Necrotizing enterocolitis and cytomegalovirus infection in a premature infant

The Nonbacterial Microbiome: Fungal and Viral Contributions to the Preterm Infant Gut in Health and Disease

The intestinal microbiome is frequently implicated in necrotizing enterocolitis (NEC) pathogenesis. While no particular organism has been associated with NEC development, a general reduction in bacterial diversity and increase in pathobiont abundance has been noted preceding disease onset. However, nearly all evaluations of the preterm infant microbiome focus exclusively on the bacterial constituents, completely ignoring any fungi, protozoa, archaea, and viruses present. The abundance, diversity, and function of these nonbacterial microbes within the preterm intestinal ecosystem are largely unknown. Here, we review findings on the role of fungi and viruses, including bacteriophages, in preterm intestinal development and neonatal intestinal inflammation, with potential roles in NEC pathogenesis yet to be determined. In addition, we highlight the importance of host and environmental influences, interkingdom interactions, and the role of human milk in shaping fungal and viral abundance, diversity, and function within the preterm intestinal ecosystem.

Postnatal Cytomegalovirus Infection of Preterm and Very-low-birth-weight Infants Through Maternal Breast Milk: Does It Matter?

BACKGROUND

Postnatal infection with cytomegalovirus (CMV) in very-preterm and very-low-birth-weight infants, transmitted through breast milk (BM), is potentially associated with adverse outcomes. This study aimed to investigate the incidence and clinical significance of postnatal CMV infection in a tertiary neonatal intensive care unit.

METHODS

Infants of CMV-seropositive mothers born in a neonatal intensive care unit in Melbourne, Australia, were observed for 14 weeks from birth in a prospective cohort study. Maternal BM and infant urine were tested weekly for CMV by culture and polymerase chain reaction, respectively. Clinical and laboratory data were collected and analyzed in relation to the infants' CMV infection status.

RESULTS

Data from 65 infants of 56 CMV-seropositive mothers were available for analysis. Of these mothers, 88% (49/56) shed CMV in their BM. Of the 58 infants exposed to CMV-positive BM, 27 (47%) became urine polymerase chain reaction CMV-positive. There was no significant difference in gestational age, birth weight, incidence of bronchopulmonary dysplasia, or necrotizing enterocolitis between the CMV-positive and CMV-negative groups. However, CMV-positive infants had a longer length of hospital stay and more episodes of prolonged neutropenia. Of the CMV-positive infants, 30% (8/27) remained asymptomatic, 48% (13/27) had symptoms categorized as mild and 22% (6/27) as severe.

CONCLUSIONS

About half of preterm and very-low-birth-weight infants exposed to CMV-positive BM become infected, and a fifth develop significant clinical symptoms. Future studies should address the maternal and neonatal factors that determine the risk of mother-to-infant CMV transmission, as well as those leading to clinical deterioration and long-term sequelae.

Recommendations for Diagnosis and Prevention of Cytomegalovirus-Associated Necrotizing Enterocolitis in Breast-Fed Preterm Infants

We present the case of a breast-fed preterm infant with postnatally acquired cytomegalovirus (CMV) and severe necrotizing enterocolitis (NEC) associated with CMV. The infant had persistent severe thrombocytopenia with clinical deterioration despite multiple platelet transfusions and maximal medical treatment. Surgical intervention was not feasible owing to the instability of the infant's condition. Upon identification of CMV in urine, intravenous ganciclovir was initiated with significant clinical improvement. We also present a literature review of cases of CMV-related NEC or other gastrointestinal complications in preterm and term infants.

