Bacterial toxins and enteral feeding of premature infants at risk for necrotizing enterocolitis

Necrotizing enterocolitis: Pathophysiology from a historical context

Necrotizing enterocolitis (NEC) continues to afflict approximately 7% of preterm infants born weighing less than 1500g, though recent investigations have provided novel insights into the pathogenesis of this complex disease. The disease has been a major cause of morbidity and mortality in neonatal intensive care units worldwide for many years, and our current understanding reflects exceptional observations made decades ago. In this review, we will describe NEC from a historical context and summarize seminal findings that underscore the importance of enteral feeding, the gut microbiota, and intestinal inflammation in this complex pathophysiology.

TGF-β2, a protective intestinal cytokine, is abundant in maternal human milk and human-derived fortifiers but not in donor human milk

OBJECTIVE

This study compared cytokines (in particular transforming growth factor [TGF]-β2) and lactoferrin in maternal human milk (MHM), human-derived milk fortifier (HDMF), and donor human milk (DHM).

MATERIALS AND METHODS

MHM was randomly collected from breastfeeding mothers who had no infectious illness at the time of milk expression. HDMF and DHM were products derived from human milk processed by Holder pasteurization. MHM samples were collected at different times (early/late) and gestations (preterm/term). Lactoferrin was analyzed by western blotting, and cytokines were quantified using commercial enzyme-linked immunosorbent assays. Significance was determined using analysis of variance.

RESULTS

In the 164 samples analyzed, TGF-β2 concentrations in HDMF and preterm MHM (at all collection times) were fivefold higher than in DHM (p<0.05). Early preterm MHM had levels of interleukin (IL)-10 and IL-18, 11-fold higher than DHM (p<0.05). IL-6 in DHM was 0.3% of the content found in MHM. IL-18 was fourfold higher in early MHM versus late MHM regardless of gestational age (p<0.05). Lactoferrin concentration in DHM was 6% of that found in MHM.

CONCLUSIONS

Pasteurization decreases concentrations of most cytokines and lactoferrin in DHM. TGF-β2, a protective intestinal cytokine, has comparable concentrations in HDMF to MHM despite pasteurization.

Bacterial colonization and gut development in preterm neonates

Necrotizing enterocolitis (NEC) develops in 5-10% of preterm infants in association with enteral feeding and bacterial colonization. It remains unclear how diet and bacteria interact to protect or provoke the immature gastrointestinal tract. Understanding the factors that control bacterial colonization may provide the clue to prevent NEC, and studies in infants must be combined with animal models to understand the mechanisms of the microbiota-epithelium interactions. Analyses of infant fecal samples show that the density and distribution of bacterial species are highly variable with no consistent effects of gestational age, delivery mode, diet or probiotic administration, while low bacterial diversity and bacterial overgrowth are commonly associated with NEC. A series of recent studies in preterm pigs show that the mucosa-associated microbiota is affected by delivery method, prematurity and NEC progression and that diet has limited effects. Overgrowth of specific groups (e.g. Clostridia) appears to be a consequence of NEC, rather than the cause of NEC. Administration of probiotics either decreases or increases NEC sensitivity in preterm pigs, while in preterm infants probiotics have generally decreased NEC incidence and overall mortality. The optimal nature and amount of probiotic bacteria are unknown and host defense factors appear more important for NEC sensitivity than the nature of the gut microbiota. Host defense is improved by feeding the optimal amount of enteral diets, such as mother's colostrum or milk, that help the immature intestinal immune system to respond appropriately to the highly variable bacterial colonization.

Preterm birth and necrotizing enterocolitis alter gut colonization in pigs

Necrotizing enterocolitis (NEC) in preterm neonates is dependent on bacterial colonization, but it remains unclear whether a particular microbiota or specific pathogens are involved. We hypothesized that gut colonization differs between preterm and term neonates and that overgrowth of Clostridium perfringens predisposes to NEC. By using terminal-RFLP and FISH, we characterized the gut microbiota of preterm, caesarean-delivered, formula-fed pigs (n = 44) with or without NEC and of formula- or colostrum-fed term, and vaginally born pigs (n = 13). A different microbiota with high C. perfringens abundance was observed in preterm pigs with NEC compared with healthy individuals. However, immunization against C. perfringens toxins did not prevent NEC, and C. perfringens inoculation (3.6 × 10 cfu/d) failed to induce NEC (n = 16), whereas prophylactic broad-spectrum antibiotics treatment prevented NEC (n = 24). Colonization in both groups of term pigs differed from preterm pigs and was dominated by Lactobacilli spp. In conclusion, gestational age (GA) and NEC influence neonatal gut colonization, whereas diet has minor effects. C. perfringens is more abundant in pigs with NEC but rather as a consequence than a cause of disease. The general bacterial load and underdeveloped gut immune responses in preterm neonates seem more important for NEC development than specific pathogens.

