Clostridioides difficile infection (CDI) continues to be a leading cause of antibiotic associated and healthcare related diarrhea in the United States (Lessa et al., 2015; Magill et al., 2018). With an estimated half million annual infections, approximately 30,000 deaths in the United States and a crude overall incidence rate of 121.2 cases per 100,000 persons, the burden of CDI is tremendous (Lessa et al., 2015; Centers for Disease Control and Prevention (CDC), 2019; Guh et al., 2020). One of the biggest challenges in managing CDI is recurrence. After an initial episode, recurrence rates have been estimated to be as high as 35%, and this rate increases with each successive episode, including up to 45% after the first recurrence and ~60% after the second recurrence (McFarland et al., 2002; Pepin et al., 2005; McDonald et al., 2018).

The treatment of CDI has historically included antimicrobials as a singular therapy. The American College of Gastroenterology guidelines published in 2021 recommend the use of vancomycin, fidaxomicin or metronidazole for non-severe initial infection (Kelly et al., 2021a). For severe initial infection (defined as a white count of >15,000 cells/mm³ and/or serum creatinine >1.5 mg/dL) treatment with either vancomycin or fidaxomicin is recommended. For an initial recurrence, a taper-pulse regimen of vancomycin or a course of fidaxomicin should be considered. As technology and research have led to a better understanding of the pathophysiology of CDI, we have come to appreciate the role of the microbiota in preventing recurrence and with this, additional therapeutics have been developed supplementing the standard of care (SOC) antimicrobial resulting in a reduction of rates of recurrence (Seekatz and Young, 2014).

The vegetative phase of C. difficile releases toxins causing symptoms including abdominal pains, fevers and diarrhea. Standard of care antimicrobials, such as vancomycin and fidaxomicin, control the vegetative phase of the infection. The spore phase is more resilient transferring from patient to patient, and also remaining within a patient’s system after SOC antimicrobials for extended periods. Antimicrobials have little to no impact on the spore phase, however, a healthy diverse microbiota can eradicate this phase decreasing the likelihood that it reconverts, or germinates, back into the vegetative phase, causing a recurrence (Seekatz and Young, 2014).

With this in mind, the treatment landscape for recurrent CDI (rCDI) has evolved dramatically in the last ten to fifteen years. A disruption of the gut microbiome composition, known as dysbiosis, allows spore germination, reduces the inherent resistance against colonization with C. difficile allowing vegetative growth with toxin production (Seekatz et al., 2016). Prior studies have demonstrated that low microbial diversity, particularly a decrease of the bacterial phyla Bacteroidetes and Firmicutes, plays a vital role in the pathogenesis of CDI (Chang et al., 2008). Thus, while antimicrobial therapy is necessary for treatment of CDI, these treatment modalities cannot directly address or correct the underlying dysbiosis. With SOC antimicrobials alone, we rely on natural regrowth of the deficient Bacteroidetes and Firmicutes, which frequently does not occur appropriately resulting in the high rates of recurrence of CDI. In fact, vancomycin itself can further deplete the diversity of the microbiota leaving patients prone to getting CDI recurrence (Louie et al., 2009; Ajami et al., 2018). In the light of this, therapies targeting intestinal microbial restoration following antimicrobials, thereby treating both phases of the infection, have rapidly evolved and are now widely available.

Therapies replenishing the gut microbial diversity are designed to engraft desirable microorganisms that are deficient, regaining the so called “colonization resistance.” This was originally achieved via fecal microbiota transplantation (FMT) but in recent times, the more sophisticated approach of live biotherapeutic products (LBPs) have become available. Broadly, FMT involves the transfer of stool from a healthy screened donor into the intestinal tract of the recipient, whereas the United States Food and Drug Administration (FDA) approved LBPs widely screen donors, sample the stool for consistency of microbes to be transferred and have standardized approaches for administration. This pivotal change in management underlies the progress achieved within the therapeutics for rCDI. This manuscript will outline the differences between thes two FDA approved LBPs, and clinical trial evidence related to the efficacy of LBPs.

