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.
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!
Bacteria use the motility provided by flagella during chemotaxis, which is movement in response to chemical gradients. Bacteria can sense nutrients and toxins in the environment. When the swimming bacterium senses the concentration of nutrients increasing, it will swim towards it. If the concentration decreases, the bacterium can stop, spin in place, and set off in a new direction. This is often repeated until the cell is headed in the right direction, straight towards the nutrients.
Chemotaxis using flagella can also be used to avoid toxic substances. Sometimes, simply swimming away from a threat is enough to save the bacterium's life. But what about bacteria that can't swim? Or what if the threat can also move, like cells of the immune system? Some species of bacteria solve this problem by putting on a coat of armor.
A glycocalyx is a layer of polysaccharides, with or without proteins, that coats the outer surface of some bacterial cells. This layer gives the bacteria a moist, sticky appearance. A thick glycocalyx bound to the cell surface is called a capsule. If the glycocalyx layer is loose and more spread-out, it is called a slime layer.
In either case, the main job of the glycocalyx is protection. The thickness and moisture content buffer the cell from its environment, protecting it from damage like drying out. Some pathogenic bacteria, like Bacillus anthracis, the causative agent of Anthrax, are able to thwart the immune system with their capsule. White blood cells and macrophages fight infections by engulfing bacterial cells. The Bacillus anthracis capsule makes it hard for white blood cells to engulf it. The current theory is that the glycocalyx and white blood cells are both strongly negatively charged, creating a repulsion between the two cells.
This sticky glycocalyx can also aid in bacterial attachment. When cells of Streptococcus mutans come into contact, the polysaccharides found in the capsule can become entangled, resulting in the accumulation of cells on a surface. Layers of Streptococcus bacteria on your teeth are a major source of tooth decay.
Let's take a minute and look back at these external bacterial structures. First we looked at pili. These long, thin tubes act as attachment points between bacteria and environmental surfaces like host tissue. Some pili target other bacteria, connecting the cytoplasm of two cells for the purpose of transferring DNA in a process called conjugation. In the same vein as pili are fimbriae. These structures are shorter and more numerous than pili but perform the similar function of attachment.
Adhesins on the surfaces of pili and fimbriae are able to identify target host tissues, ensuring the bacteria attach to the correct substrate.
A flagellum is a long, whip-like filament attached to a bacterial cell used for locomotion. These structures spin, propelling the bacteria towards nutrients or away from toxins during the process of chemotaxis.
For defense, some bacteria have a glycocalyx. This polysaccharide layer can be a tight, firm capsule or a loose, diffuse slime layer. The glycocalyx can aid in attachment to surfaces or can be used to prevent phagocytosis by white blood cells during the host's immune response to an infection.
After finishing this video, you should be able to:
- Identify the external structures found on bacteria
- Define pili, fimbriae, adhesins, flagellum and glycocalyx
- Describe chemotaxis and bacterial responses to our immune system