strengthens the intestinal epithelial mucosal barrier and is involved in the digestion of food, absorption of nutrients, metabolism of drugs, and defense against pathogens and toxins6,7. It also motivates intestinal ripening by releasing mucus, promoting angiogenesis, thickening the villi, widening the mucosal surface, and supporting cell proliferation8. Besides these actions, gut microbiota produces a variety of vital substances such as vitamins B12, folate, biotin, pyridoxine and thiamin, as well as metabolites including short-chain fatty acids (SCFA), bacteriocins, and microbial amino acids, which are involved in a variety of biological metabolic activities6,7. Furthermore, the gut microbiota promotes growth by inducing insulin-like growth factor-1 signaling9.
On the other hand, the gut microbiota activates and regulates a range of immune cells such as innate hematolymphoid cells, natural killer cells, and helper lymphocytes, reflecting its essential contribution in driving the host immune function10. SCFA, polysaccharide A, α-galactosylceramide, and tryptophan metabolites from the gut microbiota activate interleukin-22, interleukin-17, IgA, and Reg3γ which participate in immune regulation11. SCFA also stimulates the production of the anti-inflammatory factors interleukin-10, interleukin-21; murein lipoprotein, a cell wall component of gut microbiota, and can also promote IgG release12. Conceivably, healthy colonization of gut microbiota during infancy has profound implications for future immunity and metabolism13,14.
Notably, the gut microbiota’s significance probably dictates its rapid establishment shortly after birth but may be influenced by prenatal non-sterile intrauterine conditions, a special placental microbiota15. In the early stages of life,Proteobacteria and Actinobacteria are significant members of the gut microbiota; over time, the diversity and abundance of the intestinal microbiota evolved until early childhood acquired a microbial composition similar to that of adults16. By this stage the microbiota consists mainly of Proteobacteria ,Firmicutes ,Actinobacteria , Verrucomicrobia , andBacteroidetes at the phylum-level, with Firmicutes and Bacteroidetes accounting for 90 percent of the total present4,6. Other studies have shown that Gram-positive cocci , Enterobacteriaceae orBifidobacteriaceae are the major components of infant gut microbiota, which gradually transition to a predominance ofBifidobacteriaceae 17. In contrast, the gut microbiota of preterm infants in the neonatal intensive care unit (NICU) were more likely to show a sequential switch from Bacillito Gammaproteobacteria toClostridia 18,19. Overall, this dynamic transition in the gut microbiota is likely an adaptive alteration undertaken by the evolving neonatal population. However, this adds to the challenge of understanding the gut microbiota during the neonatal period. Therefore, it is not surprising that some studies have reported differing or even opposing results in gut microbiota composition during the neonatal period.
It is noteworthy that throughout gut microbiota development, its composition is extremely susceptible to a variety of elements as shown in Figure 1 15,16. Antibiotics are presumed to be one of the most important and sensitive factors in causing insult to the gut microbiota. For example, early administration of oral antibiotics to newborn rats resulted in significant gut microbiota changes, indicating that Proteobacteria and Bacteroidetesreplaced Firmicutes and Actinobacteria , and the concomitant descent in the proportion of Clostridia andBacilli was accompanied by an increment inGammaProteobacteria 20. Furthermore, remarkable changes in gut microbiota diversity were observed in infants exposed to antibiotics both prenatally and during the intrapartum stages, especially in terms of decreases observed in Bacteriodetes andBifidobacteria and an increase inProteobacteria 21. Another study showed that postnatal antibiotic exposure was associated with significantly lower levels of Enterococcus and Lactobacillus in the intestines of preterm infants, with Enterococcus thought to be associated with immunomodulation22 and Lactobacillusexhibiting powerful anti-inflammatory properties23.
Previous studies have shown that early and prolonged antibiotic exposure increases BPD risk in deficient birth weight infants24. One possible explanation is that antibiotics lead to alterations in the taxonomy and functional diversity of the gut microbiota, prolonging the time to restore healthy colonization and increasing foreign pathogen invasion opportunities25. Antibiotics can also whittle down the concentrations of SCFA20, which are considered to have antibacterial and anti-inflammatory properties26. Besides this, antibiotics diminish the abundance ofLactobacillus , which may delay host weight gain27 and further affect BPD28.
