The purpose of this study is to demonstrate the efficacy and safety of RBX7455 for the treatment of recurrent CDI in subjects who have had at least one recurrence after a primary episode (i.e., at least two episodes) and have completed at least two rounds of standard-of-care oral antibiotic therapy.

Inclusion criteria:

1. ≥ 18 years old.
2. Medical record documentation of recurrent CDI, defined as at least one recurrence after a primary episode and has completed at least two rounds of standard-of-care oral antibiotic therapy. Study subjects will have their diarrhea resolved, i.e., would be having less than 3 watery bowel movements at the time of study enrollment for 48 hours or more.
3. A positive stool test for the presence of C. difficile within 30 days prior to enrollment and standard C. difficile treatment.
4. Willing and able to swallow capsules.
5. Agrees to abstain from non-dietary probiotics for the duration of the study.
6. Agrees to abstain from vancomycin, metronidazole, fidaxomicin, rifaximin, nitazoxanide and IVIG for the duration of the study unless prescribed to treat recurrent CDI.
7. Agrees to stop proton pump inhibitors or H2 blocker medications.
8. Agrees to practice a form of effective contraception during study participation.
9. Has a negative urine pregnancy test at the time of enrollment (females of child-bearing potential only).
10. Willing and able to provide informed consent and HIPAA authorization.
11. Willing and able to complete the required Subject Diary.
12. Willing and able to meet all study requirements, including attending all assessment visits and phone calls.

Exclusion criteria:

1. A known history of continued CDI diarrhea, despite being on a course of antibiotics prescribed for CDI treatment.
2. Requires continuous antibiotic therapy for a condition other than CDI.
3. Previous fecal transplant.
4. Previous treatment with RBX2660.
5. Diagnosis of inflammatory bowel disease (IBD), e.g., ulcerative colitis, Crohn's disease, or microscopic colitis.
6. Diagnosis of irritable bowel syndrome (IBS) as determined by Rome III criteria.
7. History of chronic diarrhea.
8. History of celiac disease.
9. Disease symptoms caused by a confirmed intestinal pathogen other than C. difficile.
10. Unable to stop proton pump inhibitors or H2 blocker medications.
11. Colostomy.
12. Intra-abdominal surgery within the last 60 days.
13. Evidence of active, severe colitis.
14. History of short gut syndrome.
15. Requires the regular use of medications to manage bowel hypermotility.
16. Planned therapy in the next 3 months that may cause diarrhea (e.g., chemotherapy).
17. Planned surgery requiring perioperative antibiotics within 3 months of study enrollment.
18. Life expectancy of < 6 months.
19. Compromised immune system (e.g., HIV infection; AIDS-defining diagnosis or CD4 <200/mm3; inherited/primary immune disorders; immunodeficient or immunosuppressed due to a medical condition or medication; current or recent (< 90 days) treatment with chemotherapy; or current or recent (< 90 days) treatment with immunosuppressant medications.
20. Taking systemic steroids (≥ 20 mg a day or prednisone-equivalent) or is expected to be on steroids after enrollment through 8 weeks after completing the assigned study treatment.
21. An absolute neutrophil count of <1000 cells/µL.
22. Known or suspected current (< 90 days) illicit drug use. Note: marijuana use is allowed.
23. Pregnant, breastfeeding, or intends to become pregnant during study participation.
24. Participating in a clinical trial of another investigational product (drug, device or other) and has not completed the required follow-up period.
25. Subject, in the opinion of the investigator, for whatever reason, should be excluded from the study.