Dynamics of the preterm gut microbiome in health and disease

Advances in metagenomics have allowed a detailed study of the gut microbiome, and its role in human health and disease. Infants born prematurely possess a fragile gut microbial ecosystem that is vulnerable to perturbation. Alterations in the developing gut microbiome in preterm infants are linked to life-threatening diseases such as necrotizing enterocolitis (NEC) and late-onset sepsis; and may impact future risk of asthma, atopy, obesity, and psychosocial disease. In this mini-review, we summarize recent literature on the origins and patterns of development of the preterm gut microbiome in the perinatal period. The host-microbiome-environmental factors that portend development of dysbiotic intestinal microbial patterns associated with NEC and sepsis are reviewed. Strategies to manipulate the microbiome and mitigate dysbiosis, including the use of probiotics and prebiotics will also be discussed. Finally, we explore the challenges and future directions of gut microbiome research in preterm infants.

Necrotizing Enterocolitis: LPS/TLR4-Induced Crosstalk Between Canonical TGF-β/Wnt/β-Catenin Pathways and PPARγ

Necrotizing enterocolitis (NEC) represents one of the major causes of morbidity and mortality in premature infants. Several recent studies, however, have contributed to a better understanding of the pathophysiology of this dreadful disease. Numerous intracellular pathways play a key role in NEC, namely: bacterial lipopolysaccharide (LPS), LPS toll-like receptor 4 (TLR4), canonical Wnt/β-catenin signaling and PPARγ. In a large number of pathologies, canonical Wnt/β-catenin signaling and PPARγ operate in opposition to one another, so that when one of the two pathways is overexpressed the other is downregulated and vice-versa. In NEC, activation of TLR4 by LPS leads to downregulation of the canonical Wnt/β-catenin signaling and upregulation of PPARγ. This review aims to shed light on the complex intracellular mechanisms involved in this pathophysiological profile by examining additional pathways such as the GSK-3β, NF-κB, TGF-β/Smads, and PI3K-Akt pathways.

The association between enteric viruses and necrotizing enterocolitis

Studies on necrotizing enterocolitis (NEC) have not largely focused on enteric viruses. In order to demonstrate the association between enteric viruses and NEC, stool specimens of 51 neonates with NEC and 39 "normal" neonates were collected to detect rotavirus (RV), astrovirus (ASV), sapovirus, enterovirus (EV), adenovirus (ADV), Epstein-Barr virus (EBV), cytomegalovirus (CMV), and human bocavirus (HBoV). Rotavirus A (RVA), ASV, EBV, and ADV were detected in both the NEC and control groups; however, EV and HBoV were detected only in the NEC group and CMV was not detected in either group. ASV was the most common enteric virus, but no significant differences were found between NEC and control groups, as was similarly the case for EBV and EV. The prevalence of ADV and HBoV was higher in the NEC group than in the control group (P = 0.011, P = 0.005, respectively) but RVA showed the opposite trend (P = 0.014). Virus positivity or negativity had no influence on the clinical manifestation of NEC.Conclusion: The roles of different viruses in NEC are not congruent. Some, such as ASV, may be regarded as commensal in neonates, while in NEC patients, the presence of ADV and EBV may be related to severity of disease. What is known: • The etiology of NEC remains unknown. Studies on necrotizing enterocolitis (NEC) have not largely focused on enteric viruses and the conclusions were inconsistent. What is new: • Enteric viruses are common in the gut of neonates, but not all of them are pathogenic. • The existence of ADV and EBV may be related to the severity of NEC.

Extended-spectrum beta-lactamase (ESBL) producing Enterobacterales in stool surveillance cultures of preterm infants are no risk factor for necrotizing enterocolitis: a retrospective case-control study over 12 years

Purpose

Microbial dysbiosis has been found preceding necrotizing enterocolitis (NEC) in preterm infants; thus, we aimed to investigate whether there is evidence that neonates with extended-spectrum beta-lactamase-producing Enterobacterales (ESBL-E) positive stool cultures are at higher risk for NEC at the NICU.

Methods

We included very preterm inborn infants of ≤ 32 weeks of gestational age being fecal carriers of ESBL-E and compared them with 1:1 matched (gestational age, birth weight, gender and year) controls tested negative for ESBL-E in the stool between 2005 and 2016. An association with NEC was defined as the first detection of ESBL-E before or at the time of definite diagnosis of NEC.

