Bacterial Colonization of the Neonatal Gut

The indigenous microflora of the human gastrointestinal tract are not merely innocent bystanders, but rather play a critical role in normal physiology as well as in the modulation of systemic disease states. At birth, the human gastrointestinal tract is sterile, but within hours is colonized by ingested environmental and maternal coliforms and streptococci (1–4). It is well known that animals raised in a germ-free environment lack intestinal bacterial flora (5). Evidence that colonization occurs via the oral route comes from studies of newborn infants with intestinal obstruction, which reveal a sterile distal bowel with colon-like flora proximal to the obstruction (6). Rates and patterns of neonatal colonization are influenced by the route of delivery, hygiene of the neonatal environment, maternal bacterial flora and diet (7). By one week of age, healthy full-term infants delivered vaginally are colonized by anaerobic bacteria, predominantly Bacteroides (8). However, infants delivered by Caesarian section exhibit delayed colonization by anaerobes, and are colonized by non-E. coli Enterobacteriaceae (1). Remarkably, the flora of infants delivered via C-section resembles that of the surrounding hospital environment more than that of their mothers! Formula and breast-fed infants have very different early intestinal bacterial flora (9). The predominant bacteria of breast-fed infants are lactobacilli and bifidobacteria, which may outnumber Enterobacteriaceae by 1000-fold (10,11). The stools of formula fed infants contain predominantly Enterobacteriaceae. Of interest, the flora of the formula-fed infant develops more quickly than that of the breast-fed infant (12). This may be in part due to the iron in infant formulas, which fosters the growth of a more complex flora (13). Shortly before and after weaning, the composition of the microflora shifts to a more adult pattern, with fewer E. coli and Clostridium spp, and more Bacteroides and gram-positive anaerobic cocci (14,15). The human gastrointestinal tract is the home to more than 400 species of aerobic and anaerobic bacteria, residing in specific ‘niches‘ throughout the gut. Because of peristalsis and the antimicrobial effects of gastric acid, the microflora of the stomach (with the exception of H. pylori), is composed mostly of gram-positives at a concentration of <103 per gram of intraluminal fluid (16,17). The ileum is a ‘microbiologic transition zone‘ from the sparse jejunal aerobic flora to the dense anaerobic bacteria of the colon, where gram-negatives predominate at 1010 to 1012 organisms per gram of luminal content (18). In this respect, humans are more prokaryotic than eukaryotic! The impact of this internal environment on the development of chronic gastrointestinal and systemic inflammatory diseases is only now being defined and appreciated. The maturation and integrity of the neonatal immune system is dependent upon exposure to microbial antigens. In germ free animals, bacterial colonization causes inflammation and increased cellularity of the lamina propria (19–22). The normal gut microflora may also protect the infant from enteric pathogens by competing with pathogens for nutrients, by degrading their toxins, and by interfering with their adherence and growth (4,5,23–26). Normal flora also secrete antibacterial substances which protect against pathogens (27,28). In this issue of JPGN, Bailey et al. propose that bacterial colonization of the gut might be impacted by stress in the pregnant mother (29). Stress is presumed to suppress normal mucosal immunity and enhance the growth of enteric pathogens by enhancing the secretion of stress-induced neurohormones (30–35). Previous work by Bailey and Coe (36), also in infant rhesus monkeys, suggests that the stress produced by the disruption of the maternal-infant bond may increase vulnerability to disease. The stools of infant monkeys separated from their mothers contained significantly fewer bacteria, especially lactobacilli. Behavioral and cortisol responses to separation were correlated with the microflora and the decrease in the microflora was also correlated with displays of stressful behaviors. Infant monkeys displaying stressed behaviors were more susceptible to opportunistic bacterial infection. It is also possible that physical as well as psychological stress may induce changes in gut microflora. Wu et al. recently showed that the intestinal environment of mice after surgical stress was more conducive to Pseudomonas aeruginosa colonization and sepsis, and a marked increase in mortality (37). In the present study, the authors hypothesized that the levels of protective microflora would be low in infant monkeys with stressed dams thereby increasing their risk of acquiring gram-negative enteric pathogens. Pregnant dams were stressed early or late in gestation with acoustic startle. Postpartum, the infant monkeys showed decreased concentrations of lactobacilli and bifidobacteria in their stools for the first 6 months of life compared with the offspring from undisturbed mothers. This effect was more pronounced in the offspring of dams stressed late in pregnancy. Unfortunately, the microbial population of the breast milk, which could certainly affect colonization of the infant intestine, was not measured. However, if the reduction in bacteria associated with prenatal maternal stress could be translated to humans, it might have implications for the development of later allergy, infectious diarrhea and even inflammatory bowel disease (38–43). Cortisol levels were elevated in the pregnant dams disturbed both early and late in gestation. Interestingly, this prenatal stressor did not significantly affect the length of gestation, infant birth weight or infant weight gain. Shigella infection was more common in the stressed infants, with 43% of the early stress and 30% of the late stress infants shedding Shigella at least once during the 6-month study. In fact, two of the late stress infants were excluded from the study because of Shigella-induced diarrhea that required antibiotic treatment. None of the control infants were found to shed Shigella. This study indicates that maternal stress alters the neonatal intestinal microflora and suggests the provocative conclusion that stressful conditions during pregnancy might have the effect of increasing susceptibility to enteric infection for many months postpartum. It is much too early to speculate whether these animals might be at increased risk for the development of gastrointestinal and systemic immune-mediated diseases. The mechanisms by which stress alters the microflora were not determined by this study. The authors’ speculation that the development of intestinal flora is affected by prenatal conditions is controversial but should stimulate future studies, as there are many questions that remain to be answered.