What are the Pilis and what is their function

The Pili are Filamentous structures of protein nature that are found in the surface of bacteria. These structures, which can vary in length from micrometer fractions to more than 20 micrometers, have a diameter that ranges between 2 and 11 nanometers. Composed of protein subunits called pylinas or fimbrias, the PILI are typically organized in a helical arrangement.

The assembly of the PILI is carried out through complex protein machinery located in the outer membrane or covered by both membranes in gram -negative bacteria. Although there are differences in the assembly mechanism between bacteria gram -negative and gram -positivethe general principle is that in the gram -negative, the subunits are excreted and assembled near the Pilus under construction. In contrast, gram -positive have a different assembly mechanism, where pylinas can be recycled or destroyed when the Pilus is disassembled.

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PILI in gram -negative bacteria are divided into four main groups, each with specific functions.

• PILI TYPE I: This guy is the most common and is found in strains of E. coli and other Bacteria of the Enterobacteriaceae family. They contain adhesins at their ends, which facilitates adhesion to host cells and contributes to infections such as cystitis. Type I Pili are also essential for the formation of biofilms on abiotic surfaces.
• PILI Type IV: These Pili are involved in bacterial mobility, allowing bacteria to adhere to surfaces and move through a contraction mechanism. This type of Pili generates a spasmodic movement, which facilitates migration on solid surfaces and the search for optimal areas for adhesion.
• PILI Type V: Exclusive of gram -negative bacteria, these PILI are virulence factors that contribute to the adhesion and formation of biofilms. They are divided into older Pili (0.3 and 1.6 µm) and minors (80 and 120 nm).
• Pili Curli: Composed of amyloid proteins called curlinas, these Pili also participate in cellular adhesion and biofiller formation, being of interest to biomedical applications.

PILI in gram -positive bacteria are less varied and are classified into two groups: Short and thin canesand the longer and more flexible pili. The former are found in species of Streptococcuswhile the lengths are typical of Corynebacterium and pathogenic streptococci. The biosynthesis of these Pili implies enzymes called cracks, which allow their adherence to the cell surface. Structures similar to PILI have been identified in gram -positive bacteria by electronic microscopy, suggesting their role in the adhesion and formation of biofilms. The Long Pili are composed of three covalently linked protein subunits, using specific amino acid motifs that facilitate their processing and connection to the peptidoglycan cell wall.

• Adherence: Pili are fundamental for adhesion to host surfaces and cells. They use adhesins in their tips to overcome electrostatic repulsion between bacteria and host cells, which is vital for colonization. For example, Neisseria gonorrhoeae It adheres specifically to the cervical epithelium through its Pili, which facilitates the infection.
• Motility: Some Pili, such as type IV Pili, allow bacteria to move through a mechanism known as spasm motility. This type of movement helps bacteria to move along solid surfaces and look for optimal areas for colonization, as observed in Pseudomonas aeruginosa.
• Virulence: Pili are virulence factors in many pathogenic strains. Its presence improves the ability of bacteria to join the host tissues, which increases replication rates and interaction with the body. For example, the pathogenic strains of E. coli and Vibrio Cholerae They use PILI to adhere to tissues, facilitating the colonization and development of diseases.
• Exchange of genetic material: The conjugation PILI, known as PILI F, allow the transfer of DNA between bacteria. This process is crucial for bacterial evolution and the spread of features, such as antibiotic resistance.
• Evasion of the immune system: Pili help bacteria avoid being phagocytized by white blood cells. For example, Streptococcus pyogenes use its Pili to resist ingestion by phagocytes, which contributes to their pathogenic capacity.

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