The fetus in utero is sterile until birth. Colonization of the human body with a heterogenous collection of microorganisms from the birth canal begins at delivery. The Lactobacillus species constitute the major population of the vaginal microbiota and thus provide the initial inoculum to the infant during birth. In the case of caesarean section or premature infants, most microbes that are transferred to the newborn can be traced from the environment, i.e., from other infants via the air, equipment and nursing staff (19). Therefore, the type of delivery (passage through the birth canal versus caesarean section) as well as the type of diet (breast versus formula feeding) might affect the pattern of microbial colonization.
The general pattern observed was that the facultative microorganisms appeared first and were subsequently followed by a limited number of anaerobes during the first two weeks (20). The types of bacterial strains that are capable of populating the GI tract are regulated through the limitation of the intestinal milieu, which changes with the successive establishment of the different bacteria. Hence, bacteria that are capable of oxidative metabolism, such as enterobacteria, streptococci and staphylococci, are among the first to proliferate in the gut. As the numbers of the facultative bacteria increase, they consume oxygen and lower the redox potential to negative values. These conditions are favorable for the anaerobic bacteria to multiply and reach much higher levels than that of the first week. Populations of bifidobacteria, Bacteroides and clostridia, the commonly found anaerobes, increase with subsequent change of conditions in the GI tract. By the fourth week, the fecal microbiota of the breast-fed infants consists mainly of bifidobacteria and other groups to a lesser extent including enterobacteria, clostridia, and Bacteroides. However, in formula-fed infants, bifidobacteria do not beome so dominant and a more complex microbiota develops. The differences between the breast-fed and formula-fed infants gradually disappear with the intake of solid food. By the twelfth month, the number of facultative anaerobes declines as the anaerobes begin to increase and form a stable population, resembling that of adults in numbers and in composition. By the age of two, the profile resembles that of an adult (19). In adults, the ratio of anaerobic to aerobic bacteria is 1000:1 (21).
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