Inclusion Criteria:  Diagnosis of axSpA (according to ASAS classification criteria) or RA (according to 2010 ACR/EULAR classification criteria); Indication for bDMARD therapy, according to the Portuguese recommendations for the use of biological therapies in patients with axSpA and RA; Oral corticosteroids (equivalent to prednisolone ≤ 10mg/day) and/or nonsteroidal anti-inflammatory drugs allowed at stable dose ≥4 weeks before baseline; Conventional DMARDs allowed at stable dose ≥12 weeks before baseline; Ability to provide informed consent.  Exclusion Criteria:  History of rheumatic disorder other than axSpA or RA; History of Inflammatory Bowel Disease; Previous treatment with bDMARD; Current pregnancy or breastfeeding; Malignancy (except for completely treated squamous or basal cell carcinoma); Any uncontrolled medical condition (e.g., uncontrolled diabetes mellitus, unstable ischemic heart disease); History of any documented gastrointestinal disease or tract surgery leaving permanent residua (e.g., gastrectomy, bariatric surgery, or colectomy); Intraarticular injections of extra-axial joints and tendons within 28 days before or at baseline; Recent (<3 months prior) use of any antibiotic therapy, current extreme diet (e.g., parenteral nutrition or macrobiotic diet), current consumption of probiotics.  Control group will be healthy participants, and the same inclusion and exclusion criteria will be applied except for rheumatic disease diagnosis.