Results

During the study period, we diagnosed 217 infants with a total of 270 ESBL-E. We identified ten different species with ESBL-producing Klebsiella oxytoca being the most common one (46%) followed by Klebsiella pneumoniae (19%), and Citrobacter freundii (17%). Ten out of 217 infants had any kind of NEC in the case group compared to two of the controls (p < 0.01), but only four cases with predefined criteria were associated with NEC ≥ stage IIa (1.8 vs. 0.5%, p = 0.089, OR 4.1, CI95% 0.45–36.6). NEC mortality rate was 2/8 (25%).

Conclusions

We observed a threefold increase of ESBL-E in stool surveillance cultures during study time and germs were dominated by ESBL-producing Klebsiella spp. There was no evidence that preterm infants colonized with ESBL-E in the stool were at higher risk for definite NEC.

Postnatally acquired cytomegalovirus infection in extremely premature infants: how best to manage?

Postnatal cytomegalovirus (pCMV) infection is a common viral infection typically occurring within the first months of life. pCMV refers to postnatal acquisition of CMV rather than postnatal manifestations of antenatal or perinatal acquired CMV. pCMV is usually asymptomatic in term infants, but can cause symptomatic disease in preterm (gestational age <32 weeks) and very low birth weight (<1500 g) infants resulting in sepsis, pneumonia, thrombocytopaenia, neutropaenia, hepatitis, colitis and occasionally death. There are significant uncertainties regarding the management of premature infants with pCMV disease which is in part due to our limited understanding of the natural history of this disease. This review describes the current epidemiology and clinical manifestations of pCMV disease which should alert clinicians to test for CMV and also outlines a strategy to manage the condition.

Detection of Cytomegalovirus in Intestinal Tissue of Infants with Necrotizing Enterocolitis or Spontaneous Intestinal Perforation

OBJECTIVE

To determine the frequency of detection of cytomegalovirus (CMV) in surgical or autopsy intestinal tissue from infants with necrotizing enterocolitis (NEC) or spontaneous intestinal perforation (SIP) of the small bowel.

STUDY DESIGN

This was a retrospective cohort study of infants in the neonatal intensive care unit at Nationwide Children's Hospital, Columbus, Ohio, with NEC (Bell stage ≥2B) or SIP from 2000 to 2016. Paraffin-embedded surgical or autopsy intestinal tissues were examined for CMV by polymerase chain reaction (PCR) and immunohistochemistry (IHC), and clinical characteristics of CMV-positive vs CMV-negative cases were compared.

RESULTS

CMV was detected by PCR or IHC in 7 (4%) of 178 infants with surgical or autopsy- confirmed NEC (n = 6) or SIP (n = 1). Among 143 NEC cases (123 surgical, 20 autopsy), CMV was detected in 6 (4%): 4 (2 surgical, 2 autopsy) by both PCR and IHC, and 2 (surgical) by PCR only. Among 35 SIP cases (32 surgical, 3 autopsy), 1 (3%) surgical case was positive, by PCR only. CMV-associated NEC cases had lower median gestational age (24 vs 28 weeks; P = .02), birth weight (649 vs 1121 g; P = .04), and platelet count (16 000/mm³ vs 50 000/mm³; P = .018) compared with CMV-negative cases, respectively. No association was found with receipt of maternal milk, age at NEC diagnosis, male sex, cholestasis, or mortality.

CONCLUSIONS

CMV was detected in intestinal tissue from 4% of NEC or SIP cases (NEC, 4%; SIP, 3%). Lower gestational age, lower birth weight, and thrombocytopenia were significantly associated with detection of CMV in NEC or SIP cases.

Observational study of cytomegalovirus from breast milk and necrotising enterocolitis

OBJECTIVE

To evaluate the relationship between cytomegalovirus (CMV) exposure from breast milk and risk of necrotising enterocolitis (NEC).

