Staphylococcus aureus in the infantile bowel flora

The increasingly hygienic life-style in Western societies may not only have reduced infections, but may also have altered the composition of the commensal microflora, which may, in turn, have predisposed to e.g. allergy development. Here we investigated the first year s intestinal colonization pattern of a Swedish birth-cohort. Rectal swabs obtained at 3 days of age were cultured for aerobic bacteria and faecal samples obtained at 1, 2, 4 and 8 weeks and at 6 and 12 months of age were cultivated quantitatively for aerobic and anaerobic bacteria. When studying the overall colonization pattern we found that staphylococci have taken over as the first intestinal colonizer and that colonization with foremost Gram-negative bacteria has declined. When studying colonization pattern in relation to delivery mode we found that vaginally delivered infants were significantly more often colonized by E. coli during the first half year while sectio-delivered infants instead more often harbored other enterobacteria during the first weeks. Bacteroides colonization was delayed up to one year of age in sectio-delivered infants. We regard the findings as a result of reduced spread of some prototype faecal bacteria in the Swedish society today. In the absence of such traditional faecal bacteria skin bacteria like staphylococci, have have had increased chances to expand in the infantile intestinal microflora. Approxiamtely 80% of the Swedish infants studied harbored S. aureus at some time-point during the first year with a peak between two and six motnhs of age. Using the PCR-based method random amplified polymorphic DNA, RADP, we found that most infants carried a single strain that persisted several months in the microflora. Half of the infants were colonized with a S. aureus strain producing one or more superantigens (enterotoxin A-D and/or toxic shock syndrome toxin-1, TSST-1). Such colonization did not result in any overt clinical signs and it is not known whether the toxins are produced in situ in the gastrointestinal tract. Most of the S. aureus strains colonizing the infantile bowel derived from the skin flora of the parents. In almost all cases infant and parent(s) harbored the same strain. For comparison, intestinal colonization with S. aureus was studied in a cohort of 100 Italian infants. They were significantly less often colonized by S. aureus in their intestines than Swedish infants. Investigating which life-style factors predisposed to S. aureus colonization; we found that exclusive breast-feeding and having older sibling correlated positively with S. aureus colonization, while having pets correlated negatively. Adjusting for these factors we still found a significantly increased intestinal colonization by S. aureus in Swedish compared to Italian infants. The Swedish infantile intestinal S. aureus strains were investigated for susceptibility to a range of common antibiotics. Few strains were resistant to other antibiotics than penicillin V, similar to the situation for clinical S. aureus isolates from Sweden. Strains resistant to antibiotics were equally likely to persist in the infantile intestinal microflora and had similar population counts as susceptible strains. In conclusion, S. aureus has become a common persistent colonizer of the infantile bowel flora. This may be due to lack of competition from more professional faecal bacteria, such as E. coli and obligate anaerobes, whose circulation in society may have decreased due to improved hygienic standards