Nutritional factors influencing infections in preterm infants

In contrast with clinical studies in term infants or older children, it is very difficult to investigate possible immunoregulatory effects of a novel infant formula composition in preterm infants. This is mainly because of the multicausal origin of infections in this high-risk population that is usually admitted to the neonatal intensive care unit. Possible effects of nutrition composition on onset and incidence of nosocomial infections in these very small infants have to be compared with infections that may have originated in utero. The development of the gastrointestinal tract may be inhibited after severe intrauterine growth retardation, leading to functional impairment of the gut shortly after birth. This may be related to the onset of necrotizing enterocolitis of the newborn. However, this disease in very small preterm infants is possibly also related to the initiation of oral feeding and/or the amount of feeding. Specific infection risks of neonatal intensive care as a result of invasive techniques such as artificial ventilation or total parenteral nutrition using indwelling umbilical and/or Silastic lines and so-called "all-in-one" mixtures may influence the incidence of infections. Widespread use of intravenous antibiotics in the neonatal intensive care unit may create an even larger infection risk. Investigation of possible immunomodulatory effects of factors such as prebiotics and probiotics added to the nutrition of preterm infants should always be considered along with other nutritional factors known to influence the immature immune system.

Necrotizing enterocolitis of the neonate with Clostridium perfringens: diagnosis, clinical course, and role of alpha toxin

The severity of the clinical course in necrotizing enterocolitis (NEC) associated with Clostridium perfringens (Cp) may support the hypothesis of a specific disease. We conducted a case control study of infants diagnosed with NEC, who underwent surgical treatment over a 7-year period. Patient histories examined characteristics of the infants, bacterial infection as well as NEC's severity, antibiotic treatment, and clinical course. Infants infected with NEC associated with Cp were compared with NEC patients without Cp. The alpha toxin from Cp type A was detected in most of the isolated strains. Cp was identified as a causative agent of NEC in nine cases. As compared with the control group (n = 32), the onset of disease was earlier in life, the clinical course more severe, and patients had a larger extent of gangrene. Portal venous gas was evident in 77% of all Cp cases, as compared to 25% in the control group. The mortality rate was 44% in the Cp group, and only 18.7% in the control group. Type A Clostridium perfringens was identified in six cases. In each isolate alpha toxin production was proven, but without any correlation to the severity of the clinical course, the extent of intestinal gangrene or mortality. In premature infants NEC in conjunction with Cp seems to be more severe than other NEC cases; it also entails higher mortality and morbidity. Alpha toxin concentrations do not correlate with the severity of the disease. Portal venous gas is highly suggestive for the diagnosis of Cp infection.

Bacterial colonization, probiotics, and necrotizing enterocolitis

The intestinal microflora has a significant role in intestinal health and gut function. The neonatal population is unique in that intestinal colonization is not established and is known to be influenced by delivery method, feeding, gestational age, and medical interventions. The preterm infant is particularly sensitive to colonization patterns as inherent intestinal defense mechanisms are immature and immature intestinal epithelial cells are known to have exaggerated inflammatory responses to both commensal and pathogenic bacteria. These responses contribute to the development of neonatal necrotizing enterocolitis in this patient population. As certain bacteria are known to influence intestinal maturation and down-regulate intestinal inflammation, it has been suggested that influencing the intestinal flora of preterm infants may be beneficial. Clinical studies indicate that probiotic therapy may decrease the incidence of necrotizing enterocolitis and studies are ongoing to elucidate the mechanism of action of different probiotic organisms. Although concerns remain and further study is necessary, probiotics are a plausible means of optimizing intestinal colonization and influencing outcome of these vulnerable infants.

Neonatal gut barrier and multiple organ failure: role of endotoxin and proinflammatory cytokines in sepsis and necrotizing enterocolitis

BACKGROUND/PURPOSE

Failure of the gut barrier and endotoxemia have been implicated in sepsis and multiple organ failure (MOF) syndromes in adults. The contributions of endotoxin (ETX) and proinflammatory cytokines (CKs) to the pathophysiology of disease and the outcomes of infants in the neonatal intensive care unit (NICU) are not clear. We measured ETX and CK concentrations in infants who presented with clinical signs of sepsis and/or necrotizing enterocolitis (NEC) to study their impact on MOF and outcomes.

