An adhesin is a molecule on the surface of a microorganism that mediates attachment to host cells, tissues or abiotic surfaces. These proteins, lipoproteins or polysaccharide structures recognise specific receptors and initiate colonization.
Types and mechanisms
Bacterial adhesins include diverse structures such as fimbriae (pili), afimbrial outer membrane proteins and secreted matrix-binding proteins. Fimbrial adhesins are filamentous appendages composed of pilin subunits with a tip protein that binds host receptors; examples include type 1 and P fimbriae in Escherichia coli. Afimbrial adhesins are integral membrane proteins or autotransporters that project limited distances from the cell surface, such as Neisseria Opa and Opc proteins and Yersinia invasin. Gram-positive pathogens express microbial surface components recognizing adhesive matrix molecules (MSCRAMMs) like Staphylococcus aureus protein A and fibronectin-binding proteins, which bind host fibronectin, fibrinogen or collagen. Some viruses display adhesins in their envelope glycoproteins: influenza hemagglutinin binds sialic acid residues, while SARS-CoV-2 spike protein engages ACE2. Parasites also produce adhesins, for example Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1), which mediates cytoadherence of infected erythrocytes to endothelial cells. Adhesins often interact with specific carbohydrate or protein motifs on host cells, triggering signalling pathways and sometimes facilitating internalization.
Biological roles and examples
Adhesins are critical for the initial stages of infection and colonization. Uropathogenic E. coli use P fimbriae to adhere to uroepithelial cells and resist flushing by urine, contributing to urinary tract infections. Enteropathogenic E. coli employ bundle-forming pili and the intimin–Tir interaction to form attaching and effacing lesions in the intestine. Neisseria gonorrhoeae alters its pili and Opa proteins through phase and antigenic variation to evade immune responses while maintaining adherence to mucosal surfaces. In Gram-positive bacteria, MSCRAMMs facilitate biofilm formation and endocarditis by anchoring cells to host matrix proteins and implanted devices. Viral adhesins determine tissue tropism; for example, human rhinovirus binds intercellular adhesion molecule-1 (ICAM-1) on respiratory epithelium. Adhesins are targets for vaccines and therapeutics, as blocking adhesion can prevent colonization without exerting selective pressure for resistance. Agents such as cranberries contain proanthocyanidins that inhibit P-fimbrial binding, and antibodies against adhesins can neutralize infection.
Adhesins illustrate how microbial surface structures interact with host receptors to initiate infection. Understanding their specificity and function aids in developing strategies to prevent colonization, disrupt biofilms and design vaccines.
Related Terms: Fimbriae, Pili, Adhesion, Host receptor, Virulence factor