Gestational age is another factor that affects the gut microbiota. Premature infants showed reduced microbiota diversity compared to full-term infants, with decreased amounts of Bifidobacterium andBacteroides and increased amounts of Enterococcus andProteobacteria in those born early4. Other research has found a significantly increased abundance ofStaphylococcaceae in NICU preterm infants, accompanied by delayed transition to Bifidobacteriaceae ; a decrease inBifidobacteriaceae possibly results in acetate concentration and pH changes, which are also associated with premature health17. In part, preterm infants are usually transported to environmentally stringent NICU, thus limiting their contact with mothers and the surrounding environment, which results in a delayed or impaired conversion of the gut microbiota from facultative anaerobic to completely anaerobic bacteria14. This may result in an insufficient abundance of Bifidobacterium andBacteroides and lead to increased pathogenic bacteria invasion.
Taken together, the gut microbiota probably exerts a tremendous impact on health. Maintaining gut microbiota diversity and stability potentially enhances host-specific resistance to the environment, as abundant species can mutually compensate for functional deficiencies29. Conversely, once the indicators representing the relative steady state of the gut microbiota, including resistance, resilience, and functional redundancy, change dramatically, the gut microbiota may have a diminished or delayed ability to recover its original phenotype13. Once the original phenotype cannot be regained, gut microbiota dysbiosis can occur, which may result in onset of several gastrointestinal and extraintestinal diseases, as shown in Figure 1 .
ASSOCIATION OF GUT MICROBIOTA WITH BPD
Considerable numbers of studies have demonstrated lung microbiota imbalances in patients with BPD30,31. This is of interest given that the intestinal and respiratory epithelium have many similarities, both anatomically and functionally32. Several studies have investigated gut microbiota dysbiosis in BPD patients. Research found that the relative abundance ofEscherichia and Shigella increased significantly, whileKlebsiella and Salmonella declined markedly in gut microbiota from infants born transvaginally with BPD compared to those without BPD33. Furthermore, the gut microbiota of preterm infants receiving mechanical ventilation was significantly enriched in Proteobacteria with age, whilst Firmicutesnumbers declined sharply and Staphylococcus was the dominant genus at the genus level34. Another case-controlled study showed that the operational taxonomic units, relative abundance, and Shannon index of the gut microbiota were significantly reduced in BPD infants 28 days after birth35. Additionally, BPD severity probably correlated positively with the risk of gut anaerobic microenvironment disruption35.
Interestingly, it also appears that changes in the gut microbiota of BPD can be viewed from the perspective of metabonomics. Pintus et al.36 collected urine samples from 18 newborns seven days after birth and identified that the BPD and non-BPD groups displayed distinct metabolic patterns. Specifically, alanine and betaine increased, while trimethylamine-N-oxide (TMAO), lactate, and glycine decreased in the BPD group. Since the gut microbiota mediates the formation and production of TMAO to some extent37, it can be assumed that the decreased levels of TMAO in BPD patients reflect alterations in their gut microbiota.
These data directly or indirectly reveal the fact that gut microbiota dysbiosis occurs in BPD infants. In fact, gut microbiota dysregulation in turn probably also affects the progression of BPD. For instance, in the BPD model of perinatal maternal antibiotic exposure (MAE), the destruction of the gut microbiota diminished pulmonary vascular density, thickened the alveolar septum under hyperoxia, induced alveolar simplification and promoted more severe BPD characterized by pulmonary fibrosis38. It is worth mentioning that MAE was sufficient to induce pulmonary vascular obstacle even under normoxia conditions, suggesting that lung structural abnormalities are associated with microbiota dysbiosis38. Another study also showed that MAE remarkably diminished the abundance of commensal bacteria in the mouse gut, which aggravated hyperoxia-induced impairment of alveolar and angiogenesis39. Moreover, a cohort study in people showed a markedly increased risk of death or BPD in very low birth weight (VLBW) infants who received antibiotics two weeks after birth40. The increased risk of death or BPD was positively correlated with antibiotic exposure duration, meaning that each additional day of antibiotics was associated with an approximately 13% increased BPD risk40.
