FROM ASN EB 2013 SYMPOSIA Manipulation of the Intestinal Microbiome in Newborn Infants 1 , 2

The mammalian gastrointestinal tract harbors a highly diverse microbial population termed the microbiome, which plays a major role in nutrition, metabolism, protection against pathogens, and development of the immune system. It is estimated that at least 1000 different bacterial species cohabit the human intestinal tract. Herein we provide a brief review of the developing intestinal microbiome, with the understanding that its development often begins before birth and that disturbance in the microbiome during fetal life, birth, and shortly thereafter may result in adverse consequences. Postnatally, numerous environmental factors including premature delivery, mode of delivery, antibiotic usage, and diet can play an important role in how the intestinal microbiome of infants is shaped. The fact that human milk contains microbes is likely to have important ramifications. We discuss where these microbes come from and their potential role. Adv. Nutr. 5: 114–118, 2014. The Developing Microbiome With the advent of the Human Microbiome Project, where new high-throughput technologies are used to evaluate the taxonomy and function of populations of microbes residing in various regions of the body, there is increasing recognition of the scope of the intestinal microbiota. The total number of bacterial cells residing in the human intestine far exceeds the number of host cells. More than 3 million genes have been identified in this “microbiome,” which is >100-fold of our own human genes (1). But interestingly, most of themicrobiome in the human intestine is restricted to 4 dominant phyla: Firmicutes, Bacteroides, Actinobacteria, and Proteobacteria. Although commonly thought to be related to disease as pathogens, the majority of intestinal microbes are commensals and symbionts that provide beneficial effects in terms of nutrition, development of the immune system, and postnatal maturation of the intestine. They perform functions that cannot be performed by the human host itself. The amniotic fluid (AF) from mothers of infants born prematurely is often colonized by microbes. The quantity of microbial DNA correlates inversely with gestational age, and directly with IL-6 and white blood cells in AF (2,3). Although evidence supports that the majority of microbes found in AF are of vaginal origin, seeding the AF via translocation through the chorioamniotic intracellular junctions (4), other origins such as maternal intestinal translocation or hematogenous spread from periodontal tissue are also possible. Figure 1 shows a broad depiction of the most commonly studied modes of transfer of microbes from mother to infant. The infant’s ultimate microbiome is influenced by many factors, which can be categorized into prenatal, perinatal, and postnatal periods (Table 1). The fetus swallows large quantities, ~400 mL/d during the last trimester of pregnancy, of such microbe-containing AF (6–8), which subsequently reaches the distal small intestine, an organ that was initially found in rodents and later in humans to be significantly more sensitive to inflammatory stimuli than postnatal infant intestine (9). An injection of endotoxin or IL-1 into AF invokes a strong inflammatory response in the ileum of the 125-d ovine fetus (10,11). The fetal intestine responds to bacterial components such as LPS more robustly than to mature intestine when isolated in culture (2). Fetal intestinal inflammation and fetal inflammatory response syndrome induced by ureaplasma is blunted with intraamniotic injection of IL-1b receptor antagonist (IL1RA) (10,11). Thus, the fetal intestine is a likely origin of fetal inflammatory response syndrome that is responsible for triggering preterm labor. 1 Presented at the symposium “Bioactive Compounds and Other Milk Constituents” held 20 April 2013 at the ASN Scientific Sessions and Annual Meeting at Experimental Biology 2013 in Boson, MA. The symposium was sponsored by the American Society for Nutrition (ASN) and partially funded by Medela, Inc., McHenry, IL. A summary of the symposium was published in the November 2013 issue of Advances in Nutrition. 2 Author disclosures: N. Cacho and J. Neu, no conflicts of interest. * To whom correspondence should be addressed. E-mail: neuj@peds.ufl. edu. 3 Abbreviations used: AF, amniotic fluid; CD, cesarian delivery; IL-1RA, IL-1 receptor antagonist; LOS, late-onset sepsis; NEC, necrotizing enterocolitis. 114 ã2014 American Society for Nutrition. Adv. Nutr. 5: 114–118, 2014; doi:10.3945/an.113. 004820. Downloaded from https://academic.oup. com/advances/article-abstract/5/1/114/4557995 by guest on 30 March 2018 AF is difficult to obtain routinely for the purpose of research investigations. However, meconium and infant stools are readily available, and collection is noninvasive. The first stool in the newborn (meconium) reflects the in utero fetal intestinal environment. If microbes in AF enter the intestine of the fetus and cause an inflammatory response, then microbial remnants as well as some of the markers of inflammation would be expected to be present in the meconium of these infants. Indeed, in our ongoing studies in premature infants at risk of necrotizing enterocolitis (NEC), we found significant quantities of microbial DNA in the meconium of these infants (12). We speculate that a relation exists between the fetal microbiome as evaluated in meconium and spontaneous preterm labor. The mechanisms of this relation remain poorly understood, but research in the next few years is likely to yield information that should lead to microbehost based interventions for prevention. In a recent review it was shown that mode of delivery, cesarean section or vaginal, is thought to play a role. In the United States, the rate of cesarean delivery (CD) has increased by 48% since 1996, reaching a level of 31.8% in 2007 (13). This trend is reflected in many parts of the world, with the most populous country in the world, China, approaching 50% (14), and some private clinics in Brazil approaching 80% (15). Although a significant number of CDs are performed for obstetric indications, some are simply due to maternal request or convenience for the physician and family. In some cases, CD is due to wanting a child born on a certain date thought to be auspicious. Concurrent with the trend of increasing CDs, there has been an epidemic of autoimmune diseases such as type 1 diabetes, Crohn disease, and multiple sclerosis and allergic diseases such as asthma, allergic rhinitis, and atopic dermatitis (16,17). The mechanism of this relation between CD and increased risk of these diseases remains poorly understood. However, during vaginal delivery, the contact with the maternal vaginal and intestinal microbiota is an important source for the start of the infant’s colonization. During CD, this direct contact is absent, and nonmaternally derived environmental bacteria play an important role in infants’ intestinal colonization (18,19). There is accumulating evidence that intestinal bacteria play an important role in the postnatal development of the immune system (20). Thus, the development of the immune system depends on the intestinal microbiota, which, if altered from its usual condition due to CD rather than vaginal birth, may incur aberrations in its normal development. Obesity has also been shown to be more prevalent in infants born by CD (21). Whether this is due to differences in intestinal microbiota remains speculative, and investigations are ongoing to determine potential mechanisms (22). The Microbiome in Late-Onset Sepsis and NEC Late-onset sepsis (LOS) is typically defined as the identification of pathogenic organisms from blood culture acquired FIGURE 1 Potential transfer of microbiome from mother to infant prenatally, perinatally, and postnatally. Adapted from Reference (5), with permission. GI, gastrointestinal. TABLE 1 Examples of factors that affect the infant microbiome