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.