Paradoxically, however, Althouse et al.41 reported that although MAE caused gut microbiota dysbiosis, this did not dramatically exacerbate the hyperoxia lung injury phenotype. Another study showed that the lung microbiota is more likely to influence BPD severity than the gut microbiota42. One possibility is that in addition to BPD, there are likely multiple other factors influencing gut microbiota, this increases the uncertainty of both associations, suggesting that the correlation between gut microbiota and BPD needs further investigation.
Hyperoxia is known to be a high-risk factor for BPD. One possible reason is that hyperoxia alters the gut microbiota, leading to the pathogen invasion and inflammation involved in BPD development. For example, hyperoxia exposure dramatically elevated amounts ofEnterobacteriaceae 43, Proteobacteria andEpsilonbacteraeota 44 in mouse intestines. Another murine intestinal tract showed that hyperoxia reversed the antibiotic-induced decreases in Bacteroidales andAlistipes and incerases ofAkkermansia 41. The gut of rats exposed to hyperoxia conditions also showed enrichment of Streptococcus andGammaproteobacteria and Proteusdeficiencies45. Furthermore, Ashley et al.46 showed that following 72 hours of hyperoxia exposure, the intestinal tracts of mice exhibited pronounced decreases in the Firmicuts and Ruminococcaceae families and significantly increased Bacteroidetes were present; this gut microbiota dysbiosis possibly correlated with the degree of lung inflammation. Notably, the authors demonstrated that dysregulation of the lung and gut microbiota occurred prior to lung injury, suggesting that the microbiota dysbiosis contributed to the formation of hyperoxia-induced lung injury in mice46. In contrast, hyperoxia-exposed germ-free mice showed diminished structural and functional lung damage and had attenuated inflammatory infiltration compared to non-germ-free mice, suggesting a role for the microbiota in the development of BPD47.
Another aspect to consider is that hyperoxia disrupts intestinal epithelial cells, causing changes to the secretory component proteins, thus affecting mucosal immunity48. The intestine of hyperoxia-exposed rats showed significantly increased diamine oxidase, intestinal fatty acid binding protein, liver-type fatty acid binding protein, and concomitant decreased tight junction protein, suggesting impairment of the intestinal mucosal barrier49. Besides these differences, hyperoxia markedly elevated intestinal permeability and upregulated inflammatory markers such as toll-like receptor 4 (TLR4), nuclear factor kappa-B, tumor necrosis factor-α (TNF-α), interleukin-10, and interferon-γ49,50. These results presumably facilitated the translocation of intestinal bacteria into the lungs, leading to an increase in pulmonary cytokines which affected lung health51.
Metabolically, important considerations include fecal volatile organic compounds (VOCs), which are potentially intimately linked to BPD. VOCs probably affect lung function by altering the gas-liquid interface properties of pulmonary surfactant52, stimulating the production of pro-inflammatory factors53, and exacerbating oxidative stress54. Furthermore, VOCs alter the microRNA profile of lung tissue, which in turn impairs lung health55. Research has identified that VOCs can potentially predict the risk of lung injury in swine exposed to hyperoxia56. Similarly, VOCs can help diagnose and predict the onset of acute respiratory distress syndrome57 and BPD58 early. One study revealed that several fecal VOCs, such as tetradecane, N-nitrosopyrrolidine, and trichloretene, were significantly elevated in BPD patients59. Other studies confirmed that changes in fecal VOCs were intimately related to BPD severity60.
Notably, fecal VOC analysis is considered to be an accurate diagnostic tool for gastrointestinal diseases, and transformations in the gut microbiota play an etiological role. In other words, fecal VOCs largely reflect the composition, function, and interaction of gut microbiota61. Thus, from this perspective, gut microbiota dysbiosis likely causes changes in fecal VOCs, which further affects BPD.
Briefly, gut microbiota and BPD presumably mutually influence each other, but extensive experiments are needed to elucidate the specific mechanisms regarding their bidirectional effects. It is also necessary to establish the intrinsic link between gut microbiota dysbiosis and BPD through more direct experiments, rather than relying on antibiotic exposure, as antibiotics can damage microbiota in other parts of the host. Furthermore, it remains to be fully understood whether other gut microbiota members such as archaea, viruses, eukaryotes, and protozoans influence BPD.