*3.3. Colonization in Infants*

In fecal samples from newborns and infants, the presence of *C. difficile* rates around 70% [42].

Environment, hospitalization, and prematurity have been associated with the colonization of the bacterium. Some studies showed that vaginal delivery and maternal genital tract are not a risk for newborn acquisition. Vaginal swabs of mothers just before delivery

have been examined, and they were all negative for *C. difficile* by culture, but their infants were positive.

Penders et al. [43] associated increased colonization with *C. difficile* with birth by cesarean delivery. The newborns had lower numbers of Bifidobacterium and Bacteroidetes and were more often colonized with *C. difficile* compared with vaginal delivery infants.

Rousseau et al. observed that some microbial taxa such as *Bifidobacterium longum* were protective and negatively correlated to *C. difficile* colonization. In contrast, *Ruminococcus gravus* and *Klebsiella pneumoniae* were susceptible microbial taxa [44].

Plus, the feeding method used has been reported as a factor that can influence *C. difficile* colonization. Breast milk is a more protective factor than formula milk. That can be explained, as some studies showed, by an increase in the acidity in the intestine contents, which may facilitate sporulation and reduce vegetative forms [45].

All of this evidence only reiterates how the gut microbiota begins to form and perform its functions from the earliest days of life, and again, there are multiple factors involved in the development and regulation of the microbiome.

Proteins of human milk play an inhibitory role in toxin TcdA [46].

Moreover, secretory IgA has shown neutralizing activity against toxin A [47].

On the other hand, formula-fed infants are often colonized with *Escherichia coli*, *C. difficile*, Bacteroides, and Lactobacilli compared with breast-fed infants [43].

What is important to underline is that toxigenic and non-toxigenic *C. difficile* has been isolated in infants with a higher percentage of the non-toxigenic one. However, even with the presence of toxigenic strains, colonization seems not to be associated with *CDI*.

So, the infant gut appears to be resistant to CD toxins. That could be explained by the absence of toxin receptors, poorly developed cellular signaling pathways because of the immature gut mucosa, or the presence of protective factors in the infantile gut that remains at the moment unknown [47].

As we have just seen, this fundamental interaction plays an essential and delicate role from an early age.
