Altered Innate Defenses in the Neonatal Gastrointestinal Tract in Response to Colonization by Neuropathogenic Escherichia coli

Abstract

d Two-day-old (P2), but not 9-day-old (P9), rat pups are susceptible to systemic infection following gastrointestinal colonization by Escherichia coli K1. Age dependency reflects the capacity of colonizing K1 to translocate from gastrointestinal (GI) tract to blood. A complex GI microbiota developed by P2, showed little variation over P2 to P9, and did not prevent stable K1 colonization. Substantial developmental expression was observed over P2 to P9, including upregulation of genes encoding components of the small intestinal (␣-defensins Defa24 and Defa-rs1) and colonic (trefoil factor Tff2) mucus barrier. K1 colonization modulated expression of these peptides: developmental expression of Tff2 was dysregulated in P2 tissues and was accompanied by a decrease in mucin Muc2. Conversely, ␣-defensin genes were upregulated in P9 tissues. We propose that incomplete development of the mucus barrier during early neonatal life and the capacity of colonizing K1 to interfere with mucus barrier maturation provide opportunities for neuropathogen translocation into the bloodstream. The newborn infant is particularly vulnerable to systemic bacterial infection during the first 4 weeks of life, and mortality and morbidity associated with neonatal bacterial meningitis (NBM) and accompanying sepsis remain significant despite advances in antibacterial chemotherapy and supportive care (1, 2). In the developed world, Escherichia coli and group B streptococci are responsible for the majority of cases of NBM, and bacteria isolated from the cerebrospinal fluid of infected neonates invariably elaborate a protective polysaccharide capsule. Of neuroinvasive E. coli isolates, 80 to 85% express the K1 capsule (3, 4), a homopolymer of ␣-2,8-linked polysialic acid (polySia) that mimics the molecular structure of the polySia modulator of neuronal plasticity in mammalian hosts (5) and enables these strains to evade detection by a neonatal innate immune system undergoing a process of age-dependent maturation (6).Risk factors for NBM include obstetric and perinatal complications, premature birth, and low birth weight, particularly in low socioeconomic groups (7), but predisposition to infection is critically dependent on vertical transmission of the causative agent from mother to infant at or soon after birth (8). Although many aspects of the pathogenesis of E. coli K1 in NBM are unclear, maternally derived E. coli K1 bacteria are known to colonize the neonatal gastrointestinal (GI) tract (8, 9, 10), which is sterile at birth but rapidly acquires a complex microbiota that eventually converges toward a profile characteristic of the adult GI tract (11). E. coli K1 bacteria then translocate from the lumen of the small intestine or colon into the systemic circulation before entering the central nervous system (CNS) across the blood-brain barrier at the cerebral microvascular endothelium of the arachnoid membrane (12) or the blood-cerebrospinal fluid (CSF) barrier at the choroid plexus epithelium (13).Many of the temporal and spatial aspects of NBM can...

Non-Invasive Model of Neuropathogenic <em>Escherichia coli</em> Infection in the Neonatal Rat

“…The bacteria are ingested, colonize the GI tract, translocate into the blood compartment via the mesenteric lymph nodes before establishing organ-specific disease with associated inflammation of the brain 24 . Importantly, the model displays strong age dependency; as shown in Figure 3, two-dayold (P2) rat pups are highly susceptible to invasive disease but over a seven-day period the animals become progressively more refractory to infection, but not to GI tract colonization 17 . After transit from the site of GI colonization to the blood compartment, the bacteria can be visualized in blood samples by fluorescence microscopy (Figure 4) before entering the CNS predominantly at the choroid plexus 25 .…”

“…The pups rapidly develop a complex microbiota and within two days of birth the GI tract is colonized with a wide range of bacteria from the phyla established as the most abundant microbes in the infant and adult gut. Pups that have not been fed E. coli A192PP do not carry E. coli K1 in the GI tract 17 and so determination of rates of colonization is relatively straightforward. However, the neuS-based qPCR method for detection of colonizing E. coli K1 is far more sensitive that traditional culture methods and is strongly recommended 17 .…”

“…Pups that have not been fed E. coli A192PP do not carry E. coli K1 in the GI tract 17 and so determination of rates of colonization is relatively straightforward. However, the neuS-based qPCR method for detection of colonizing E. coli K1 is far more sensitive that traditional culture methods and is strongly recommended 17 .…”

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“…The results of previous investigations have shown that different inflammatory factors may contribute to the pathogenesis of intestinal barrier injury, and most studies have focused on the pathological roles of cytokines and biologically active mediators produced by enterocytes and macrophages 13,14 . However, the relationships between vasoregulation and the protective mechanisms of the gastrointestinal barrier have not been investigated.…”

“…The protective function of the intestinal mucosal barrier depends on the expression and properties of mucins 8 and antimicrobial defensive factors, which are involved in the regulatory system and generate the main chain of host defence against pathogens 9 . Mucins are actively expressed in the epithelium of the gastrointestinal tract and form the high-molecularweight viscoelastic layer, which is the protective barrier between the mucosal surface of the abdominal contents of the gastrointestinal tract, representing a nourishing environment for vital activity of commensal bacteria of the intestine 10,11 .…”