**Relationship between Gut Microbiota and** *Clostridium difficile* **Infection**

**Healthy carriers**—Up to 17.5% of adults are healthy carriers of *Clostridium difficile*, who do not develop the disease protected by commensal bacterial flora.

**Colonization in infants**—In fecal samples from newborns and infants, the presence of *Clostridium difficile* rates around 70%; the infant gut appears to be resistant to *Clostridium difficile* toxins.

**Disruption of the microbiome and CDI risk factors**—Alterations in the microbiota can lead to the onset of CDI. Risk factors that can lead to this include antibiotic use, age, PPI use, and presence of IBD, while having been affected by CDI is a serious risk factor for recurrence.

**Fecal microbiota transplantation in CDI**—Used for some relapsed forms, precisely because this therapy aims to resolve the dysbiosis that led to the onset of the infection.

*C. difficile* grows in the digestive tract of infants, where it does not develop an infection but rather a colonization, until during growth when new microbial species take over, providing protection against *C*. *difficile* itself, although any new imbalance may again stimulate its growth [37].

The composition of the gut microbiota then, after a rapid change in early life, remains nearly stable throughout adulthood, until undergoing new changes with advancing age.

Colonization resistance refers to the peculiar ability on the part of the gut microbiome to resist colonization against pathogenic organisms, including *C*. *difficile*.

Several mechanisms underlying this resistance to colonization against *C*. *difficile* have been hypothesized, ranging from stimulation of host immune defenses, competition for nutrients, production of a protective physical barrier against the intestinal mucosa to production of inhibitory substances such as secondary bile acids and bacteriocins [38].
