Bacterial Structures and Their Functions

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  • 0:05 External Structures
  • 0:48 Pili
  • 2:11 Fimbriae
  • 3:16 Flagella
  • 5:36 Glycocalyx
  • 7:03 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

A bacterial cell is not smooth like a balloon. Bacteria can be covered with a wide range of structures like pili and capsules that give each species of bacteria different abilities. In this lesson, you will learn about several of these key external structures of bacteria.

External Structures

Have you ever wondered how you 'catch' pneumonia? It might surprise you to learn that it is probably more accurate to say that the pneumonia 'catches' you. Every time you breathe in and out, you bring in, then exhale, many thousands of bacteria. Some species of bacteria probably entered and exited in approximately the same amount. There are other bacterial species that you inhaled that were able to adhere tightly to your respiratory surfaces and remain behind to cause illnesses like pneumonia. How is this possible? The answer lies in the external structures those bacteria possess. Let's take a look at a few of the major structures and examine their impact on the life of bacteria.


The first external structure is the pilus (plural: pili). A pilus is a thin, rigid fiber made of protein that protrudes from the cell surface. The primary function of pili are to attach a bacterial cell to specific surfaces or to other cells. But how does the pilus know exactly what surface to attach to? Along the length of the pilus are adhesin proteins. The word 'adhesin' should remind you of the word 'adhesive'! These molecules aid in the attachment of the pilus and are specific to the target surface.

Bordetella pertussis is the bacteria that causes whooping cough. Bordetella has pili coated with adhesins that can identify the mucosal surface of the respiratory tract and will stick to only that surface, allowing it to adhere to and infect those cells.

Pili can also aid in attachment between bacterial cells. Some bacteria are able to produce conjugation pili that allow for the transfer of DNA from one bacterial cell to another. Bacteria have evolved the process of conjugation as a way to increase genetic variability. The cell with the conjugation pilus attaches to another cell, connecting the cytoplasm of each cell and allowing molecules of DNA to pass through the hollow pilus.


Closely related to pili are structures called fimbriae (singular: fimbria). These are short, filamentous structures, present in large numbers that aid in cell adherence to surfaces. A bacterium that has fimbriae is usually covered with short hair-like fibers. In contrast, pili are much longer, and a cell usually only has one or two pili. Pathogenic bacteria can have adhesins on the fimbriae that allow them to attach to the target tissues of their host.

To draw an analogy to the human body, most people have hair on their body that is similar to fimbriae - many individual strands that are relatively short compared to the total length of the body. In contrast, pili are more similar to your arms - there are only two and they are usually longer than any of your hair.

One important note on fimbriae: there are some microbiologists that use the terms pili and fimbriae interchangeably. Both structures are similar and perform similar functions, blurring the distinction between the two.


A structure that looks similar to a pilus but has a different function is a flagellum. Flagella (singular: flagellum) are long, thin, whip-like appendages attached to a bacterial cell that allow for bacterial movement (also known as motility). Different bacterial species have different flagella arrangements, from a single flagellum to one on each end to tufts of many.

The long, filament portion of the flagellum is composed of a protein called flagellin. These proteins form long chains that give the flagellum a helical shape. At the cell membrane, the flagellum gets wider and attaches to a ring of proteins known as the flagellar motor. The motor is embedded in the membrane, anchoring the flagellum.

It is no accident that the base of the flagellum is called a motor. Just think of the last time you used a hand mixer for baking. Electricity powers a motor that spins the mixing beaters. The flagellum is very similar! The flagellar motor is plugged into the cell membrane, where it can be powered by capturing the energy of chemical gradients. This turns the motor, and that spin is translated to the rest of the flagellum, which results in propelling the cell. This tiny motor is able to generate up to 1,500 revolutions per minute!

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