Pili are slender, hair-like appendages extending from the surface of many bacteria that are composed of polymerised pilin proteins. They are distinct from flagella and enable attachment, genetic exchange, and specialized movement.
Structure and Functions
Most pili are formed by the polymerisation of a major pilin subunit into a helical fibre that is anchored in the cell envelope. In Gram-negative bacteria, thousands of short pili known as fimbriae mediate adhesion to host tissues and abiotic surfaces via tip adhesins. Type I fimbriae help enteric bacteria bind to intestinal mucosa, while P-pili in uropathogenic Escherichia coli recognise glycoprotein receptors in the urinary tract. Type IV pili are longer, flexible filaments that can extend and retract through the action of ATPases, enabling twitching motility and social gliding. They also serve as receptors for certain bacteriophages and mediate natural competence for DNA uptake. Conjugative pili, such as the F pilus encoded by fertility plasmids, form a hollow channel that facilitates the transfer of DNA between bacterial cells during conjugation. In Gram-positive bacteria, sortase enzymes assemble pili by covalently linking pilin subunits to the peptidoglycan, providing adherence to host tissues. Through these varied architectures, pili contribute to colonisation, biofilm formation, host specificity, and horizontal gene transfer.
Examples and Biological Roles
Conjugative F pili are essential for the spread of plasmids carrying antibiotic resistance genes among enteric bacteria. Uropathogenic E. coli use P pili and type 1 fimbriae to establish urinary tract infections by binding to epithelial receptors. Neisseria gonorrhoeae and Neisseria meningitidis express Type IV pili that undergo antigenic variation to evade immune responses and promote adherence to mucosal surfaces. Pseudomonas aeruginosa relies on Type IV pili for twitching motility and biofilm development on medical devices. Certain Gram-positive pathogens, including Corynebacterium diphtheriae and Streptococcus pneumoniae, produce sortase-assembled pili that facilitate colonisation of airway epithelia. Beyond pathogenesis, pili also play roles in microbial ecology; cyanobacteria use pili to maintain buoyancy and access light, and some bacteria harvest electrons from surfaces via conductive pili. These examples illustrate how pilus-mediated interactions shape microbial behaviour and evolution.
Pili are versatile surface structures that enable bacteria to attach to hosts, exchange genetic material and move across surfaces. Their structural diversity and dynamic properties make them key determinants of colonisation, biofilm formation and virulence. Understanding pilus biology informs strategies to prevent infection and limit the spread of resistance genes.
Related Terms: Fimbriae, Conjugation, Type IV pilus, Adhesin, Horizontal gene transfer