METHODS

Blood samples from infants with signs of NEC and/or sepsis were collected for culture and determination of complete blood cell counts and concentrations of CKs (interleukin [IL]-1beta, tumor necrosis factor [TNF] alpha, and IL-6) and ETX at the onset of illness. Infants with signs of sepsis but without those of NEC were classified by blood culture results into a confirmed sepsis group (ie, positive culture) or a control group (ie, negative culture). Endotoxin concentrations were determined by chromogenic Limulus amebocyte lysate assay, and CK levels were quantitated by enzyme-linked immunoassay. Data are expressed as mean +/- SD and as odds ratios (ORs) with 95% confidence intervals (CIs). P values lower than .05 were considered to be significant.

RESULTS

There was no demographic or clinical difference among the NEC (n = 27), sepsis (n = 44), and control (n = 56) groups, except that fewer (P = .02) infants in the NEC group (11%) had received maternal milk feedings as compared with infants in the sepsis group (23%) and those in the control group (39%). Endotoxin concentrations were higher (P < .0001) in the NEC group (3.30 +/- 2.11) as compared with the sepsis group (0.67 +/- .86) and the control group (0.09 +/- 0.24). Generalized linear regression analysis using formula feeding, mechanical ventilation, and gram-negative bacteremia as covariates demonstrated that NEC increased ETX concentrations independently (r = .80; P < .0001). Endotoxemia correlated with higher concentrations of all 3 CKs (P < .0001). There was an inverse association between ETX and both platelet count (r = -0.30; P = .0003) and absolute neutrophil count (r = -0.29; P = .0009). Infants who died of MOF had higher concentrations of ETX (2.83 +/- 3.04 vs 0.67 +/- 1.04 EU/mL; P < .0001), IL-1beta (509 +/- 493 vs 106 +/- 223 pg/mL; P < .0001), IL-6 (416 +/- 308 vs 99 +/- 165 pg/mL; P < .0001), and TNF-alpha (503 +/- 449 vs 126 +/- 237 pg/mL; P < .0001) as compared with those without MOF. Eighty-six percent of the infants with MOF died. Multivariate logistic regression analysis demonstrated that higher ETX concentrations (OR = 2.47; 95% CI = 1.39-4.40; P = .002) and lower gestational age (OR = 1.41; 95% CI = 1.12-1.77; P = .003) predicted mortality.

CONCLUSIONS

Neonatal endotoxemia and release of proinflammatory CKs are important contributors to MOF and mortality in the NICU. Endotoxemia was most severe at the onset of illness among the infants with NEC, suggesting that gut barrier failure plays an important role in adverse outcomes in the NICU.

Potential prophylactic value of bovine colostrum in necrotizing enterocolitis in neonates: an in vitro study on bacterial attachment, antibody levels and cytokine production

Necrotizing enterocolitis (NEC) is an important disease of low birth-weight neonates. The immaturity of the gut mucosa may result in close contact between the host epithelium and microorganisms which are normally confined to the gut lumen. Damage of the mucosa due to endotoxin, cytokine production or other factors is believed to then occur. The aim of this study was to determine whether spray-dried bovine colostrum demonstrated potential in vitro as a prophylactic for NEC. Antiadherence was measured using a tissue culture assay and antibody levels against Enterobacteriaceae were determined by ELISA. The effect of bovine colostrum on the production of cytokines implicated in NEC was determined by a multiplex bead assay. Enterobacter cloacae, Klebsiella oxytoca, Escherichia coli, Serratia marcescens and Klebsiella pneumoniae ssp. pneumoniae were common in both NEC positive and NEC negative infants and IgA and IgG1 antibodies to these species were present in the bovine colostrum. Pretreatment with bovine colostrum produced a significant decrease (P<0.001) in attachment of bacteria to HT-29 cells. Bovine colostrum significantly increased the production of IL-8 in HT-29 cells and IL-8, IL-6 and TNF-alpha in THP-1 cells (P<0.001). The potential of bovine colostrum to increase the production of inflammatory mediators could limit its usefulness.

