Intestinal Microflora Composition

Infant feeding practice may affect the intestinal colonization pattern in two ways: by influencing which microbes that are available, and by influencing the intestinal milieu to become hostile to some bacteria but favourable for others.

During breast-feeding the infant will suckle not only milk, but also bacteria on or around the nipple. Thus, the infant swallows typical skin flora bacteria, such as Staph. epidermidis and Staph. aureus. Such bacteria are also found in banked mother's milk32. Conversely, the bottle fed infant may ingest bacteria contaminating the feeds, such as enterobacteria, especially in developing countries with poor hygienic conditions31.

We have recently summarized the results of25 studies that compare the intestinal flora ofbreast-fed and bottle-fed infants23. The results from six of these studies are shown in Table 2. The most consistently observed differences are that bottle-fed infants have more enterococci and more clostridia than breast-fed infants. Instead, breast-fed infants tend to have more staphylococci, especially at an early age.

In some studies, breast-fed infants have lower counts of enterobacteria than bottle-fed infants, but more consistently they have less enterobacteria other than E. coli, e.g. Klebsiella, Enterobacter and Citrobacter, as compared with bottle-fed infants. Breast-fed infants also have fewer E. coli strains at a certain time point and over a period of time as compared with bottle-fed infant33-35. Among E. coli strains, those expressing type 1 fimbriae with mannose-specific adhesins seem to be selectively favoured in the suckling infant36- 37while E. coli with adhesins conferring mannose-resistant hemagglutination and P-fimbriated E. coli are disfavoured by breast-feeding35' 38 . There are also studies showing that breast-fed infants less often than bottle-fed ones tend to be colonized withE. coli strains that are resistant to the bactericidal effect of human serum39, or carry the K1 capsule33, the latter being a virulence factor for neonatal sepsis/meningitis40. Collectively, these factors contribute to a less virulent composition ofthe enterobacterial flora ofthe breast-fed infant.

Already a century ago, Tissier reported that breast-fed infants had a microflora dominated by bifidobacteria, while this was not the case with artificially fed infants41. Such a pronounced difference has not been noted in most recent studies, for unknown reasons23.

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