Bacterial Cell Surface Structures and Appendages (Flagella, Fimbriae and Pili)


Fimbria vs pilus

Cell Surface Appendages of Bacteria
(Flagella vs Fimbriae vs Pili of Bacteria)

What are cell surface appendages?

Cell surface appendages (aka filamentous appendages) are proteinaceous tubular or fibrous structures found on the surface of bacterial cells. They extend from the surface of the bacterial cell wall and can have many functions such as locomotion, attachment, adhesion and assisting in genetic exchange.

What are the three types of cell surface appendages of bacteria?

The THREE types of cell surface appendages are present on bacteria. The classification is based on the relative length of the appendages, composition and function. The three cell surface appendages of bacteria are

(1).  Flagella

(2).  Fimbriae

(3).  Pili

(1). Flagella

Bacterial Flagella definition: Bacterial flagella are long whip-like filamentous structures present in some bacteria. The most important function of flagella is to assist in locomotion. Flagella can also act as a sensory organ to detect temperature and the presence of certain chemicals. Even though the flagella are present in prokaryotes and eukaryotes, both are entirely different in their structure, formation and mechanism of propulsion.

What are the characteristics of bacterial flagella?

Ø  Flagella are long whip-like filamentous structures.

Ø  Flagella are many times longer and thicker than Fimbriae and Pili.

Ø  Approximate length of flagella varies from 15 to 20 µm.

Ø  Flagella are present in both Gram-positive and Gram-negative bacteria.

Ø  Examples of bacteria having Flagella: Escherichia coli, Helicobacter pylori, Salmonella typhimurium.

Ø  Flagella usually show a distinct pattern of distribution. Distribution may be polar, lateral or throughout the surface.

Ø  Flagella are made up of a protein called flagellin.

Ø  Flagella are helical and non-straight in nature.

Ø  Flagella are hollow tubular structure with lumen.

Ø  Flagella are interior in their origin, originated from the cytoplasmic membrane.

Ø  The formation of flagella is controlled by genes present in the nucleoid region.

Ø  The main function of flagella is locomotion.

Ø  Some flagella also act as the sensory organ to sense temperature and chemicals.

Ø  Flagella do not have any role in surface attachment.

Ø  Bacterial flagella are always associated with a complex motor system at the base for propulsion.

Difference between Fimbriae and Flagella

(2). Fimbriae

Fimbriae definition: Fimbriae are bristle-like short fibres occur on the surface of some bacteria. Fimbriae enable the bacterial cell to stick to the surface of host cells. They also help in the formation of pellicles or biofilms. (Pellicle: thin sheet of cells on the surface of a liquid).

What are the characteristics of bacterial Fimbriae?

Ø  Fimbriae are bristle-like short fibres.

Ø  Fimbriae are present on both Gram-positive and Gram-negative bacteria.

Ø  Examples of bacteria having fimbriae: Salmonella typhimurium, Shigella dysenteriae.

Ø  Fimbriae are made up of fimbrillin protein.

Ø  Fimbriae are comparatively shorter in length than pili and flagella.

Ø  Approximate length of fimbriae is 0.03 to 0.14 µm.

Ø  Fimbriae are evenly distributed on the entire surface of the cell.

Ø  The total number of fimbriae per cell may range from 200 – 400.

Ø  Similar to Flagella, the formation of fimbriae is controlled by the genes present in the nucleoid region.

Ø  Fimbriae are solid structures without a lumen.

Ø  Fimbriae are comparatively thinner and fragile than flagella and pili.

Ø  The main function of fimbriae is surface attachment.

Ø  Fimbriae do not have any role in bacterial mobility and conjugation.

(3). Pili

Pili definition: Pili are long hair like tubular micro-fibres like structures present on the surface of some Gram-negative bacteria. They are comparatively shorter than flagella and longer than fimbriae. There are many classes of pili based on their structure and function.




What are the characteristics of bacterial Pili?

Ø  Pili are long hair like tubular microfibres like structures.

Ø  Pili are present only on some Gram negative bacteria.

Ø  Examples of bacteria having pili: Escherichia coli, Neisseria gonorrhoeae, Pseudomonas.

Ø  Pili are made up of pilin protein.

Ø  Pili are comparatively longer than fimbriae and shorter than flagella

Ø  Approximate length of pili is 0.5 – 2 µm.

Ø  Pili are randomly distributed on surface of the cell.

Ø  Number of pili per cell is very less. It is usually 1 – 10 per cells.

What is sex pilius

Ø  The formation of pili is controlled by the gene present in plasmids.

Ø  Pili are more rigid than fimbriae.

Ø  The main function of pili is gene transfer and attachment.

Ø  A special type of pili called sex pili facilitates bacterial gene transfer by conjugation.

Ø  Some pili such as Type IV pili show the twitching type of movements.

Ø  The majority of pili can act as the receptors for some virus.


Review questions…

(1).  What are cell surface appendages?
(2).  What are the three types of cell surface appendages of bacteria?
(3).  What are the characteristics of bacterial flagella?
(4).  What are the functions of bacterial flagella?
(5).  What are the characteristics of bacterial fimbriae?
(6).  What are the functions of bacterial fimbriae?
(7).  What are the characteristics of bacterial Pili?
(8).  What are the functions of bacterial pili?
(9).  What are sex pili? What is its function?
(10). What are the difference between fimbriae and flagella?
(11).  What are the difference between fimbriae and pili?
(12).  What are the difference between pili and flagella?


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