From vancomycin to fidaxomicin: shifting paradigms in Clostridioides difficile management.

Current evidence on pathogenesis and virulence factors

The pathogenesis of C. difficile infection is closely linked to the production of its main virulence factors: toxins A (TcdA) and B (TcdB). Both toxins are glycosyltransferases that inactivate Rho-GTPases in the actin cytoskeleton of intestinal epithelial cells, causing disruption of tight junctions, increased epithelial permeability, local inflammation, and cell death[6].

Although toxin A was historically considered the primary determinant of disease, the most recent experimental and clinical evidence has shown that toxin B plays a central role in the bacterium’s virulence[7]. Studies using isogenic mutants have shown that strains producing exclusively toxin B retain the ability to induce significant tissue damage, while those expressing both toxins exhibit greater virulence, suggesting a synergistic effect in disease development[7].

These findings have had direct implications for the development of new therapeutic strategies, particularly those aimed at neutralizing toxin B, and have helped redefine the classical understanding of the pathogenesis of CDI.

Additionally, the emergence of hypervirulent strains, such as NAP1/BI/027, has been associated with increased toxin production, greater sporulation capacity, and higher rates of recurrence and mortality[8]. However, the distribution and actual impact of these strains in Latin America remain poorly characterized, which limits the direct extrapolation of data from North America and Europe.

Clinical manifestations and disease severity

C. difficile infection presents with a broad clinical spectrum ranging from mild watery diarrhea to severe conditions such as pseudomembranous colitis, fulminant colitis, and toxic megacolon[9]. Diarrhea remains the cardinal symptom; however, the severity of the clinical presentation depends on multiple factors, including bacterial load, toxin production, the host’s immune status, and the presence of comorbidities[10].

In mild to moderate cases, patients present with diarrhea, abdominal pain, and nonspecific systemic symptoms[11]. In contrast, severe forms are characterized by marked leukocytosis, compromised general condition, fluid and electrolyte imbalances, and a high risk of life-threatening complication[11]. In fulminant cases, transmural inflammation of the colon can lead to paralytic ileus and cessation of diarrhea, which delays diagnosis and worsens the prognosis[12].

Available evidence confirms that advanced age, recent antibiotic exposure, and prolonged hospitalization are consistently associated with greater disease severity and a higher probability of recurrence, establishing CDI as a condition of particular relevance in high-risk hospital populations[12]. The clinical manifestations of CDI can be grouped into distinct clinical phenotypes with different prognostic implications (Table 1).

Diagnostic advances and limitations of available tests

The diagnosis of CDI is based on the integration of clinical criteria with laboratory tests aimed at detecting toxigenic strains or their toxins in stool samples[13]. Available evidence demonstrates that no single diagnostic test offers optimal performance in all clinical scenarios, which has driven the use of combined diagnostic algorithms[14].

Molecular tests, such as polymerase chain reaction (PCR) and loop-mediated isothermal amplification (LAMP) techniques, have demonstrated high sensitivity and rapid diagnostic performance; however, their main limitation lies in their inability to distinguish between colonization and active infection, which can lead to overdiagnosis in asymptomatic patients[15]. The main tests used for the diagnosis of CDI have advantages and limitations that determine their clinical utility (Table 2).

On the other hand, toxin detection assays using enzyme-linked immunosorbent assay (ELISA) offer greater clinical specificity, although with limited sensitivity, which increases the risk of false negatives. The cytotoxicity assay continues to be considered a gold standard, but its technical complexity and the time required to obtain results have limited its use in daily clinical practice[16].

Glutamate dehydrogenase (GDH) is a constitutive enzyme produced by all strains of C. difficile, both toxigenic and non-toxigenic; therefore, its detection is used as an initial screening test due to its high sensitivity, although with limited clinical specificity[17].

Therapeutic evidence and emerging strategies

Historically, metronidazole was considered the first-line treatment for mild to moderate cases of C. difficile infection. However, accumulating evidence has shown that it is less effective in real-world clinical settings, particularly when compared with oral vancomycin, with lower rates of clinical resolution and a higher risk of recurrence. This finding has been consistent across observational studies and clinical trials, especially in patients with moderate to severe disease[18].

Additionally, an increase in the minimum inhibitory concentration (MIC) of metronidazole has been documented in some strains, suggesting a progressive decline in bacterial susceptibility, although the mechanisms of resistance are not fully defined. This phenomenon, coupled with its lower clinical efficacy, has contributed to the displacement of metronidazole as the preferred therapeutic option[19]. In this context, the updated guidelines from the Infectious Diseases Society of America and the Society for Healthcare Epidemiology of America (2021) recommend oral vancomycin or fidaxomicin as first-line treatment, relegating metronidazole to specific situations where no other alternatives are available[19].

The guidelines from the Infectious Diseases Society of America and the Society for Healthcare Epidemiology of America (2021) recommend tapering and/or pulsing regimens for vancomycin, which have been shown to reduce the likelihood of recurrence by enabling more effective eradication of residual spores. These regimens consist of progressively spaced administration of the antibiotic following a standard course, promoting the recovery of the gut microbiota. A common regimen includes 125 mg every 6 hours for 10–14 days, followed by a gradual dose reduction and administration every other day or every 2–3 days for several weeks[20].

