Why Don't We Bud?

The transition from the microbe-free existence of intrauterine life to the extrauterine world occurs via the most bacteria-rich environment in the human body. Such an abrupt confrontation with the potentially hostile microbial world appears to be universal in nature. Compared with comfortable and sterile budding (e.g., through the abdominal wall), this route may provide the individual with a good germinating (literally) start for a healthy life. This is well demonstrated in incubator-hatched chicks, which do not come into contact with the normal flora of the mother fowl. These microbe-defective animals are susceptible to colonization by Salmonella, but can be saved by oral inoculation of intestinal bacterial homogenate immediately after hatching, the so-called Nurmi principle (1). Similarly, fish in fish hatcheries grow significantly better and are healthier in natural water when compared with extremely well sanitized water. In the healthy human, a critical balance is established and maintained between protective mucosal immunity, including a vigorous immune response to pathogenic antigens, and nonresponsiveness to resident microorganisms and ubiquitous antigens, such as food. The immature and inexperienced immune system needs to learn this lesson from its environment. One plausible hypothesis here could thus be that the normal flora acquired during delivery is needed for the development of the systemic and mucosal immunity of the newborn. These extreme educational measures invite speculation that timing is of essence. Gut Microflora The immunophysiologic regulation in the gut depends on the establishment of an indigenous microflora. Nature has not left this crucial developmental process to take place at random (2–5). Initially, facultative gram-positive cocci (staphylococci, streptococci, and enterococci) and enterobacteria dominate. Anaerobic colonization occurs during the second day of life. Bifidobacteria then take the lead, followed by bacteroides. Once the initial bacterial ecosystem has established itself, the relative frequencies and levels of bacteria remain stable during the period of exclusive breastfeeding. After weaning, the composition of the microflora becomes more complex, resembling that of the adult flora, when the mucosal surfaces harbor more bacterial cells that the total cell count in the human body. What Is the Immunologic Impact of the Establishment of the Gut Microflora? The gut-associated lymphoid tissue is an important element in the immunologic homeostasis of the host. Its hallmark is oral tolerance, which maintains unresponsiveness to a myriad of antigens encountered by the enteric route. Not all intraluminal antigens induce oral tolerance. Microbial antigens elicit specific immune responses at local and systemic levels (6–8). This has been explained by their capacity to bind to epithelial cells and thus to allow antigen entry via enterocytes and escape tolerance induction in Peyer's patches. The stronger the adhesion, the greater the gut immune response. Viruses and bacteria, which invade the cells of the small intestinal epithelium, stimulate the production of inflammatory cytokines and affect phagocytosis together with the anti-inflammatory cytokines. This process generates the immunoglobulin (Ig) A response, and the net result is the eradication of the pathogen. For the gut microflora bacteria a distinct type of IgA response has been shown recently (9), which may allow the maintenance of the resident microbiota and provide tonic immune responses in the gut-associated lymphoid tissue controlling its milieu. Indeed, Duchmann et al. (10) have shown that healthy individuals are tolerant to their own microflora, and that such tolerance is abrogated in patients with inflammatory bowel disease. For the newborn, it is vital that intestinal colonization act as an effective antigenic stimulus for the maturation of the gut-associated lymphoid tissue (11,12). The capacity to generate IgA-producing cells increases progressively during the successive stages in the establishment of the gut microflora (12,13). This microflora also acts as the key mediator in the induction and maintenance of oral tolerance. The role of the intestinal microflora in the induction of oral tolerance to the IgE response was recently investigated in germ-free mice (14). These microflora, unlike their colonized counterparts, continued to produce IgE antibodies to orally administered ovalbumin. The sustained IgE response in germ-free mice could be corrected by the reconstitution of gut microflora at the neonatal stage only. These results suggest that intestinal colonization is an important prerequisite for the generation of a T-helper-2 population susceptible to oral tolerance induction, and further: timing is essential. Clinical Implications Many of the immunoregulatory