DESIGN

Secondary analysis of a multicentre, observational cohort study. Maternal breast milk and infant serum or urine were serially evaluated by nucleic acid testing at scheduled intervals for CMV. Infants with evidence of congenital infection were excluded. Competing-risks Cox models, with adjustment for confounders, were used to evaluate the relationship between breast milk CMV exposure or postnatal CMV infection and NEC.

SETTING

Three neonatal intensive care units in Atlanta, Georgia.

PATIENTS

Infants with a birth weight≤1500 grams.

EXPOSURES

Maximal CMV viral load in breast milk in the first 14 days after birth or postnatal CMV infection. Two different approaches were used to assess the timing of onset of CMV infection (midpoint or early).

MAIN OUTCOME MEASURES

NEC, defined as Bell stage II or greater.

RESULTS

Among 596 enrolled infants, 457 (77%) were born to CMV seropositive mothers and 33 developed postnatal CMV infection (cumulative incidence 7.3%, 95% CI 5.0% to 10.1%). The incidence of NEC was 18% (6/33) among infants with CMV infection, compared with 7% (37/563) among infants without infection (adjusted cause-specific HR (CSHR): 2.81; 95% CI 0.73 to 10.9 (midpoint); 6.02; 95% CI 1.28 to 28.4 (early)). Exposure to higher breast milk CMV viral load was associated with a higher risk of NEC (adjusted CSHR per twofold increase 1.28; 95% CI 1.06 to 1.54).

CONCLUSIONS

CMV exposure from breast milk may be associated with the development of NEC in very low birth weight infants.

Detection of cytomegalovirus in saliva from infants undergoing sepsis evaluation in the neonatal intensive care unit: the VIRIoN-C study

Objective To determine the frequency of detection of cytomegalovirus (CMV) among infants evaluated for late-onset sepsis in the neonatal intensive care unit (NICU). Methods This study was a prospective cohort study. Results During the 13-month study, 84 infants underwent 116 sepsis evaluations, and CMV DNA was detected in saliva in three (4%) infants (median: gestational age 28 weeks, birth weight 950 g), representing 5% (n=6) of all sepsis evaluations. One infant had CMV DNA detected in saliva in all four sepsis evaluations. Two infants had acquired CMV infection, while the timing of CMV acquisition could not be determined in one infant. Two of the three infants had concomitant Gram-negative bacteremia and urinary tract infections (UTIs), two developed severe bronchopulmonary dysplasia (BPD) and none died. Conclusion Detection of CMV DNA in saliva occurred in 4% of infants and 5% of sepsis evaluations. Persistence of CMV DNA shedding in saliva made attribution of clinical illness difficult to ascertain.

Cytomegalovirus Enterocolitis in a Term Neonate

Background

Cytomegalovirus (CMV) enterocolitis is rare in term neonates.

Case characteristics

A term newborn with persistent pneumonia from birth developed enterocolitis on day 18 of life.

Outcome

Polymerase chain reaction (PCR) for CMV DNA was positive in urine sample. Antiviral therapy for six weeks resulted in successful treatment without any stricture formation.

Message

CMV enterocolitis should be considered as a differential diagnosis in atypical cases of necrotizing enterocolitis in neonates.

High prevalence of cytomegalovirus infection in surgical intestinal specimens from infants with necrotizing enterocolitis and spontaneous intestinal perforation: A retrospective observational study

BACKGROUND

Necrotizing enterocolitis (NEC) is a severe, often fatal gastrointestinal emergency that predominantly affects preterm infants, and there is evidence that neonatal cytomegalovirus (CMV) infection may in some cases contribute to its pathogenesis.

OBJECTIVES

This study aimed to evaluate the prevalence of CMV in infants with NEC.

