II. THE PROKARYOTIC CELL: BACTERIA
B. PROKARYOTIC CELL STRUCTURE
4. STRUCTURES LOCATED OUTSIDE THE CELL WALL
c. Pili
The overall purpose of this Learning Object is:
1) to learn the chemical makeup and functions associated with bacterial pili;
2) to introduce the relationship between components of bacterial pili and body defenses;
3) to introduce the relationship between bacterial pili and pathogenicity.
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.
Pili (Fimbriae) (def)
A. Structure and Composition
Pili are thin, protein tubes originating from the cytoplasmic membrane. They are found in virtually all gram-negative bacteria but not in many gram-positive bacteria. 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).
There are two basic types of pili:
1) short attachment pili, also known as fimbriae, that are usually quite numerous (see Fig. 2), and
2) long conjugation pili, also called "F" or sex pili (see Fig. 3), that are very few in number.
B. Functions and Significance of Pili to Bacteria Causing Infections
The short attachment pili or fimbriae are organelles of adhesion allowing bacteria to colonize environmental surfaces or cells and resist flushing. . 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 1 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).
Highlighted Bacterium: Neisseria gonorrhoeaeClick on this link, read the description of Neisseria gonorrhoeae, and be able to match the bacterium with its description on an exam.
Scanning electron micrograph E. coli with pili; courtesy of Dennis Kunkel's Microscopy.
Some bacteria can produce a special pilus called a conjugation or sex pilus that enables conjugation. As will be seen in Unit 6, conjugation is the transfer of DNA from a donor or male bacterium with a sex pilus to a recipient or female bacterium to enable genetic recombination. Scanning electron micrograph E. coli with a conjugation pillus; courtesy of Dennis Kunkel's Microscopy.
E-Medicine article on infections associated with organisms mentioned in this Learning Object. Registration to access this website is free.
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