aberrations favoring inflammatory response exert their major effects in early infancy, when the main load of new antigens is encountered. The predisposition to gastrointestinal infection is associated with immaturity and dysfunction of the gut defence barrier. The microflora provide potent stimuli to build and strengthen this barrier. The impact of the gut microflora, however, extends beyond the gut. The postnatal period is immunologically characterized by suppression of cell-mediated immunity and a concurrent move toward humoral immunity, which is mediated by a switch in the T-helper cell population of the fetoplacental unit to produce T-cell cytokines of the T-helper-2 phenotype, including interleukin-4 (15). A deficiency in T-helper-1–related processes, such as the immune response to microbial antigens, may increase the risk of consolidating the T-helper-2 phenotype responsible for IgE production, eosinophilia and atopic disease. It appears, moreover, that the longer such deficiency continues, the greater the risk will be. Gut Microflora As Therapeutic Tool The demonstration that the gut microflora is an important promoter of the intestine's defence barrier has given rise to the concept of probiotic therapy: therapeutic application of consumption of cultures of potentially beneficial microorganisms as probiotics. A probiotic has been defined as “A live microbial food ingredient that is beneficial to health”(5). The criteria for a probiotic include that the strain is of human origin, is safe in human use and stable to acid and bile, and adheres to the intestinal mucosa. Different capacities for adhesion to epithelial cells have been reported, and available probiotics have been classified according to this property. The most frequently used genera are lactobacilli and bifidobacteria. Among the possible mechanisms of probiotic therapy is promotion of the gut defence barrier: normalization of increased intestinal permeability and unbalanced gut microecology (5). Further mechanisms include improvement of the intestine's immunologic barrier, particularly intestinal IgA responses, and alleviation of intestinal inflammatory response by controlling the balance of proinflammatory and anti-inflammatory cytokines (6–8,16–18). The multicenter study of the ESPGHAN working group coordinated by Guandalini (19) substantiates the notion of the gut microflora as a source of novel therapeutic modalities. In demonstrating the safe administration of a probiotic in an oral rehydration solution, a significant reduction in the duration of diarrhoea and hospitalisation, and prevention of the evolution of rotavirus diarrhoea toward a protracted course, the study provides a sound database backing the use of probiotics for the treatment of rotavirus diarrhoea in infants. Recent data indicate differences between candidate probiotic bacteria with respect to their immunomodulatory effects. Distinct effects have been detected in healthy subjects and patients with inflammatory diseases (20). These observations raise two questions: how safe is probiotic therapy, and what are the effects, if any, on the balance of the gut ecosystem? Active research is in progress to characterize the specific immunomodulatory properties of probiotic bacteria and to develop probiotic functional foods for specific clinical applications. These measures include rigorous safety assessment (5). The second question is more difficult. More than 500 species of bacteria are represented accounting for 35% to 50% of the volume of the contents of the human colon, but the number of nonculturable bacteria far exceeds that of the known microbes. These microbes may attach to the mucus, adhere to intestinal epithelial cells, aggregate together or with pathogenic bacteria and be metabolically active either on the mucosa or in the gut contents, or, following translocation, in other parts of the body. They are constantly dividing, some are even budding. So what do we actually know of our “buddies”? Not very much. What controls and selects the normal gut microflora of a human individual is also not known. Host factors, such as innate and acquired immunity, are involved. Most probably human leukocyte antigen (HLA) type is also important. Microbial factors render this relationship a mutual one. From this point of view, the selection of the microbes in the infant's own mother might be most fitting for the baby. Yet the most dramatic changes in our environment—nutrition, sanitary conditions and antimicrobial therapy—clearly have an impact on our buddies and their well being. New information on their habits, population dynamics, other activities, and the species of unculturable microbes may provide novel explanations for many gut-related health problems in our western civilization, ranging from allergies to autoimmune and inflammatory diseases.