The principle of FMT, as it relates to CDI, is to replenish the entire microbial diversity supplementing the deficiencies of desirable bacteria within the intestinal tract, particularly Bacteroidetes and Firmicutes. This introduction of a broad array of microbes aims to reverse the dysbiosis created by CDI establishing metabolic equilibrium that minimizes the likelihood of future recurrent episodes. Guidelines from the Infectious Disease Society of America and Society of Healthcare Epidemiology of America in 2021, along with the American College of Gastroenterology 2021, recommend the use of FMT after two or more recurrences of CDI that have been treated with appropriate antibiotics (Johnson et al., 2021; Kelly et al., 2021a). The American Gastroenterological Association guideline discussing fecal microbiota-based therapies published in 2024 advised consideration for fecal-microbiota based therapies, such as FMT and LBPs, in adults with “recurrent C. difficile” (Peery et al., 2024). This effectively includes patients with first recurrence, expanding the indications for these treatments.

Although the clinical trials considering FMT are heterogenous in reporting of methodology and design there are now many studies considering FMT for the prevention of recurrence of CDI (Feuerstadt et al., 2022a). The efficacy for FMT to prevent rCDI has been seen to vary, being more than 85% in many early observational studies but closer to 72% in controlled clinical trials (McGovern et al., 2021). An observational study of the largest stool bank in the United States that utilized centralized screening and processing, assessed 5,344 patients between 2014 and 2018 reporting a clinical resolution rate of 78% (Osman et al., 2022). Another study reported prospectively collected data from the AGA FMT National Registry, reporting that across 259 patients, they had 1 month follow up data on 222 with clinical resolution rates of 90% at that time. Of the 112 patients that had data available for 6-month follow up, 96.4% remained without recurrence (Kelly et al., 2021b).

A recent systematic review and meta-analysis evaluated the efficacy of FMT and LBPs for the prevention of rCDI in prospective clinical trials and compared this to the data from randomized clinical trials (Tariq et al., 2023). Across 19 clinical trials, clinical resolution with FMT or LBP was seen in 78% of patients, and among these, a resolution rate of 72% was observed in the 10 trials that included a control arm. While data from most studies have been encouraging, there has been significant heterogeneity in study design, screening of donors, composition of microbiota that was administered, assessment of outcomes and mode of administration among these clinical trials. Given this lack of standardization across studies with FMT, efforts focusing on microbiota therapeutics that are standardized with consistent microbial consortia have created the sub-category of LBPs ( Table 1 ). This advancement of technology has resulted in better studies and resulted in the FDA approval of two products that will be discussed below.

RBL was the first FDA approved LBP consisting of a broad consortium of spore and non-spore forming bacteria, including a minimum threshold of Bacteroides. It is a single, rectally administered dose containing 150mL of therapeutic material and 10⁷ microbes per mL or 15 x 10⁸ microbes per treatment.

Vowst (VOS) is distinct from RBL as this is a microbiota-based LBP that is derived from donor stool and subjected to an ethanol purification process that results in the isolation of Firmicutes spores. This is a consortium of microorganisms that only produce spores. VOS is administered orally as four capsules a day for three consecutive days after the patient has completed a course of SOC antimicrobials and a bowel lavage.

With an understanding of the clinical trials for RBL and VOS, it is also important to consider some key factors that might impact these trials, and how these factors might affect outcomes measured. It is pertinent to note that in each trial, both the intervention arm and the placebo arm received SOC antibiotics prior to randomization. The LBP was an “add-on” to the standard of care to improve its efficacy. The “placebo” arm frequently implies that patients did not receive further active intervention following SOC antibiotics. The duration of antimicrobials did differ among the trials ranging from ten to twenty-one days in the VOS trial and a minimum of 10 days in the RBL. It is unclear how this might impact outcomes, but longer courses of antimicrobials, such as vancomycin, might not be better, given the impact of vancomycin on the microbiota (Vrieze et al., 2014).

