Microbiology, part 12: Prokaryotes - Glycocalyx, Fimbriae & Pili, & Flagella

Hi. I'm Cathy with Level Up RN. In this video, I'm going to continue my coverage of prokaryotic cells. Specifically, I'll be talking about structures found outside the cell wall, including the glycocalyx, fimbriae, pili, and flagella. At the end of the video, I'm going to give you guys a quiz to test your understanding of some of the key points I'll be covering, so definitely stay tuned for that. And if you have our Level Up RN microbiology flashcards, go ahead and pull out your flashcards so you can follow along with me, and pay close attention to the bold red text on the back of the cards because those are the things that you are likely to get tested on.

Outside the cell wall, some but not all prokaryotic cells have a glycocalyx. This is a sugar coat that surrounds the exterior of the cell and allows the cell to adhere to surfaces. The two main types of glycocalyxes include a capsule and a slime layer. A capsule is an organized and firmly attached layer of polysaccharides. Bacteria with capsules have increased pathogenicity, which means they have an increased ability to cause disease. This is because a capsule helps the bacterial cell evade the immune system. For example, a capsule will help protect the bacterium from phagocytosis. A slime layer is an unorganized, loosely attached layer composed of polysaccharides, glycoproteins, and/or glycolipids. Slime layers aid in the formation of biofilms, which are communities of microorganisms that stick together. And biofilms play a significant role in causing infectious diseases.

Next, let's talk about appendages that some bacterial cells have, which include fimbriae, pili, and flagella. Fimbriae are short protein appendages that help the cell attach to surfaces. So they help with adherence. Pili are similar to fimbriae, but they are longer and less numerous. One specific type of pilus to be familiar with is an F pilus, which is also called a sex pilus or conjugation pilus. The F pilus allows for the transfer of DNA from one bacterial cell to another in a process called conjugation. And we will be talking about conjugation in detail in another video.

Now we're going to talk about flagella, which are structures that allow for motility of the cell. Flagella in prokaryotic cells rotate in a propeller-like fashion. This is different than flagella in eukaryotic cells, which move in a whip-like fashion. A flagellum has three main structural elements, including the basal body, the hook, and the filament. The basal body is the motor of the flagella, and it anchors the flagellum in the cell wall and plasma membrane. The hook connects the basal body to the filament, and the filament is the structure that propels the organism. Different bacteria will have different arrangements of flagella. A bacterium without any flagella is called atrichous. So A means without, and trichous means having hair or being haired. So atrichous means without hair. Bacteria that are monotrichous have a flagellum located at one end of the cell. So mono means one. So we have one flagellum. A bacterium that is amphitrichous has a flagellum on each end of the cell. So amphi means both sides. So we have flagella on both sides of the cell. A bacterium that is lophotrichous has a tuft of flagella at one end of the cell. So our cool chicken hint to help you remember this is lophotrichous equals lopsided. So both lophotrichous and lopsided start with lop. And finally, a bacterium that is peritrichous has flagella covering the entire cell. So peri means around or surrounding. So we have flagella all around the cell.

Movement of a bacterium in response to an external stimulus is called taxis. Chemotaxis is movement in response to a chemical stimulus. Phototaxis is movement in response to light. And magnetotaxis is movement in response to a magnetic field. When a bacterium is moving in one direction, this is referred to as a run, which is achieved through counterclockwise rotation of the flagella. When the bacterium needs to change directions, it tumbles, which is achieved through clockwise rotation of the flagella. So when a bacterium is moving towards a chemical attractant or other stimuli, the lengths of runs will increase, and the lengths of tumbles will decrease. However, when there is no stimulus present, the lengths of the runs and the lengths of the tumbles will be roughly equal.

All right. It's quiz time, and I have five questions for you. Question number one, what do you call an organized and firmly attached layer of polysaccharides on the outside of a bacterial cell? The answer is, a capsule. Number two, what kind of pilus allows for the transfer of DNA from one bacterium to another? The answer is, an F pilus, which is also referred to as a sex pilus or conjugation pilus. Question number three, what part of the flagellum connects the basal body to the filament? The answer is, the hook. Number four, what arrangement of flagella is characterized by a flagellum at each end of the cell? The answer is, amphitrichous. Number five, is rotation of the flagellum in a clockwise direction associated with a run or tumble? The answer is, a tumble.

All right. I hope you did great with that quiz, and I hope you found this video to be helpful. Take care, and thank you so much for watching.

[BLOOPERS]

That is composed of polysaccharides. And [magnetostaxis?]-- I can't say it. Magnetotaxis, not staxis. I'm not sure how I just said that. My goodness.