Immunobiology Research Programme, Department of Bacteriology and Immunology, University of Helsinki, Helsinki, Finland. katri.korpela@helsinki.fi. European Molecular Biology Laboratory, Heidelberg, Germany. katri.korpela@helsinki.fi. Immunobiology Research Programme, Department of Bacteriology and Immunology, University of Helsinki, Helsinki, Finland. Department of Veterinary Biosciences, Faculty of Veterinary Medicine, University of Helsinki, Helsinki, Finland. Department of Microbiome Science, Max Planck Institute for Developmental Biology, Tübingen, Germany. Institute of Biotechnology, University of Helsinki, Helsinki, Finland. Skin and Allergy Hospital, Department of Paediatrics, Helsinki University Hospital, Helsinki, Finland. Children's Hospital, University of Helsinki and Helsinki University Hospital, Helsinki, Finland. Laboratory of Microbiology, Wageningen University, Wageningen, the Netherlands.