The intestinal bacterial colonisation in preterm infants: a review of the literature

The aim of this study is to review the normal development of the intestinal microflora of preterm infants and the factors influencing its development. Preterm infants have an increased intestinal permeability, which may lead to bacterial translocation to systemic organs and tissues. In combination with immaturity of the immune system the risk to systemic infections might be increased. Especially potential pathogenic bacteria are able to translocate. The intestinal microflora of breast-fed term infants, dominated by bifidobacteria and lactobacilli, is thought to suppress the growth of potentially pathogenic bacteria. Many attemps have been made to stimulate the presence of bifidobacteria and lactobacilli with changes in the diet and ingredients-like prebiotics and probiotics. After selection, six studies were included reviewing the intestinal bacterial colonisation of preterm infants. In general, these studies show that the intestinal bacterial colonisation with beneficial bacteria is delayed in preterm infants. The number of potentially pathogenic bacteria is high. Antibiotics influence the intestinal colonisation. Many preterm infants receive prophylactic antibiotics at birth. As antibiotics delay the normal intestinal colonisation, caution should be given to the treatment with broadspectrum antibiotics in preterm infants at birth and every attempt has to be made to restrict the period of treatment.

Evidence for clostridial implication in necrotizing enterocolitis through bacterial fermentation in a gnotobiotic quail model

Despite extensive research, the pathogenesis of neonatal necrotizing enterocolitis (NEC) remains elusive. The aim of our work was to investigate the role of bacterial strains involved in NEC in gnotobiotic quails as experimental model. Six groups of germ-free quails that were fed a lactose diet were associated with Klebsiella pneumoniae, Clostridium perfringens, C. difficile, C. paraputrificum, or C. butyricum (two strains). Implantation level, incidence of cecal lesions, production of short-chain fatty acids, and histologic lesions of the cecal wall were investigated. Whatever the strain, the implantation level was high (10(9) UFC/g). Neither K. pneumoniae nor C. difficile induced any cecal lesions. In contrast, the four other clostridial strains led to cecal NEC-like lesions with a variable occurrence: four of 12 quails for C. perfringens, eight of 12 quails for C. paraputrificum, and the same highest value, nine of 12 quails and eight of 10 quails for both C. butyricum strains. Gross aspects of the lesions may be linked to the short-chain fatty acid profiles and/or concentrations: thickening of the cecal wall (C. butyricum and C. perfringens) with high proportion of butyric acid, hemorrhages (C. paraputrificum) with high proportion of iso-butyric acid, and presence of other iso-acids. In addition, C. butyricum was characterized by pneumatosis, linked to a high gas production. Microscopic aspects confirmed the presence of edemas and intramucosa hemorrhages. Clostridia species, whose role is controversial, seem to be strongly implicated in NEC through excessive production of butyric acid as a result of colonic lactose fermentation. These results call for anaerobe detection in feces of infants who have NEC.

Neonatal necrotizing enterocolitis: diagnosis, management, and pathogenesis

The diagnosis of neonatal necrotizing enterocolitis is one of great concern to pediatric and neonatal clinicians. Intravenous access remains an integral part of the medical and surgical management of infants with this diagnosis, and the infusion nurse is intimately involved in the care of these patients. This article discusses the definition of necrotizing enterocolitis, presents current knowledge regarding its basic pathophysiology, and identifies common and rare sequelae of this oftentimes devastating disease of premature infants. Medical and surgical management goals of therapy are described. This overview will aid the infusion nurse in caring for these patients.

Endotoxin inhibits intestinal epithelial restitution through activation of Rho-GTPase and increased focal adhesions

Diseases of gut inflammation such as neonatal necrotizing enterocolitis (NEC) result after an injury to the mucosal lining of the intestine, leading to translocation of bacteria and endotoxin (lipopolysaccharide). Intestinal mucosal defects are repaired by the process of intestinal restitution, during which enterocytes migrate from healthy areas to sites of injury. In an animal model of NEC, we determined that intestinal restitution was significantly impaired compared with control animals. We therefore sought to determine the mechanisms governing enterocyte migration under basal conditions and after an endotoxin challenge. Here we show that the cytoskeletal reorganization and stress fiber formation required for migration in IEC-6 enterocytes requires RhoA. Enterocytes were found to express the endotoxin receptor Toll-like receptor 4, which served to bind and internalize lipopolysaccharide. Strikingly, endotoxin treatment significantly inhibited intestinal restitution, as measured by impaired IEC-6 cell migration across a scraped wound. Lipopolysaccharide was found to increase RhoA activity in a phosphatidylinositol 3-kinase-dependent manner, leading to an increase in phosphorylation of focal adhesion kinase and an enhanced number of focal adhesions. Importantly, endotoxin caused a progressive, RhoA-dependent increase in cell matrix tension/contractility, which correlated with the observed impairment in enterocyte migration. We therefore conclude that endotoxin inhibits enterocyte migration through a RhoA-dependent increase in focal adhesions and enhanced cell adhesiveness, which may participate in the impaired restitution observed in experimental NEC.