STUDY DESIGN

Seventy intestinal specimens from 61 infants with NEC, spontaneous intestinal perforation (SIP), or related surgical complications were collected at Karolinska University Hospital and Uppsala University Hospital, Sweden. Ten specimens from autopsied infants without bowel disease served as controls. Samples were analyzed for CMV immediate-early antigen (IEA), CMV late antigen (LA), 5-lipoxigenase (5LO) and CMV-DNA by immunohistochemistry (IHC) and in situ hybridization (ISH), respectively. In 10 index samples, CMV DNA was analyzed with Taqman PCR after laser capture microdissection (LCM) of cells positive for CMV IEA by IHC.

RESULTS

CMV IEA was detected by IHC in 57 (81%) and CMV LA in 45 (64%) of 70 intestinal specimens from index cases; 2 (20%) of 10 control specimens were positive for both antigens. 5LO was detected in intestinal tissue section obtained from all examined index and controls. CMV DNA was detected in 4 of 10 samples (40%) after LCM. By ISH, all 13 IHC-IEA-positive samples were positive for CMV DNA; however, 3 of 5 IHC-IEA-negative samples (60%) were also positive.

CONCLUSIONS

CMV-specific antigens and CMV DNA were highly prevalent in intestinal specimens from infants with NEC, SIP, and related surgical complications. Our findings provide further evidence that neonatal CMV infection contributes to the pathogenesis of these diseases and may affect patient outcome.

Intestinal manifestations of postnatal and congenital cytomegalovirus infection in term and preterm infants

To review the relevant literature on cytomegalovirus-(CMV-)related intestinal problems in neonates, supplemented by two own cases of volvulus, a PubMed search and separate additional searches with characterizing terms were performed. 46 hits were found, 15 of which had to be excluded because they did not report clinical cases, yielding a total of 47 infants. Symptoms in both preterm and term infants with proven postnatal infection (n=16) included abdominal distension, bloody diarrhoea, necrotising enterocolitis (NEC) with perforation and intestinal stricture, with lethal outcome in 3 patients. Manifestations in congenital or unclearly allocated CMV-infections, including volvulus, were similar. We report on 2 additional cases with volvulus in extremely low gestational age neonates (ELGANs) with proven postnatal CMV-infection and concurrent detection of CMV positive cells within the resected intestinal stroma. As a conclusion volvulus should be added to other known intestinal manifestations of breastmilk (BM) related postnatal CMV disease in extremely preterm infants.

Cytomegalovirus and other common enteric viruses are not commonly associated with NEC

AIM

Changes in gut microbiota may contribute to NEC, but most studies focus on bacteria. Case reports suggest a link between cytomegalovirus (CMV) or other enteric viruses and NEC, but there are few case series systematically looking at common potential viral causes. We aimed to assess the presence of candidate viruses in blood or stool of a case series of infants with NEC managed in one surgical centre.

METHODS

We identified 22 infants diagnosed with NEC (from November 2011 to March 2014): 17 had suitable blood stored, of whom 14 also had suitable stool samples stored. Blood was analysed with polymerase chain reaction (PCR) for CMV, Epstein-Barr virus (EBV) and adenovirus, and stool by PCR for norovirus, sappovirus, astrovirus, adenovirus and rotavirus.

RESULTS

All samples were negative.

CONCLUSION

Although case reports indicate an episodic association of enteric viruses in NEC, the inability to detect any of these viruses in our 17 NEC infants suggests that a viral aetiology is unlikely to be causative for most sporadic forms of NEC.

Infectious causes of necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is the most common gastrointestinal emergency among premature infants. Although a large body of research has focused on understanding its pathogenesis, the exact mechanism has not been elucidated. Of particular interest is the potential causative role of infectious culprits in the development of NEC. A variety of reports describe bacterial, viral, and fungal infections occurring in association with NEC; however, no single organism has emerged as being definitively involved in NEC pathogenesis. In this review, the authors summarize the literature on infectious causes of NEC.

