Difference Between Flagella and Pili

Flagella and pili are two different types of cellular protrusions found in bacteria. They perform vital roles on the cell surface of bacteria and are involved in adhesion, motility, identification of chemicals, and transportation of protein and DNA in and out of the bacterial cell.

These structures have several times the cell's capacity for growth, the ability to create and sustain significant stresses, and the ability to quickly form and disassemble in response to environmental signals.

Although both flagella and pili look like thread structures and are protruding from the bacterial surface, they both are different from each other in their structure as well as in functions.

Flagella and Pili. Image created in BioRender.com

Flagella

Flagella (Singular flagellum) are extremely thin, long, and thread-like appendages. They are rigid and have little curved structure. Flagella are normally found in bacilli and spiral-shaped bacteria. Cocci bacteria very rarely have flagella. They are also found in archaea and some eukaryotes.

Structure of flagella

They come out through the cell wall of bacteria and originate from the basal body which is a structure just beneath the cell membrane in the cytoplasm. The thread-like structure that extends from the cell membrane is called axial structure. The axial structure is divided into three parts called hook, filament, and rod.

The hook is a small curved structure and helps in the rotation of the flagellum. The filament is a thick and helical structure made up of 20,000 subunits of a protein called flagellin. The rod is a complex helical structure that penetrates the peptidoglycan layer and the outer membrane and works to reduce friction during the rotation of the flagellum with the cell surface.

The length of flagella is usually from 5-20 μm and they are 10-30 nm in diameter. The flagellum rotates at a speed of 200–300 Hz to help bacteria swim.

Functions of flagella

Flagella performs several functions in bacteria including:

1. They help in the motility of bacteria from one place to another by either rotating clockwise or anti-clockwise.
2. They help in the detection and movement of bacterial cells in response to chemical signals such as pH and nutrients. This type of behavior is called chemotaxis.
3. Flagella are also known to serve in the adhesion process. Flagella helps in bacterial adherence and invasion into host cells. So they are found in pathogens such as Escherichia coli and Pseudomonas aeruginosa.

Types of Flagella

A single bacteria can have not too many flagella. Different types of flagella according to their number and arrangement on the bacterial surface are:

1. Atrichous

Bacteria without any flagellum

2. Monotrichous

Bacteria have a single polar flagellum

3. Lophotrichous

Bacteria have a tuft (collection of hair) of flagellum present only at one pole of their surface.

4. Amphitrichous

Bacteria have a single flagellum at both poles of their cell surface.

5. Peritrichous

Flagella surround the whole bacterial cell.

Video Lesson on Flagella

Pili

Pili (singular pilus) comes from the Latin word which means hair. They are hollow, nonhelical, filamentous appendages found in bacteria. 

They are less rigid and straight in structure. Present in gram-negative bacteria (do not retain the crystal violet stain used in the Gram staining and appear pink under the microscope) and archaea.

Structure of pili

Pili are smaller and thinner compared to flagella. The pili are 0.3–1.0 μm in length and about 7 nm in diameter. They are present all over the cell surface of bacteria. Pili are made up of thousands of subunits of pilin protein.

Functions of pili

Several functions performed by pili in bacteria include:

1. They are involved in the attachment of bacteria to various surfaces, a process called adhesion. They are involved in cell-to-cell and cell-to-host attachments, thus involved in the pathogenicity and colonization of bacteria.
2. Conjugative pili allow the transfer of plasmid DNA between bacteria in the process called conjugation.
3. Pili are responsible for virulence in the pathogenic strains of many bacteria.
4. Some pili are involved in the gliding and twitching movement of bacteria.

Types of pili

There are two main types of pili.

1. Short pili or fimbriae

They are short and many. They are involved in the adhesion process.

2. Long conjugation pili or sex pili

They are longer and thicker. A bacterial cell can have 1-4 sex pili.

Video Lesson on Pili

Difference between flagella and pili

The main difference between flagella and pili are compiled in the table below:

Flagella	Pilli
Thick, long, and thread-like appendage	Thin, hollow, nonhelical, filamentous appendages
Found in bacteria, archaea, and some eukaryotes	Found in bacteria and archaea
Found in gram-positive and gram-negative bacteria	Found in gram-negative bacteria only
5-20 μm in length and 10-30 nm in diameter	0.3–1.0 μm in length and about 7 nm in diameter
Made up of flagellin protein	Made up of pilin protein
Originate from cytoplasm	Originate from the cell wall
Involved in swimming motility of bacteria	Involved in gliding and twitching motility of bacteria
Involved in chemotaxis	Involved in conjugation and adhesion
Types of flagella include atrichous, monotrichous, lophotrichous, amphitrichous, and peritrichous	Two main types of pili include short pili or fimbriae and long conjugation pili or sex pili.
Maybe polar, lateral, or present throughout the cell	Present throughout the cell surface of bacteria
Involved in the growth and survival of bacteria	Involved in new genetic combinations and pathogenicity

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Difference between flagella and pilli

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Some Questions and Answers

1. What are flagella and pili used for?

A. Flagella are involved in motility and chemotaxis, while pili are involved in adhesion and conjugation.

2. Is flagella thicker than pili?

A. Pili are thicker and shorter compared to flagella.

3. What are the two types of pili?

A. Two main types of pili include short pili or fimbriae and long conjugation pili or sex pili.

4. do pili help in motility?

A. Pili are involved in the gliding and twitching movement of bacteria.

5. What are the three parts of the axial structure of the flagellum?

A. Axial structure of flagellum is divided into three parts called hook, filament, and rod.

6. Which structure helps in conjugation in bacteria?

A. Sex pili are involved in the conjugation process.

7. Do eukaryotes have flagella or pili?

A. Some eukaryotes have flagella but pili are not found in any eukaryotes.

8. What is flagella and where is it located?

A. Flagella (Singular flagellum) are extremely thin, long, and thread-like appendages. They are found in bacteria, archaea, and some eukaryotes.

9. What are flagella made of?

A. Flagella are made up of a protein called flagellin.

10. which structure is involved in the pathogenicity of bacteria?

A. Pili are involved in the pathogenicity of bacteria.