II. THE PROKARYOTIC CELL: BACTERIA

B. PROKARYOTIC CELL ANATOMY

4. STRUCTURES LOCATED OUTSIDE THE CELL WALL

c. Fimbriae and Pili

Fundamental Statements for this Learning Object:

1. Fimbriae and pili are thin, protein tubes originating from the cytoplasmic membrane found in virtually all Gram-negative bacteria but not in many Gram-positive bacteria. Pili are typically longer and fewer in number than fimbriae.
2. The short attachment pili or fimbriae are organelles of adhesion allowing bacteria to colonize environmental surfaces or cells and resist flushing.
3. Antibodies made against bacterial pili may block colonization and/or promote opsonization.
4. The long conjugation pilus enables conjugation in Gram-negative bacteria.
5. The pilus has a shaft composed of a protein called pilin with an adhesive tip structure at the end having a shape corresponding to that of specific receptors on a host cell.
6. The same bacterium may switch the adhesive tips of the pili in order to adhere to different types of cells and evade immune defenses.
7. Type IV pili not only allow for attachment but also enable a twitching motility that enables bacteria to “crawl” or “walk” over the surfaces to which they have attached by extending and retracting their type IV pili.

LEARNING OBJECTIVES FOR THIS SECTION


In this section on Prokaryotic Cell Anatomy we are looking at the various cellular parts that make up a bacterium. As mentioned in the introduction to this section, a typical bacterium usually consists of:

We will now look at bacterial pili.


Fimbriae (def) and Pili (def)

 

A. Structure and Composition

Fimbriae and pili are thin, protein tubes originating from the cytoplasmic membrane of many bacteria. Both are able to stick bacteria to surfaces, but pili are typically longer and fewer in number than fimbriae. 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.

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.

 

Proteins associated with pili function as antigens and initiate adaptive immunity. An antigen (def) is defined as a molecular shape that reacts with antibody molecules and with antigen receptors on lymphocytes. We recognize those molecular shapes as foreign or different from our body's molecular shapes because they fit specific antigen receptors on our B-lymphocytes and T-lymphocytes, the cells that carry out adaptive immunity.

The actual portions or fragments of an antigen that react with antibodies and with receptors on B-lymphocytes and T-lymphocytes are called epitopes (def). An epitope is typically a group of 5-15 amino acids with a unique shape that makes up a portion of a protein antigen, or 3-4 sugar residues branching off of a polysaccharide antigen. A single microorganism has many hundreds of different shaped epitopes that our lymphocytes can recognize as foreign and mount an adaptive immune response against.

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 3 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 gonorrhoeae

Click on this link, read the description of Neisseria gonorrhoeae, and be able to match the bacterium with its description on an exam.

 

 

One class of pili, known as type IV pili (def), 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.

 

Movie of twitching motility of Pseudomonas
Courtesy of Dr. Howard C. Berg
from the Roland Institute at Harvard.
Retraction of pili of Pseudomonas used in twitching motility
Courtesy of Dr. Howard C. Berg from the Roland Institute at Harvard.

 

 

 

C. Significance of Fimbriae and Pili in the Initiation of Body Defense

Initiation of Adaptive Immunity

Proteins associated with bacterial fimbriae and pili function as antigens and initiate adaptive immunity. An antigen (def) is defined as a molecular shape that reacts with antibody molecules and with antigen receptors on lymphocytes. We recognize those molecular shapes as foreign or different from our body's molecular shapes because they fit specific antigen receptors on our B-lymphocytes and T-lymphocytes, the cells that carry out adaptive immunity.

The actual portions or fragments of an antigen that react with antibodies and with receptors on B-lymphocytes and T-lymphocytes are called epitopes (def). An epitope is typically a group of 5-15 amino acids with a unique shape that makes up a portion of a protein antigen, or 3-4 sugar residues branching off of a polysaccharide antigen. A single microorganism has many hundreds of different shaped epitopes that our lymphocytes can recognize as foreign and mount an adaptive immune response against.

The body recognizes an antigen as foreign when epitopes of that antigen bind to B-lymphocytes (def) and T-lymphocytes (def) by means of epitope-specific receptor molecules having a shape complementary to that of the epitope. The epitope receptor on the surface of a B-lymphocyte is called a B-cell receptor and is actually an antibody molecule. The receptor on a T-lymphocyte is called a T-cell receptor (TCR).

There are two major branches of the adaptive immune responses: humoral immunity and cell-mediated immunity.

1. Humoral immunity (def): Humoral immunity involves the production of antibody molecules in response to an antigen (def) and is mediated by B-lymphocytes. Through a variety of mechanisms, these antibodies are able to remove or neutralize microorganisms and their toxins after binding to their epitopes. For example, antibodies made against the adhesive tips of bacterial pili can prevent bacteria from adhering to and colonizing host cells (see Fig. 7A and Fig. 7B). Antibodies made against pili antigens can also stick bacteria to phagocytes, a process called opsonization (def).

 

2. Cell-mediated immunity (def): Cell-mediated immunity involves the production of cytotoxic T-lymphocytes, activated macrophages, activated NK cells, and cytokines in response to an antigen (def) and is mediated by T-lymphocytes. These defense cells help to remove infected cells and cancer cells displaying foreign epitopes.

Adaptive immunity will be discussed in greater detail in Unit 6.

 

 

 

 

Medscape article on infections associated with organisms mentioned in this Learning Object. Registration to access this website is free.

 


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