II. THE PROKARYOTIC CELL: BACTERIA
B. PROKARYOTIC CELL STRUCTURE
4. STRUCTURES LOCATED OUTSIDE THE CELL WALL
c. Fimbriae and Pili
The overall purpose of this Learning Object is:
1) to learn the chemical makeup and functions associated with bacterial fimbriae and pili;
2) to introduce the relationship between bacterial fimbriae and pili and bacterial pathogenicity.
LEARNING OBJECTIVES FOR THIS SECTION
In this section on Prokaryotic Cell
Structure we are looking at the various organelles or structures that make up
a bacterium. As mentioned in the introduction to this section, a typical bacterium
usually consists of:
Structures located outside the cell wall of bacteria include the glycocalyx (capsule), flagella, and pili. We will now look at bacterial pili.
Fimbriae and Pili (def)
A. Structure and Composition
Fimbriae and pili are thin, protein tubes originating from the cytoplasmic membrane of many bacteria. They are found in virtually all gram-negative bacteria but not in many gram-positive bacteria. The fimbriae and pili have a shaft composed of a protein called pilin. At the end of the shaft is the adhesive tip structure having a shape corresponding to that of specific glycoprotein or glycolipid receptors on a host cell (see Fig. 1).
There are two basic types of pili:
1. Short attachment pili, also known as fimbriae, are usually short and quite numerous (see Fig. 2) and enable bacteria to colonize environmental surfaces or cells and resist flushing.
- Scanning electron micrograph E. coli with pili; courtesy of Dennis Kunkel's Microscopy.
- Electron micrograph of Salmonella showing both flagella and pili
from the Wiki Biodiversityserene.
2. Long conjugation pili, also called "F" or sex pili (see Fig. 3), that are longer and very few in number. The conjugation pilus enables conjugation. As will be seen later in this unit, conjugation is the transfer of DNA from one bacterium to another by cell-to-cell contact. In gram-negative bacteria it is typically the transfer of DNA from a donor or male bacterium with a sex pilus to a recipient or female bacterium to enable genetic recombination.
B. Significance of Pili to Bacterial Pathogenicity
The short attachment pili or fimbriae are organelles of adhesion allowing bacteria to colonize environmental surfaces or cells and resist flushing. The pilus has a shaft composed of a protein called pilin. At the end of the shaft is the adhesive tip structure having a shape corresponding to that of specific glycoprotein or glycolipid receptors on a host cell (see Fig. 1).
Because both the bacteria and the host cells have a negative charge, pili may enable the bacteria to bind to host cells without initially having to get close enough to be pushed away by electrostatic repulsion. Once attached to the host cell, the pili can depolymerize and enable adhesions in the bacterial cell wall to make more intimate contact.
Bacteria are constantly losing and reforming pili as they grow in the body and the same bacterium may switch the adhesive tips of the pili in order to adhere to different types of cells and evade immune defenses (see Fig. 4). This will be discussed in detail later in Unit 2 under Bacterial Pathogenesis.
Bacteria that use pili to initially colonize host cells include Neisseria gonorrhoeae (inf), Neisseria meningitidis (inf), uropathogenic strains of Escherichia coli, and Pseudomonas aeruginosa (inf).
One class of pili, known as type IV pili, not only allow for attachment but also enable a twitching motility. They are located at the poles of bacilli and allow for a gliding motility along a solid surface such as a host cell. Extension and retraction of these pili allows the bacterium to drag itself along the solid surface (see Fig. 5). In addition, bacteria can use their type IV pili to "slingshot" the bacterium over a cellular surface. In this case, as the pili contract they are thought to become taut like a stretched rubber band. When an anchoring pilus detaches, the taut pili "slingshot" the bacterium in the opposite direction (see Fig. 6). This motion typically alternates with the twitching motility and enables a more rapid motion and direction change than with the twitching motility because the rapid slingshotting motion reduces the viscosity of the surrounding biofilm.
This enables bacteria with these types of pili within a biofilm to move around a cellular surface and find an optimum area on that cell for attachment and growth once they have initially bound. Bacteria with type IV pili include Pseudomonas aeruginosa, Neisseria gonorrhoeae, Neisseria meningitidis, and Vibrio cholerae.
- Electron micrograph of type IV pili of Neisseria gonorrhoeae from Magdalene So, University of Arizona
You Tube movie showing twitching motility in Pseudomonas due to type IV pili
Courtesy of Dr. Lori Burrows You Tube videos
You Tube movie showing Pseudomonas using type IV pili to "walk" on end following binary fission. Courtesy of Gerard Wong, UCLA Bioengineering, CNSI
E-Medicine article on infections associated with organisms mentioned in this Learning Object. Registration to access this website is free.
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