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Figure 1. Recognition and interaction of F18⁺ E. coli and Gram-positive bacteria with epithelial cells through PRRs. The F18 fimbriae, unique to F18⁺ E. coli, bind to the F18 receptor via the FedF protein. Additionally, the diffusely adhering adhesin binds to an integral N-glycosylated membrane protein. These adhesins enhance bacterial adhesion and promote colonization. LPS, a component of the F18⁺ E. coli outer membrane, is recognized by PRRs. Initially, LPS binds to LPS-binding protein and is then transferred to CD14, which delivers it to the TLR4/MD2 complex. This interaction activates the MyD88-dependent pathway, leading to the release of NF-κB. Activated NF-κB promotes the secretion of pro-inflammatory cytokines and increases intestinal permeability, leading to diarrhea in pigs and enhanced epithelial cell proliferation. In contrast, peptidoglycan from Gram-positive bacteria is recognized by TLR2. Teichoic acid and LTA, unique to Gram-positive bacteria, are also recognized by TLR2. These recognitions activate the MyD88-dependent pathway and stimulate the ERK pathway, resulting in the secretion of anti-inflammatory cytokines and reduced inflammation. Created in BioRender. Son, J. (2025) https://BioRender.com/950e0xm. E. coli: Escherichia coli; PRRs: pattern recognition receptors; LPS: lipopolysaccharide; CD14: cluster of differentiation 14; TLR4: Toll-like receptor 4; MD2: myeloid differentiation factor 2; MyD88: myeloid differentiation primary response 88; NF-κB: nuclear factor kappa B; TLR2: Toll-like receptor 2; LTA: lipoteichoic acid; ERK: extracellular signal-regulated kinase.