Long-time persistence of superantigen-producing Staphylococcus aureus strains in the intestinal microflora of healthy infants

Staphylococcus aureus has been isolated at an increasing rate from infants' stools during the last decades, but it is not known whether this species c an colonize and persist in the intestinal microflora. To investigate this, 49 Swedish infants were followed prospectively from birth until 12 months o f age. S. aureus was identified in a rectal swab obtained 3 d after deliver y and in quantitative cultures of fecal samples collected at 1, 2, 4, and 8 weeks and at 6 and 12 months of age. A random amplified polymorphic DNA (R APD) method was developed to distinguish individual S. aureus strains from one another and the strains were tested for production of enterotoxins A-D and TSST-1. By 3 days of age, 16% of infants had S. aureus in their intesti nes, which increased to 73% by 2-6 months, whereafter it decreased slightly to 53%. At the same time S, aureus population counts in colonized infants declined from an average 10(6.8) CFU/g feces during the first months of lif e to 10(4.0) CFU/g feces by 12 months. Colonized infants usually harbored o ne or two S. aureus strains in their microflora for long periods of time. F ew strains were transient passengers and the median time of persistence of S. aureus strains in the microflora was several months. Of the 75 S. aureus strains identified, 43% produced one or more toxins: 13% SEA, 7% SEE, 23% SEC, 4% SED, and 11% TSST-1. Altogether, 47% of the investigated infants we re colonized by a toxin-producing S. aureus during their first year of life . Despite this they were apparently healthy and did not have more gastroint estinal problems than noncolonized infants. This report is the first to sho w that S. aureus may be a resident member of the normal intestinal microflo ra in infancy.