Background
Staphylococcus aureus may worsen already established atopic dermatitis (AD), but its primary role in the aetiopathogenesis and severity of AD is unclear.

Objectives
To compare the prevalence of S. aureus colonization in early infancy in children who developed AD during the first 2 years of life with children who did not.

Methods
In this prospective birth cohort study, which included 450 infants, we analysed bacterial swabs collected from cheek skin at 0 and 2 months of age. The development of AD, and its severity, was diagnosed by a physician and monitored prospectively for 2 years. Information on parental atopy, filaggrin gene mutation status and use of antibiotics and emollients was included in the analyses.

Results
At birth, the occurrence of S. aureus colonization was similar in infants who developed subsequent AD and those who did not. At 2 months of age, S. aureus colonization was more common in children who later developed AD (adjusted hazard ratio 1.97, 95% confidence interval 1.21–3.19; P = 0.006). No association was found between S. aureus colonization and AD severity or age at onset.

Conclusions
It remains unknown whether colonization with S. aureus may directly increase the risk of AD, or whether it should be considered as secondary to skin barrier impairment or a skewed immune activity, but according to our findings, S. aureus colonization is more commonly increased at 2 months of age in children who later developed AD.