Fidaxomicin has emerged as a relevant therapeutic alternative, especially in patients at high risk of recurrence[21]. Several studies have documented significantly lower recurrence rates compared to vancomycin, which is attributed to its lesser impact on the gut microbiota. In cases of multiple recurrences, fecal microbiota transplantation (FMT) has established itself as one of the most effective therapeutic strategies. Its rationale lies in the restoration of gut microbial diversity, which is altered following antibiotic exposure. From a methodological standpoint, FMT involves rigorous donor selection, clinical and microbiological screening, as well as standardized stool processing; the stool can be administered via colonoscopy, enema, nasogastric tube, or oral capsules[21].

Accumulated evidence, including clinical trials and real-world studies, has reported resolution rates exceeding 80–90% in recurrent infections, establishing it as a standard-of-care option in this setting. However, challenges remain regarding protocol standardization, long-term safety, and regulatory oversight[22].

In this context, regulatory-approved microbiota-based therapies have emerged, such as Rebyota (a rectal suspension of fecal microbiota) and Vowst (an oral preparation of purified spores), both approved by the Food and Drug Administration for the prevention of C. difficile infection recurrence. These therapies represent an evolution toward standardized products with controlled safety profiles and regulated manufacturing processes, overcoming some of the limitations of conventional FMT[23]. Recent data from Europe support the use of these strategies, highlighting a significant reduction in recurrence and adequate tolerability, which reinforces their emerging role within the current therapeutic arsenal[23].

Other innovative strategies include the use of monoclonal antibodies, such as bezlotoxumab, directed against toxin B, which have been shown to significantly reduce recurrence rates when used as adjuvant therapy. Furthermore, therapies aimed at preserving the gut microbiota during antibiotic use represent a promising line of research in the primary prevention of CDI[24].

Colonization, risk factors, and prevention strategies

Asymptomatic colonization by C. difficile is a common phenomenon, particularly in hospital settings and among at-risk populations. It is estimated that 3–8% of healthy adults may be carriers, a figure that can rise to 20–50% among hospitalized patients or residents of long-term care facilities. Although these individuals do not exhibit clinical symptoms, they act as potential reservoirs for the transmission of the microorganism[25].

The progression from colonization to clinical infection depends on multiple host and environmental factors. Key risk factors include prior exposure to broad-spectrum antibiotics, advanced age, prolonged hospitalization, the presence of comorbidities, immunosuppression, and the use of proton pump inhibitors. These factors contribute to alterations in the gut microbiota and reduce resistance to colonization[26].

From a pathophysiological perspective, intestinal dysbiosis resulting from the use of antimicrobials allows C. difficile spores to germinate and toxin A- and B-producing vegetative forms to proliferate. These toxins induce epithelial damage, disruption of tight junctions, and a local inflammatory response that leads to the development of clinical symptoms[26].

Prevention strategies are based on interrupting transmission and preserving the intestinal microbiota. Key measures include the rational use of antimicrobials through optimization programs (PROA in Spanish), hand hygiene with soap and water, isolation of symptomatic patients, and environmental disinfection with sporicidal agents. Additionally, the implementation of appropriate diagnostic protocols helps prevent overdiagnosis and unnecessary treatment.

Epidemiological implications and the burden on health care systems

Available epidemiological evidence confirms a sustained increase in the incidence and prevalence of C. difficile infection (CDI) globally, particularly in hospital and long-term care settings[27]. This increase is associated not only with higher morbidity and mortality but also with a considerable economic impact, resulting from prolonged hospital stays, frequent readmissions, and costs associated with managing recurrences and serious complications[27].

In Europe, C. difficile infection continues to represent a significant cause of healthcare-associated infection. In Spain, multicenter studies have documented variable incidence rates across hospitals, attributed to differences in diagnostic algorithms, microbiological surveillance, and antimicrobial use policies[27]. Recent data indicate that the infection predominantly affects elderly, hospitalized patients with prior exposure to antibiotics or proton pump inhibitors. Furthermore, an increase in the identification of community-acquired cases and a significant persistence of recurrences have been reported, which keeps this infection a current clinical and epidemiological problem in Spain[28].

In the Spanish context, the gradual implementation of antimicrobial optimization programs (PROA) and infection control measures has helped strengthen the preventive approach, although diagnostic challenges and interhospital heterogeneity persist.

Complementarily, the European Society of Clinical Microbiology and Infectious Diseases has published updated recommendations that agree on prioritizing fidaxomicin and vancomycin over metronidazole as initial treatment. However, the ESCMID guidelines take a more conservative approach to the use of fecal microbiota transplantation, reserving it primarily for carefully selected cases of multiple recurrences, while also emphasizing the need for individualized assessment based on clinical severity, therapeutic availability, and the local epidemiological context[29].

In contrast, the recommendations from the Infectious Diseases Society of America and the Society for Healthcare Epidemiology of America advocate for earlier incorporation of therapies aimed at preventing recurrences, including monoclonal antibodies and microbiota-based products.

These findings underscore the need to implement comprehensive strategies for prevention, prudent use of antimicrobials, and infection control, with the aim of reducing both the clinical burden and the economic impact of C. difficile infection on healthcare systems[30]. Various factors have been associated with both the development of the first episode and the recurrence of C. difficile infection, with notable differences between the two scenarios (Table 3).