When considering treatment of a disease, if the disease is not properly diagnosed, one can’t expect a treatment targeting that disease to work. Therefore, diagnostic testing plays an essential role. Unfortunately, the diagnostic tools we have for CDI are sub-optimal. The polymerase chain reaction (PCR) assay, detecting the genes that code for the toxin, is the most commonly used assay for CDI in the United States but has a tendency to over-diagnose the infection, being positive when the C. difficile is not necessarily releasing toxin (Magill et al., 2018). Alternatively the enzyme immunoassay (EIA), a test that detects the toxin itself, has an unacceptably low sensitivity (Khanna et al., 2017). Therefore, a negative test does not necessarily rule out CDI. Finally, a cell culture cytotoxin neutralization assay is an accurate tool for diagnosis, but not widely available in clinical practice. Testing in the clinical trials need to balance accuracy while also mimicking what is available for the majority of practitioners. Ultimately, the diagnostics used in the trial have an impact on accurate patient selection and therefore the ability of interventions to show they work in the correct patients.

The number of recurrent episodes used for initial enrolment was not uniform among the LBP trials. The pivotal trial for RBL included patients with one or more recurrent episodes while the VOS trial included patients with two or more rCDI episodes. It is believed that with each recurrence, the dysbiotic state of the patient becomes more severe. Therefore, if we are trying to use a therapy that restores the deficiencies, one might argue that with more recurrences, it might require more therapy to achieve the same effect in getting the microbiota above the threshold where it can effectively resist germination of C. difficile and a recurrence.

A washout period is the time between completion of antimicrobials and administration of the LBP. The purpose of the washout period is to ensure that the SOC antibiotics have been eliminated from the patient so they do not significantly alter the newly administered organisms given within the LBPs. This washout period ranges between 24 and 72 hours, which could further be confounded by the specific antimicrobial therapy that was received prior to LBP.

Further, only the VOS trial used a bowel purge to further eliminate the SOC antimicrobial from the patient. This was performed with magnesium citrate. Dosing of each LBP also varies significantly. While RBL is a 150mL enema given once and is a broad consortium of microorganisms including a minimum threshold of Bacteroidetes, VOS is administered in the form of capsules over 3 days and is a narrow consortium of Firmicutes microorganisms.

For years, treatment avenues for CDI have been limited to SOC antimicrobial therapies, including vancomycin and fidaxomicin. While these antibiotics are effective and are supported by current clinical guidelines (Johnson et al., 2021; Kelly et al., 2021a), they only treat a portion of the infection and can further contribute to the disruption of the gut microbial environment. They also cannot act effectively on the C. difficile spores, which can remain in a patient’s system and if the microbiota is unable to properly regrow and rediversify, these spores can germinate causing a recurrence.

With the discovery of the pivotal role that the intestinal microbiota plays in the pathogenesis of CDI, recent therapeutics sought to modulate the microbiome and its resultant metabolome hindering recurrence of CDI. Fecal microbiota transplantation was the first microbial remodulation therapy to be developed. While evidence for FMT was reassuring, the lack of standardization across studies raised concerns with its safety along with risks of transmission of other pathogenic organisms (DeFilipp et al., 2019). With the need for more uniform therapies, that a wider array of providers would feel comfortable using, the LBPs were developed, which were created in a more regulated fashion having much more rigidly structured clinical trials testing their safety and efficacy. Clinical trial data for both RBL and VOS was encouraging, proving not only their clinical success, but also their sustained efficacy at a microbial and metabolomic level ultimately resulting in FDA approval of both for the prevention of recurrence of CDI. With this approval, there should be much wider access for these effective therapies for both providers and patients. As the frequency of their administration increases in clinical practice, both products should result in a significant reduction in the rates of rCDI and hopefully the burden of CDI on our patients and healthcare system.

KS: Conceptualization, Data curation, Writing – original draft. PF: Conceptualization, Supervision, Writing – review & editing.