Background. The hygiene hypothesis suggests that high hygiene standards have led to an immune dysfunction and an increase in allergic diseases. Farming-related exposures are associated with a decreased risk of asthma. Since the gut microbiota may be a pivotal component in the hygiene hypothesis, we studied whether perinatal exposure to pets, doctor's diagnosed wheezy bronchitis (WB), and compositional changes in the gut microbiota are interrelated among urban infants. Methods. Data were collected prospectively from a mother-infant nutrition study. Data on perinatal pet ownership, WB, and the microbiota composition of faecal samples of the infants assessed by quantitative PCR at 1 month were compared. Results. None of the 30 infants exposed to pets had suffered from WB by 24 months, whereas 15 of the 99 (15%) nonexposed infants had had WB ( ). The counts of Bifidobacterium longum were higher in samples ( ) from nonwheezing infants with pet exposure compared to those ( ) in wheezing infants without pet exposure (8.59/10.44 versus 5.94/9.86, resp. (median/upper limit of range, bacteria(log)/g of stool); ). B. breve was more abundant in the wheezing infants ( ). 1. Introduction The development of asthma and allergic diseases is a result of complex interactions between genetic predisposition and multiple environmental influences [1]. The hygiene hypothesis, first proposed by Strachan in 1989 [2] and subsequently supported by epidemiological studies [3], suggests that the higher hygiene standards adopted during the last few decades have led to dysfunction in the immune system, giving rise to allergic and autoimmune diseases as seen especially in affluent societies. Modern infants living in the developed countries may thus lack stimulation of the mucosal immune system sufficient to generate a tolerogenic immune milieu and be prone to develop diseases of inflammatory origin. In support of such a conception, differences in the neonatal gut microbiota have been shown to precede the development of atopy; for example, infants in whom atopy is developing harbour fewer bifidobacteria than their nonatopic peers [4]. The original hygiene hypothesis has been extended to a microbiota hypothesis emphasising the importance of the indigenous intestinal microbiota [5]. Farming-related exposures such as exposure to animal sheds and raw milk consumption have been shown to protect against the development of asthma and allergies [3]. Children living on farms are exposed to a wide range of microbes, and the diversity of microbial exposure is inversely associated with the risk of
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