The neonatal bowel microbiome in health and infection

PURPOSE OF REVIEW

In newborns, interactions between the host and the microbiome operate synergistically, modulating host immune function and shaping the microbiome. Next generation molecular sequencing methodologies in tandem with modeling complex communities allow insights into the role of the microbiome in health and disease states. Infection-related disease states in which dysbiosis is integral include late-onset sepsis (LOS) and necrotizing enterocolitis (NEC), which still cause deaths and morbidity. Understanding microbiomic interactions may lead to alternative prevention, monitoring or treatment strategies, and modulation of long-term health outcomes especially in the preterm population. Recent studies have advanced understanding of the microbiome in NEC and LOS.

RECENT FINDINGS

Mechanisms of host-microbiome interaction have been demonstrated. Patterns of microbiomic change in association with NEC and LOS have been observed, with community changes dominated by Proteobacteria and Firmicutes appearing to precede NEC, and very early microbiomic signatures influencing LOS. Data on viral and fungal elements are emerging.

SUMMARY

Greater understanding of the neonatal bowel microbiome may allow tailored clinical practice and therapeutic intervention. Data handling and interpretation is challenging. Mechanistic studies of clinical interventions that affect the gut microbiome are important next steps.

Necrotizing enterocolitis in newborns: update in pathophysiology and newly emerging therapeutic strategies

While the survival of extremely premature infants with respiratory distress syndrome has increased due to advanced respiratory care in recent years, necrotizing enterocolitis (NEC) remains the leading cause of neonatal mortality and morbidity. NEC is more prevalent in lower gestational age and lower birth weight groups. It is characterized by various degrees of mucosal or transmural necrosis of the intestine. Its exact pathogenesis remains unclear, but prematurity, enteral feeding, bacterial products, and intestinal ischemia have all been shown to cause activation of the inflammatory cascade, which is known as the final common pathway of intestinal injury. Awareness of the risk factors for NEC; practices to reduce the risk, including early trophic feeding with breast milk and following the established feeding guidelines; and administration of probiotics have been shown to reduce the incidence of NEC. Despite advancements in the knowledge and understanding of the pathophysiology of NEC, there is currently no universal prevention measure for this serious and often fatal disease. Therefore, new potential techniques to detect early biomarkers or factors specific to intestinal inflammation, as well as further strategies to prevent the activation of the inflammatory cascade, which is important for disease progression, should be investigated.

Update in pathogenesis and prospective in treatment of necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is among the most common and devastating diseases in neonates and, despite the significant advances in neonatal clinical and basic science investigations, its etiology is largely understood, specific treatment strategies are lacking, and morbidity and mortality remain high. Improvements in the understanding of pathogenesis of NEC may have therapeutic consequences. Pharmacologic inhibition of toll-like receptor signaling, the use of novel nutritional strategies, and microflora modulation may represent novel promising approaches to the prevention and treatment of NEC. This review, starting from the recent acquisitions in the pathogenic mechanisms of NEC, focuses on current and possible therapeutic perspectives.

[Cytomegalovirus colitis in an immunocompetent child]

Cytomegalovirus (CMV) colitis is uncommon in young infants. Here we report a case of a 10-week-old breastfed infant admitted to the emergency room for rectal bleeding. He had had stools streaked with red blood for four days, was in good general condition, and was apyretic. Blood count, C-reactive protein, stool bacteriology and virology, abdominal X-ray examination, and abdominal ultrasounds did not reveal any abnormality. Cow's milk protein allergy was suggested despite the negativity of specific immunoglobulins E (IgE) and a diet excluding cow's milk protein was initiated in the mother. In a context of persistent rectal bleeding and good general condition, an endoscopic examination found diffuse hemorrhagic colitis and an erythematous aspect of the fundus. CMV was revealed by histological examination (typical cytopathic effect and immunostaining) and high viral load in the blood. The patient's immune balance was normal and the mother did not have any form of CMV infection. The diagnosis of primary CMV infection was retained in this immunocompetent child. Rectal bleeding disappeared without specific antiviral treatment. This observation points out that with rectal bleeding in an infant, cow's milk protein allergy should not be the only cause considered.

