So we said that most bacteria have cell walls and then some bacteria have an additional structure outside the cell wall composed of something called glycocalyx and glycocalyx is a sugar coat. It's viscous sticky kind of gelatinous and it comes in two main forms. So it can come as a capsule which is fairly rigid or sort of showing the capsule down here as being fairly well defined and it can also as a slime layer.
So this is showing you the slime layer. It's the same molecule, it's just a much more looser, less well-ordered structure. And just to give a sense of where we are, here was our internal cell, here was the cytoplasmic membrane, the plasma membrane, here was our peptidoglycan cell wall, and then here we are looking at this glycocalyx here.
Capsules and slime layers, I've got a long list of things that they do. One of the main things we think of them as useful for is just the contribution of virulence. And that's partly because they often will help cells with capsules evade phagocytosis. So they actually sort of prevent your immune system from finding these bacteria and digesting them and chewing them up.
They can also help with attachment. And they're also useful to help cells from drying out. One of the things that's really interesting about... slime layers is that they are associated with the formation of biofilms. And biofilms themselves are associated with both virulence and also a reduced sensitivity to antibiotics and detergents.
They really protect cells. So this is just a little example here of a capsule is actually so rigid that it prevents an India ink stain from getting in. So you can see the capsule negatively stains in the presence of India ink, whereas the slime layer isn't nearly as rigid.
So the the stain is able to get into the bacteria. And then the slime layer is looser and then if you have many bacteria present and their slime layer all together form this structure called a biofilm, which we'll be talking about after we talk about viruses, it'll be after the first exam. There's kind of a fun mnemonic that sometimes people use because we do have some pretty serious pathogens that have capsules. This cartoon here is showing it. macrophage trying to eat a cat a bacteria with a capsule you can see the bacteria sticking its tongue out at the macrophage because it can't eat it anyway but here the the mnemonic is even some super killers have pretty nice big capsules and this is one of the ways that physicians remember e coli streptococcus pneumoniae salmonella club zl a pneumonia hamafluous influenzae cinnamones aerogenesis Syria meningitis Bacteroides fragilis and the yeast Cryptococcus neoformans all have pretty nice big capsules.
So hopefully you find that entertaining and I'm not going to make you memorize those, but if you ever do need to memorize them someday, maybe this will help. Another structure that you find on the outside of bacteria and also a structure that can be associated with virulence for sure is the flagella. If there's more than one, it's, excuse me, flagella is plural. But there's a single flagellum. And this is a filamentous protein that allows bacteria to move.
And they come in different flavors. So you might just have one flagella on one end. So we would refer to that as monotrichus and polar.
You might have a flagella on either end. So this would be amphitrichus and polar. If you have multiple flagella on one end, it's lobotrichus and polar. And if the flagella are just all over the place, we say it's peritrichus.
And they allow bacteria to move. can be quite modal when they have flagella. And again, when we do our ID project later in the semester, some of you will have unknowns that are modal and some of you will have unknowns that are not. Over the next few weeks though, we're actually going to look at bacteria that have both characteristics in classes in laboratory as a group.
So you'll get a chance to see some of these characteristics up close and in person if you haven't already. When we think about flagella, you know, what causes the bacteria to move? And well, it's usually in response to either chemicals or light.
So we talk about phototaxis or chemotaxis. In the case of chemotaxis, the bacteria are moving either towards or away from something they either want to get to, an attractant or a repellent. And the way this works with bacteria is that the flagella can rotate in two different directions.
So if they rotate in one direction, you get a run. And so it propels the bacteria forward. And then if the bacteria finds that it's not going the direction it wants, it will reverse the rotation of the flagella and it causes it to tumble. And then the tumbling will turn it in another direction. So then it will reverse rotation again and do a run.
And then it'll reverse and do a tumble. And then it'll reverse and do a run. And ultimately what you see is it's able to, over time, orient itself so it's moving overall towards the... the area that it wants to get to.
So this is showing a chemical gradient. One can imagine maybe there's food or nutrients that the bacteria attracted to and moving towards in this particular case. Some bacteria exhibit swarming characteristics. These bacteria are highly modal. This is a picture of Proteus mirabilis.
We use this organism in lab and so you should have had a chance, if not yet, very soon to actually see Proteus mirabilis and view its swarming characteristics. On a plate it almost looks like it forms waves. as the bacteria divide and then move across the surface of a plate.
And if Proteus mirabilis is your unknown bacteria, you'll have to grow it separately from your other unknown because it will take over the plate. You can't inoculate it and have it stay in one place. It will swarm and fill the entire petri plate.
It's really incredible. And it smells terrible, just as an aside. So swarming characteristics are something we associate with bacteria in the Proteus family, particularly.
Mirabilis and Vulgaris are the ones we work with in class, but they are very motile and very smelly. There are also some bacteria that have filaments. Instead of flagella on the outside, they have these axial filaments that are inside the cell. And a couple of examples would be Lyme disease, which the organism Borrelia burgdorferi, and then the organism that causes syphilis, which is Treponema pallidum.
I just thought I would show you quickly what this looks like. It's kind of cool to watch. So you can see them moving with their axial filaments from one direction to another.
Super cool. The person who's using the microscope has to move because the bacteria keep moving and so they they're changing their field of view so that you can see them. Anyway, I think that's a pretty cool little video worth spending a moment on. Other structures that are on the outside of cells include Fimbriae and Peli.
When we talk about Fimbriae, we tend to be referring very specifically to structures that help bacteria adhere to surfaces. These are also useful in biofilms, allowing bacteria to attach to a structure and then a biofilm will form around them. Sometimes Fimbriae are a virulence factor.
So for example, the organism that causes gonorrhea Neisseria gonorrhoeae, that particular organism gets into the urogenital tract and then these fimbriae attach very, very strongly to the cells in the urogenital tract and they attach by the fimbriae. And that's how the bacteria first begins to invade the tissue. It starts by attaching with this these fimbriae and then it's able to get inside your tissues and cause an infection.
In the case of pili, these are associated primarily with something called lateral gene transfer. And this is a way we were talking earlier in this lecture set about plasma DNA. We said that it was a little extra piece of circular DNA outside the chromosome.
And that plasma DNA is actually something that bacteria can share with one another. And the way they do it is they make copies of the plasma DNA. And then a bacterium that can make a pillus will make this pillus. That's what this is, the tube.
And that plasma can travel through this tube and go into this bacteria. So if this bacteria, for example, had a plasmid that made it antibiotic resistant, and this bacterium was not antibiotic resistant, this plasmid could actually share that trait of antibiotic resistance by sending the plasmid through the pillus into this organism. And it's pretty effective. If you start with one bacteria in a culture that can form a pillus and has a plasmid, and you wait a few generations, all of them will have the plasmid.
after a short period of time. So this is a really significant way that bacteria share traits, not just by passing them down to their offspring bacteria, but also in the same generation. It would be like if I could share one of my genes with you.
It's basically that idea. That's why we refer to it as lateral gene transfer. And this actually plays a huge role in the problem of antibiotic resistance in bacteria.
So Fimbriae are critical in attachment. Pillai are really important in lateral gene transfer. All right, our next lecture, we're going to talk about eukaryotic cells and their primary substructures.