Digging deeper into necrotizing enterocolitis: bridging clinical, microbial, and molecular perspectives

Abstract

Necrotizing Enterocolitis (NEC) is a severe, life-threatening inflammatory condition of the gastrointestinal tract, especially affecting preterm infants. This review consolidates evidence from various biomedical disciplines to elucidate the complex pathogenesis of NEC, integrating insights from clinical, microbial, and molecular perspectives. It emphasizes the modulation of NEC-associated inflammatory pathways by probiotics and novel biologics, highlighting their therapeutic potential. We further critically examine dysbiotic alterations within the gut microbiota, with a particular focus on imbalances in bacterial and viral communities, which may contribute to the onset of NEC. The intricate interactions among toll-like receptor 4 (TLR4), microvascular integrity, immune activation, and the inflammatory milieu are meticulously summarized, offering a sophisticated understanding of NEC pathophysiology. This academic review aims to enhance the etiological comprehension of NEC, promote the development of targeted therapeutic interventions, and impart the significant impact of perinatal factors on the formulation of preventive and curative strategies for the disease.

Keywords: Necrotizing enterocolitis (NEC); dysbiosis; mechanism; microbes; preterm infants; toll-like receptor 4 (TLR4).

Figure 1.

Clinical perspective of NEC. (a) Etiology. NEC is a multifactorial disease. Prematurity, dysbiosis, hypoxia-ischemia, and non-breast milk feeds are common factors contributing to the development of NEC. (b) Clinical presentation. Abdominal distention, vomiting, and bloody stool are typical signs of NEC. (c) Laboratory tests. Complete blood count, blood gas, electrolytes, and blood culture are routine tests of NEC. (d) Diagnosis. Bell staging system, together with clinical manifestations and abdominal X-ray findings are usually used to diagnose and assess the severity of NEC. In response to the limitations of bell criteria, multiple newer diagnostic definitions of NEC have been proposed. Ultrasound and biomarkers remain an ongoing pursuit. (e) Treatments. Conventional treatment includes halting all oral feedings, performing nasogastric decompression, administering broad-spectrum antibiotics, providing supplemental fluids intravenously, and surgical intervention if necessary. Probiotics in protecting against NEC is an enthralling and burgeoning area, and novel biologic agents continue to be a compelling frontier in medical research. (f) Complications. Secondary organ involvement, particularly lung and brain injuries, is commonly observed in addition to the primary intestinal effects.

Figure 2.

Microbial perspective of NEC. (a) Gut microbiota in preterm infants without NEC. In premature infants without NEC, beneficial bacteria such as the genera bifidobacterium and lactobacillus predominate, whereas potential pathogenic bacteria like Klebsiella, Escherichia, and Enterobacter are less prevalent. Under these conditions, LPS levels are lower, resulting in reduced TLR4 signaling and minimal inflammation. (b) Gut microbiota in preterm infants with NEC. In preterm infants with NEC, there is an increase in the abundance of potential pathogenic bacteria such as Klebsiella, Escherichia, and Enterobacter, together with a reduction in beneficial bacteria like bifidobacterium and Lactobacillus. This shift is associated with heightened production of LPS, which, upon interaction with TLR4, substantially activates the TLR4 signaling pathway. This in turn initiates a robust inflammatory cascade, predisposing to the development of NEC. In addition, a large number of potential pathogens can migrate to the intestinal lamina propria through a compromised intestinal barrier, which can also lead to gut inflammation, and, ultimately, the potential development of NEC.

Figure 3.

Molecular perspective of NEC. (a) Immune system. Neutrophils, rapidly migrate to sites of injury or infection upon the detection of tissue damage or the release of alarmins. They perform their function through the formation and release of NETs. In the context of NEC, the macrophage population increases in the intestinal tissue, where they play a crucial role in clearing bacteria that breach the intestinal epithelial barrier. TGF-β2 in the macrophage microenvironment enhances the inhibition of LPS-induced cytokine production and facilitates the maturation of immature macrophages. The immunomodulatory effect is correlated with a significant decrease in the incidence of NEC. In the intestine of NEC, T lymphocytes are extremely abundant, particularly CD4⁺ Th17 cells. These cells can induce a proinflammatory phenotype under appropriate conditions. In contrast, the proportion of anti-inflammatory regulatory T (Treg) cells relative to effector T cells is notably reduced, which is important in the pathogenesis of NEC. (b) TLR4 signaling. In the premature gut, TLR4 is upregulated, and an imbalanced microbiota activates TLR4 in response to dysbiosis (TLR4 recognizes its ligand, LPS) initiating a cascade of immunological and cellular events. Specifically, TLR4 signaling activation induces enterocyte demise through mechanisms of apoptosis, autophagy, and necroptosis. The cumulative impact of these pathological processes results in irreversible harm to the intestinal mucosa, a defining pathogenesis feature of NEC. Mutations, deficiencies, or inhibition of the TLR4 gene can confer protection against NEC. NOD2 and MDP (a NOD2 agonist) have a protective effect on NEC through the dampening of TLR4 signaling. HMOs can inhibit the LPS binding site on TLR4, which in turn reduces TLR4 signaling and consequently attenuates intestinal inflammation. EGF and LPS-neutralizing peptides, which inhibit TLR4 signaling, mitigate the pathological features of NEC. (c) Microvascular system. The relative hypoxia in utero serves as a potent stimulus for HIF-mediated signaling, which promotes VEGF synthesis, and activates VEGF/VEGFR2 signaling, culminating in the induction of fetal angiogenesis. Exposure to oxygen-rich conditions can result in supraphysiologic oxygen concentrations, leading to a reduction in HIF-mediated signaling, which may impair VEGF synthesis, and predispose premature infants to NEC. Intestinal mucosal damage enables bacterial translocation into the mesenteric microvasculature and triggers a cascade of interactions, notably with TLR4 on endothelial cells. This leads to a downregulation of eNOS and a subsequent reduction in nitric oxide production, which promotes vasoconstriction of the mesenteric vessels, and consequently intestinal ischemia, a hallmark of NEC.