Figure 2.

Cartoon of epithelial cell/microorganisms dynamic interactions. Bacterial recognition by TLRs expressed by epithelial cells leads to activation of local defense responses in the epithelial tissues that become colonized. TLR-dependent induction of anti-microbial substances and inflammatory mediators contribute to bacterial clearance by controlling organism survival and by triggering host local and systemic immune responses, respectively. While such processes are pivotal against pathogens, they are not desirable against the local commensal microflora. In a Yin–Yang balance of epithelial tissue homeostasis and defense/inflammatory responses, regulation of TLR signaling is crucial for inducing appropriate cell responses to microorganisms of different nature.