AIM:To investigate whether birch pollen allergy symptoms are linked with gut microbiota changes and whether probiotics have an effect on these.METHODS:Forty seven children with confirmed birch pollen allergy were randomized to receive either a probiotic combination of Lactobacillus acidophilus (L.acidophilus) NCFMTM (ATCC 700396) and Bifidobacterium lactis (B.lactis) Bl-04 (ATCC SD5219) or placebo in a double-blind manner for 4 mo,starting prior to onset of the birch pollen season.Symptoms were recorded in a diary.Blood samples were taken for analysis of cytokines and eosinophils.Fecal samples were analysed for microbiota components,calprotectin and IgA.Nasal swabs were taken for analysis of eosinophils.RESULTS:The pollen season induced a reduction in Bifidobacterium,Clostridium and Bacteroides which could not be prevented by the probiotic intervention.During the intervention,significantly higher numbers of B.lactis 11.2×107 ± 4.2×107 vs 0.1×107 ± 0.1×107 bacteria/g feces (P<0.0001) and L.acidophilus NCFMTM 3.5×106 ± 1.3×106 vs 0.2×106 ± 0.1×106 bacteria/g feces (P<0.0001) were observed in the probiotic group compared to the placebo group.During May,there was a tendency for fewer subjects,(76.2% vs 95.2%,P=0.078) to report runny nose,while during June,fewer subjects,11.1% vs 33.3%,reported nasal blocking in the probiotics group (P =0.101).Concomitantly,fewer subjects in the probiotic group had infiltration of eosinophils in the nasal mucosa compared to the placebo group,57.1% vs 95% (P=0.013).Eye symptoms tended to be slightly more frequent in the probiotic group,12.5 d [interquartile range (IQR) 6-18] vs 7.5 d (IQR 0-11.5) (P=0.066) during May.Fecal IgA was increased in the placebo group during the pollen season;this increase was prevented by the probiotics (P=0.028).CONCLUSION:Birch pollen allergy was shown to be associated with changes in fecal microbiota composition.The specific combination of probiotics used was shown to prevent the pollen-induced infiltration of eosinophils into the nasal mucosa,and indicated a trend for reduced nasal symptoms. AIM: To investigate whether birch pollen allergy symptoms are linked with gut microbiota changes and whether probiotics have an effect on these. METHODS: Forty seven children with confirmed birch pollen allergy were randomized to receive either a probiotic combination of Lactobacillus acidophilus (L. acidophilus) NCFMTM (ATCC 700396) and Bifidobacterium lactis (B. lactis) Bl-04 (ATCC SD5219) or placebo in a double-blind manner for 4 mo, starting prior to onset of the birch pollen season. Symptoms were recorded in a diary. Blood samples were taken for analysis of cytokines and eosinophils. Fecal samples were analysed for microbiota components, calprotectin and IgA. Nasal swabs were taken for analysis of eosinophils .RESULTS: The pollen season induced a reduction in Bifidobacterium, Clostridium and Bacteroides which could not be prevented by the probiotic intervention. During the intervention, significantly higher numbers of B. lactis 11.2 × 107 ± 4.2 × 107 vs 0.1 × 107 ± 0.1 × 107 bacteria / g feces (P <0.00 01) and L. acidophilus NCFMTM 3.5 × 106 ± 1.3 × 106 vs 0.2 × 106 ± 0.1 × 106 bacteria / g feces (P <0.0001) were observed in the probiotic group compared to the placebo group. During May, there was a tendency for fewer subjects, reported nasal blocking in the probiotics group (76.2% vs 95.2%, P = 0.078) to report runny nose, while during June, fewer subjects, 11.1% vs 33.3% .Concomitantly, fewer subjects in the probiotic group had infiltration of eosinophils in the nasal mucosa compared to the placebo group, 57.1% vs 95% (P = 0.013). Eye symptoms tended to be slightly more frequent in the probiotic group, 12.5 d [interquartile range (IQR) 6-18] vs 7.5 d (IQR 0-11.5) (P = 0.066) during May. Fecal IgA was increased in the placebo group during the pollen season; this increase was prevented by the probiotics (P = 0.028) .CONCLUSION: Birch pollen allergy was shown to be associated with changes in fecal microbiota composition. The specific combination of probiotics used was shown to prevent the pollen-induced i nfiltrationof eosinophils into the nasal mucosa, and a trend for reduced nasal symptoms.