Adhesins

1. Adhesins

Adhesins are specialized proteins or glycoproteins found on the surface of microorganisms, such as bacteria, viruses, and fungi, that facilitate their attachment to host cells. This attachment is a critical step in the pathogenesis of many infectious diseases, as it allows the microorganism to colonize the host, resist mechanical removal, and establish an infection.

Structure and Function

Adhesins are typically located on the surface of the microorganism and can be part of structures such as pili, fimbriae, or the cell wall. They recognize and bind to specific receptors on the host cell surface, which are often glycoproteins or glycolipids. This binding is highly specific and is often compared to a "lock and key" mechanism.

Types of Adhesins

• **Fimbrial Adhesins**: These are located on fimbriae, which are hair-like appendages on the surface of bacteria. An example is the P fimbriae of *Escherichia coli*, which bind to receptors in the urinary tract.
• **Afimbrial Adhesins**: These are not associated with fimbriae and are often part of the bacterial cell wall. An example is the M protein of *Streptococcus pyogenes*.
• **Viral Adhesins**: These are proteins on the surface of viruses that facilitate attachment to host cells. For instance, the hemagglutinin of the influenza virus binds to sialic acid residues on respiratory epithelial cells.

Role in Pathogenesis

Adhesins play a crucial role in the initial stages of infection. By adhering to host tissues, microorganisms can resist flushing mechanisms such as urination, coughing, or the movement of mucus. This adherence is often followed by colonization, invasion, and the establishment of disease.

Examples of Diseases Involving Adhesins

• **Urinary Tract Infections (UTIs)**: Caused by uropathogenic *E. coli* that use P fimbriae to adhere to the urinary tract epithelium.
• **Streptococcal Pharyngitis**: *Streptococcus pyogenes* uses M protein to adhere to the throat epithelium.
• **Influenza**: The influenza virus uses hemagglutinin to attach to respiratory epithelial cells.

Host Defense Mechanisms

The host has evolved several mechanisms to counteract the effects of adhesins. These include the production of secretory IgA antibodies that block adhesin-receptor interactions, the shedding of epithelial cells, and the production of mucus to trap and remove microorganisms.

Also see

• Pathogenesis
• Bacterial pili
• Viral entry
• Host-pathogen interactions