**2.** *Clostridium difficile*

*Clostridium difficile* is a spore-forming Gram-positive bacterium, which can cause an infection whose symptoms range from asymptomatic colonization to fearsome complications such as the onset of toxic megacolon [12].

It is this broad spectrum of clinical manifestations that makes this disease so insidious. Detection of *C. difficile* without evidence of clinical signs is termed colonization, while *Clostridium difficile* infection (CDI) is referred to when *C. difficile* is present associated with its characteristic clinical manifestations [13].

CDI is a clinical condition that doctors in different specialties very often struggle with. CDI is one of the most common nosocomial infections [8,14].

By some estimates, CDI affects about 460,000 people a year in the US [15].

These important data help us understand why this disease is increasingly proving to be one of the big public health topics.

The main risk factors for CDI appear to be older age (above 65 years), antibiotic use, and nosocomial exposure [16].

Several considerations can be drawn from these data.

One is purely epidemiological: with the inevitable increase in the geriatric population, these numbers are unfortunately bound to increase.

Another, however, is that all three of the conditions mentioned are acquired.

It is precisely these observations that underlie the involvement of the microbiome in the pathogenesis of this disease.

The onset of *CDI* occurs as a result of the oro-fecal transmission of sufficient numbers of spores of a toxin-producing strain of *C. difficile* within the colon of the host, accompanied by their overgrowth at the expense of normal commensal microorganisms [17].

The occurrence of *Clostridium difficile* infection is thus associated with an alteration in the gut microbiota by antibiotics that are not active against *C. difficile*, which thus cause uncontrolled growth [18].

A great many antibiotics are therefore associated with the occurrence of *CDI*, and those with a higher risk appear to be more commonly used antibiotics including penicillins such as amoxicillin and ampicillin, as well as cephalosporins, clindamycin, and fluoroquinolones [14]. Thus, the main risk factor for *CDI* appears to be exposure to broadspectrum antibiotics, precisely because of the changes they make to the normal human microbiome [19] (Figure 2). Asymptomatic colonization on admission to the hospital is estimated to have a prevalence of 0.6–13% [20].

**Figure 2.** Pathogenesis of *Clostridium difficile* infection (CDI). One of the most accepted theories to explain the onset of CDI is the administration of antibiotics in a healthy *C. difficile carrier* patient. Antibiotic treatment, by generating a dysbiosis in the gut microbiota, causes a loss of its defense function, leading to the onset of CDI.

These data seem to explain such a high incidence of *CDI*, the genesis of which is multifactorial.

Thus, the factors at play are asymptomatic colonization, associated with antibiotic therapy that brings an imbalance to the microbiome, which may then promote the onset of CDI.

This is why CDI is often considered the best example of the symbiotic relationship between the individual and the host and how its imbalance can lead to disastrous consequences.

*C. difficile* is rarely invasive, and its mechanism of intestinal damage appears to be mediated by its potent exotoxins: toxin A and toxin B [21].

Treatment of CDI therefore relies on the use of antibiotics such as fidaxomicin, vancomycin, and metronidazole [22].

Fidaxomicin has proven to be the most selective antibiotic against *C. difficile* [23] and has been shown to be a highly effective treatment against this disease, although more data are needed regarding safety and efficacy in children and adults [24].

Vancomycin appears to be less selective than fidaxomicin against other intestinal bacteria [23] and continues to be the cornerstone treatment of CDI along with fidaxomicin.

Previously, metronidazole was considered along with vancomycin to be the pivotal treatment for CDI, until new guidelines in 2017 considered it to be of lower efficacy than fidaxomicin and vancomycin, which thus became the main antibiotics for the treatment of CDI [25].

Prolonged use of metronidazole is also burdened by important side effects such as neurotoxicity [26].

As antibiotic treatment is burdened with an important risk of relapse, new therapeutic strategies to cope with CDI are under investigation, which we will discuss later [25].

It should also not be forgotten that for very severe patients, those with a "fulminant" form of *C.m difficile* colitis who present with major symptoms such as shock, hypotension, and megacolon, surgical therapy is also considered [27].

Precisely because *CDI* is such a complex disease, it can require such diverse treatments.

However, as we shall see, even when a *CDI* is appropriately treated with the use of antibiotics, the serious problem of recurrence can arise, which is particularly significant for this disease.

The risk of recurrence remains significant, ranging from 12 to 64 percent, with a median of 22 percent [28].

Recurrence is said to occur when, after treatment accompanied by a total disappearance of clinical symptoms, there is a recurrence of CDI within two to eight weeks after discontinuing therapy [27].

Treatment of some of these forms of this recurrent colitis relies on the use of fecal microbiota transplantation (FMT), which is the installation of treated feces collected from a healthy donor into a patient with CDI [29,30].

Regarding FMT, there are some aspects that differ in different centers, such as the most effective dose and preparation time, while some aspects are absolutely shared, such as donor exclusion criteria.

In fact, one of the crucial aspects of this treatment is the careful selection of the donor, as there is a potential risk of pathogens through the FMT procedure.

Thus, patients who carry or are at risk of transmitting infectious diseases, those with gastrointestinal diseases, or who may have recently taken certain medications (antibiotics, immunosuppressants, etc.) that alter the composition of the gut microbiota are excluded [31].

Regarding the safety of this procedure, some data show that adverse effects occurred in 9.2% of patients, including death (3.5%), infection (2.5%), and recurrence of intestinal bowel disease (0.6%) [32].

There are many routes of administration of FMT: oral capsules, procedures involving the upper gastrointestinal tract such as the nasojejunal and nasoduodenal tube, and those of the lower gastrointestinal tract such as colonoscopy or enema.

The lower route of administration appears to be more effective than the upper route, although further studies are needed in this regard [33].

The use of FMT has also been considered for the treatment of fulminant colitis supported by several pieces of evidence, including a retrospective study of 199 patients, in which FMT was associated with reduced mortality and colectomy rates [34].

Meanwhile, evidence on the use of FMT for the treatment of recurrent CDI comes from both randomized trials [35] and meta-analyses [36].

FMT represents one of the latest frontiers of CDI treatment and is an ever-evolving field, in which the role of the microbiome is of paramount importance (Figure 3).

**Figure 3.** Use of fecal microbiota transplantation (FMT). Currently, the use of FMT is indicated for the treatment of recurrent CDI, in which recurrent infections occur due to the persistence of dysbiosis. The aim of this treatment is precisely to intervene by restoring the balance of the gut microbiota.
