The goal of this longitudinal study is to (1) explore the association between the gut microbiota and inflammatory disease activity in early onset multiple sclerosis, (2) investigate whether/how gut microbial composition vary when patients experience a relapse, and (3) to assess whether the gut microbiota shows increased similarities between affected pairs of first-degree relatives within the same family when compared with discordant pairs of first-degree relatives.

Using metagenomics, as well as clinical, immunological, and radiological observations, the investigators will investigate if active relapsing-remitting multiple sclerosis patients have a more pro-inflammatory gut microbiota signature than multiple sclerosis patients with less active disease and matched healthy controls.

More specifically, the investigators will investigate whether temporal variability of the gut microbiota is related to inflammatory disease activity in multiple sclerosis, whether changes in the gut microbiota are predictive of future inflammatory disease activity in multiple sclerosis, and whether gut microbiota characteristics are predictive of the disease course after 2 years.

Inclusion Criteria patients:

• Diagnosis of MS (as defined by the 2010 McDonald criteria).
• Occurrence of symptoms no longer than 5 years before baseline.
• Aged 18-65.
• Willingness to participate in the study and to sign the informed consent.

Exclusion Criteria patients:

• Treatment with high doses of systemic steroids 2 months before baseline.
• Use of antibiotics 3 months before baseline.
• Chronic gastrointestinal disease (e.g. inflammatory bowel disease, colon cancer).
• Other immune-mediated or autoimmune diseases (e.g. rheumatoid arthritis, diabetes type 1 and 2, psoriasis).

Additional inclusion criteria for MS patients undergoing a relapse:

• Ability to provide a faecal sample within 4 weeks from onset of the first symptoms suggestive of a relapse, before cortisone treatment. A relapse is defined by a new clinical sign or clinical worsening of a previous sign/symptom persisting for >=24 hours in the absence of fever.

Additional exclusion criteria for MS patients undergoing a relapse:

• Treatment with cortisone before collection of baseline faecal sample.
• Evidence of a relapse less than 2 months before baseline.
• Switching disease modifying treatment less than 2 months before baseline.

Inclusion criteria healthy controls:

• Willingness to participate to the study and to sign the informed consent.
• Aged >=18.

Exclusion criteria healthy controls:

• Neurodegenerative disorders.
• Chronic gastrointestinal disease (e.g. inflammatory bowel disease, colon cancer) or autoimmune diseases (e.g. rheumatoid arthritis, diabetes type 1, psoriasis).
• Use of antibiotics 3 months before baseline.
• Treatment with high doses of systemic steroids 2 months before baseline.