Inclusion Criteria:
1. The participants were fully informed of the purpose, procedure and risks of the study and voluntarily signed the informed consent form.
2. Aged 18 to 65 (inclusive), no gender restrictions.
3. Meets the CSU diagnostic criteria, with a disease duration of ≥6 weeks.
4. Antihistamine resistance: Regular use of a standard dose (as recommended in the package leaflet) of a second-generation H1 antihistamine (e.g. desloratadine 5 mg, levocetirizine 5 mg, etc.) for at least 2 weeks prior to screening, but with symptoms still not effectively controlled. Defined as: a UAS7 score of ≥7 within 7 days prior to the screening visit (V1) and baseline visit (V2).
5. Agree to maintain the type and dose of the baseline antihistamine throughout the trial.
6. Agree not to consume any products that may significantly affect the gut microbiota during the trial (such as yoghurt containing probiotics, or probiotic/prebiotic supplements).
7. At the baseline visit, the treatment regimen was clearly defined, and exposure status (OM-85 exposure group or non-exposure group) could be determined based on actual prescriptions and medication records.
Exclusion Criteria:
1. Special types of urticaria: These primarily include physical urticaria (such as artificial urticaria, cold contact urticaria and cholinergic urticaria), urticarial vasculitis, and angioedema caused by hereditary or acquired C1 esterase inhibitor deficiency;
2. Systemic use of glucocorticoids (oral or injectable) within the four weeks prior to screening;
3. Exclude patients who have used immunosuppressants (such as cyclosporine, methotrexate, or Tripterygium wilfordii) within the previous four weeks;
4. Exclude patients who have used biologics (such as omazulimab, dupilumab, etc.) within the previous three months;
5. Exclude those who have used bacterial lysates (such as Panfuxu or Pidomod) or received vaccinations within the past three months;
6. Key exclusion criteria for the microbiome study: use of oral or intravenous antibiotics within the four weeks prior to screening, or continuous use of probiotic supplements for more than one week.
7. Patients with severe systemic autoimmune diseases (such as systemic lupus erythematosus, rheumatoid arthritis, inflammatory bowel disease, etc.) may interfere with the assessment of immune mechanisms;
8. A history of serious liver, kidney, heart, lung or blood disorders, or malignant tumours;
9. Individuals known to be allergic to bacterial lysates and their excipients.
10. Special populations: pregnant or breastfeeding women, or participants of childbearing age who do not intend to use contraception during the trial.
11. Other: Participants who have taken part in other clinical trials involving medicinal products within the past 30 days; or where the investigator considers that there is poor compliance, a mental health condition, or any other circumstance rendering the participant unsuitable for the trial.
