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ABSTRACT:The modern Western lifestyle may have altered the composition of the commensal microflora. Here, we investigated the first year's intestinal colonization pattern in 99 vaginally delivered Swedish infants and 17 delivered by cesarean section. Rectal swabs obtained at 3 d of age were cultured for aerobic bacteria and fecal samples obtained at 1, 2, 4, and 8 wk and at 6 and 12 mo of age were cultivated quantitatively for aerobic and anaerobic bacteria. Vaginally delivered infants more often had Escherichia coli compared with cesarean section-delivered infants, whereas the latter more frequently carried other enterobacteria, such as Klebsiella and Enterobacter. Independent of delivery mode, it took 2 mo until most infants were colonized by enterobacteria, traditionally the first colonizers. In contrast, coagulase-negative staphylococci colonized 99% of the infants from d 3 onwards. The poor adaptation of staphylococci to the gut was shown by declining population sizes after some weeks. Dominating anaerobes were initially bifidobacteria and clostridia, whereas Bacteroides initially colonized only 30% of vaginally delivered infants and increased very slowly in prevalence. Bacteroides colonization was delayed up to 1 y in cesarean section-delivered compared with vaginally delivered infants. Our results show that some "traditional" fecal bacteria are acquired late today especially in cesarean section-delivered infants, probably due to limited environmental circulation. In their absence, skin bacteria like staphylococci have become the first gut colonizers. (Pediatr Res 59: 96-101, 2006)

Colonization by Escherichia. coli in infants might have decreased in the last decades, owing to changes in hospital routines and family lifestyle. In this study, the E. coli flora was characterized in 70 healthy Swedish infants followed for the first year of life. E. coli was isolated from rectal swabs obtained at 3 d of age and quantified in fecal samples collected at 1, 2, 4, and 8 wk of age and at 6 and 12 mo of age. Strains were typed using random amplified polymorphic DNA, and their virulence factor genes were identified by multiplex PCR. Colonization by E. coli occurred late; only 61% of the infants were positive by 2 mo of age. The turnover of individual strains in the microflora was slow (1.5 strains per infant during 6 mo, 2.1 during 1 y). Environmental factors, such as siblings, pets, or feeding mode, did not influence colonization kinetics or strain turnover rate. Genes encoding type 1 fimbriae, P fimbriae, and hemolysin were significantly more common in E. coli strains persisting for at least 3 wk in the microflora than in transient strains. The P-fimbrial class III adhesin gene was more common in E. coli from children who had a cat in their homes than in E. coli from children without pets (p ϭ 0.01); this adhesin type is common in E. coli from cats. The late colonization and low E. coli strain turnover rate suggest limited exposure of Swedish infants to E. coli. Our results confirm that P fimbriae and other virulence factors facilitate persistence of E. coli in the human colonic microflora. Escherichia coli is one of the first bacterial species to colonize the infant's intestines. In the 1970s, E. coli usually appeared in the baby's feces a few days after birth (1, 2), as a sign of its establishment in the intestinal microflora (3, 4). E. coli colonizing the newborn infant may originate in the maternal fecal flora (5), but E. coli strains are also commonly spread at maternity wards via the nursing staff, especially during periods of high bed-occupancy and staff workload (6). We have recently reported that Staphylococcus aureus has become a major colonizer of the infant gut (7), which may be a sign of reduced competition from other microbes. E. coli and other fecal bacteria might be less easily spread today, because of increased hygiene in hospitals and families.Some E. coli strains persist in the intestinal microflora of an individual for months or years (resident strains), whereas others (transient strains) disappear within a few weeks (8). Resident E. coli strains display certain characteristics that enable them to persist in the intestinal microflora, e.g. the expression of P fimbriae and capacity to adhere to colonic epithelial cells (9 -12). P fimbriae are composed of a fimbrial rod with a tip adhesin that exists in three varieties, termed papG classes I, II, and III. These recognize the Gal␣134Gal␤ disaccharide, with slight differences in binding specificity (13). The class II variety of the papG adhesin is common among E. coli causing pyelonephritis (14), whereas the class III variety is comm...

Early introduction of fish into the child's diet was associated with less eczema development, and a tendency to less asthma. Sensitization was not associated with the timing of fish introduction.