The aim of this study is to assess the effect of Saccharomyces boulardii CNCM I-745 on gut microbiota in patients undergoing antibiotic therapy (in the context of erythema migrans (early skin form of Lyme borreliosis)).

The study is designed to compare the efficacy of Saccharomyces boulardii CNCM-I-745 500 mg bid to a placebo in patients undergoing antibiotic therapy in the context of erythema migrans as the early form of Lyme borreliosis.

Inclusion Criteria:

• Adult Patients, ≥18 years old.
• Who are prescribed antibiotic therapy (as per medical routine practices, amoxicillin 1000 mg bid for 14 days) in the context of erythema migrans (early skin form of Lyme borreliosis).
• Able to comply with study requirements and to provide signed informed consent before any study procedure.
• Has no condition that may interfere with the study assessments.
• Able to fulfil in the diary stool log, according to the physician's opinion.
• Regular defecation (frequency and stool consistency, with at least about three bowel movements a week).
• For women of childbearing potential:
• A negative urine pregnancy test immediately prior to starting the study treatment,
• Agreement to comply with approved methods of contraception during the whole study: unless they meet the criteria of post-menopausal, i.e. 12 months of spontaneous amenorrhea, women of child-bearing potential, defined as all women physiologically capable of becoming pregnant, including women whose career, lifestyle, or sexual orientation precludes intercourse with a male partner, should use one or more acceptable methods of contraception that should be maintained throughout the study.

Exclusion Criteria:

• History of hypersensitivity to the study treatments (active substance or excipients), brewer's or baker's yeast,
• Contraindication and special warning to the study drugs according to the SmPCs,
• History of chronic constipation with passage of fewer than 3 spontaneous bowel movements per week on average,
• History of chronic or recurrent diarrhoea with spontaneous unformed bowel movements equivalent to or more often than 3 times daily,
• Prior gastrointestinal surgery (apart from appendectomy or cholecystectomy performed at least more than one year ago),
• History of Clostridium difficile infection,
• Active gastrointestinal inflammatory disease,
• Known chronic or recurrent systemic disorder that may interfere with the study drug evaluation,
• Immunocompromised (organtransplants, leukaemia, malignant tumours, radiotherapy, chemotherapy,prolonged high dose cortisone treatment, immunosuppressant treament) or critically ill patients (such as autoimmune disease, HIV,…), patients with a central venous catheter,
• Severe hepatic or renal impairment,
• Systemic antibacterial therapy during the 2 months prior to study enrollment,
• New prescription medications during the 2 weeks prior to study enrollment,
• Use of any drug or product that alters gut microbiota or function, such as probiotics, laxatives, antiemetics, cisapride, antisecretory or adsorbent treatments (racecadotril, smectite, activated charcoal), opiates such as loperamide, atropine and other cholinergic agents, 4 weeks prior to study enrollment and during the study,
• Intake of antifungals within 14 days prior to study enrollment,
• Substantial changes in eating habits within 30 days prior to receiving the first dose of IMP product, and during the study as assessed by the Investigator,
• History or presence of drug or alcohol abuse,
• Heavy smoker (more than 10 cigarettes per day),
• Breast-feeding woman,
• Patients enrolled in another interventional clinical trial where they received an investigation treatment within the past 30 days,
• Any condition or personal circumstance that, in the opinion of the investigator, rendered the subject unlikely or unable to comply with the full study protocol

The 2 types of investigational products will be checked for blind maintenance before release (capsules will be identical in size, shape, color and appearance). The principal investigator does not have access to the randomization (treatment) code, except in exceptional circumstances for valid medical or safety reason, such as occurrence of a serious adverse event for which the knowledge of the study medication would be considered essential for treating the patient.