Asymptomatic DNAemia heralds CMV-associated NEC: case report, review, and rationale for preemption

Human cytomegalovirus (CMV) infection may be acquired in very low birth weight and extremely low birth weight (ELBW) infants from breast milk. The clinical relevance of such infections is uncertain. There is no consensus on whether screening breast milk for CMV, freezing/pasteurizing milk before feeding, or performing virological monitoring on at-risk infants is warranted. We describe an ELBW infant who acquired CMV postnatally from breast milk and developed CMV sepsis syndrome and clinical evidence of necrotizing enterocolitis (NEC) at ≈ 5 weeks of age. The availability of serial dried blood spots from day of life (DOL) 4 to 21, coincidentally obtained for a metabolic study, provided the novel opportunity to retrospectively test for and quantify the magnitude of CMV DNAemia. DNAemia was present for several weeks before the onset of severe CMV disease, first being noted on DOL 18 and increasing in magnitude daily to 4.8 log10 genomes/mL on DOL 21, approximately 8 days before the onset of abdominal distension and 15 days before the onset of CMV sepsis syndrome and NEC. After surgical resection, supportive care, and ganciclovir therapy, the infant recovered. This case underscores the importance of including CMV infection in the differential diagnosis of sepsis and NEC in premature infants. This case also suggests the value of prospective virological monitoring in at-risk low birth weight and ELBW infants. Future studies should examine the potential utility of preemptive monitoring for, and possibly treatment of, CMV DNAemia in premature infants, which may herald the onset of serious disease.

Innate immune signaling in the pathogenesis of necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is a challenging disease to treat, and caring for patients afflicted by it remains both frustrating and difficult. While NEC may develop quickly and without warning, it may also develop slowly, insidiously, and appear to take the caregiver by surprise. In seeking to understand the molecular and cellular processes that lead to NEC development, we have identified a critical role for the receptor for bacterial lipopolysaccharide (LPS) toll like receptor 4 (TLR4) in the pathogenesis of NEC, as its activation within the intestinal epithelium of the premature infant leads to mucosal injury and reduced epithelial repair. The expression and function of TLR4 were found to be particularly elevated within the intestinal mucosa of the premature as compared with the full-term infant, predisposing to NEC development. Importantly, factors within both the enterocyte itself, such as heat shock protein 70 (Hsp70), and in the extracellular environment, such as amniotic fluid, can curtail the extent of TLR4 signaling and reduce the propensity for NEC development. This review will highlight the critical TLR4-mediated steps that lead to NEC development, with a focus on the proinflammatory responses of TLR4 signaling that have such devastating consequences in the premature host.

The role of the intestinal microbiota in the pathogenesis of necrotizing enterocolitis

Development of necrotizing enterocolitis (NEC) requires a susceptible host, typically a premature infant or an infant with congenital heart disease, enteral feedings and bacterial colonization. Although there is little doubt that microbes are critically involved in the pathogenesis of NEC, the identity of specific causative pathogens remains elusive. Unlike established normal adult gut microbiota, which is quite complex, uniform, and stable, early postnatal bacterial populations are simple, diverse, and fluid. These properties complicate studies aimed at elucidating characteristics of the gut microbiome that may play a role in the pathogenesis of NEC. A broad variety of bacterial, viral, and fungal species have been implicated in both clinical and experimental NEC. Frequently, however, the same species have also been found in physiologically matched healthy individuals. Clustered outbreaks of NEC, in which the same strain of a suspected pathogen is detected in several patients suggest, but do not prove, a causative relationship between the specific pathogen and the disease. Studies in Cronobacter sakazakii, the best characterized NEC pathogen, have demonstrated that virulence is not a property of a bacterial species as a whole, but rather a characteristic of certain strains, which may explain why the same species can be pathogenic or non-pathogenic. The fact that a given microbe may be innocuous in a full-term, yet pathogenic in a pre-term infant has led to the idea of opportunistic pathogens in NEC. Progress in understanding the infectious nature of NEC may require identifying specific pathogenic strains and unambiguously establishing their virulence in animal models.