The establishment and succession of bacterial communities in hospitalized preterm infants has not been extensively studied. Because earlier studies depended on classical cultural techniques, their results were limited. This study monitored the establishment and succession of the neonatal microbiota in the first weeks of life by analyzing the 16S rDNA variety in fecal samples applying PCR-denaturing gradient gel electrophoresis (PCR-DGGE). Fecal samples from 29 preterm infants hospitalized in a neonatal intensive care unit, including samples from antibiotic-treated infants and one with neonatal necrotizing enterocolitis, were subjected to PCR-DGGE analysis. Daily DGGE profiles from all preterm infants during the first 4 wk were obtained and analyzed. In addition, feces of 15 breast-fed, full-term infants and a variety of clinical bacterial isolates were examined and compared with the PCR-DGGE profiles of the preterm infants. During the first days of life, the DGGE profiles were rather simple but increased in their complexity over time. It became obvious that not only the intraindividual band-pattern similarity increased over time, but also the interindividual. During the observation period, similarity values (C s ) increased in each preterm infant from 0 to 80%, whereas interindividual C s increased from 18.1 to 57.4%, revealing the acquisition of a highly similar bacterial community in these infants. In contrast, C s -values obtained for breast-fed, full-term infants were rather low (11.2%). Escherichia coli, Enterococcus sp., and Klebsiella pneumoniae were the bacteria most commonly found in all preterm infants. The interindividual bacterial composition in hospitalized preterm infants is more similar in comparison with breast-fed, full-term infants and is not necessarily influenced by birth weight, diet, or antibiotic treatment. The GI tract of a normal fetus is sterile. During the birth process and rapidly thereafter, microbes from the mother and the surrounding environment colonize the gastrointestinal tract of the infant until a dense, complex bacterial community is established. This enteric flora contributes not only to health by facilitating carbohydrate assimilation (1) and interaction with the developing immune system (2, 3), it also plays a significant role in disease (4 -6). A dynamic balance exists between the GI bacterial community, host physiology, and diet, all of which influence the initial acquisition, developmental succession, and eventual stability of the gut ecosystem (7). In the birth process and soon thereafter, bacterial colonization starts and facultative anaerobic bacteria such as enterobacteria appear in feces. Because at this stage the composition of the gut bacterial community is strongly influenced by the diet, a shift in the bacterial composition can be observed (3,8). A breast-fed, full-term infant shows a fecal bacterial composition in which bifidobacteria predominate over potentially harmful bacteria, whereas, in formula-fed infants, coliforms, enterococci, and bacteroides predomi...
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