So for the second of my two lectures, out of the five on bacteria and archae, I'm going to be talking about the structure of bacteria and archae, and we'll start with the cell envelope. So again, like the previous lecture, this lecture is divided into two shorter lectures. The reason they have that structure actually goes back to COVID times and lockdown, and it was thought that 20-minute lectures...
were a bit more digestible than hour-long lectures. But anyway, I've kept it in that format. So we talked about diversity last time.
And today, we're going to be talking a little bit about structures. So hopefully, at the end of this lecture or this pair of short lectures, you'll understand that bacteria and archaea have complex membranes and surface layers. So we did talk about the contrast between relatively simple bacteria and archaea. You'll sometimes hear the term prokaryote to describe those two groups. I tend to avoid it personally.
But the relatively simple structure of those cells compared to the eukaryotes. But it doesn't mean they don't have structure. There are lots of structures within bacteria and archae that are important to understand.
And you should also understand that the bacterial cell is structured by proteins that are similar to the cytoskeleton in eukaryotes. So all the things like microtubules and... and so on, they do have equivalents in bacteria, or homologs even.
So just a quick recap of the difference between bacteria and archaea. We have all these membrane-bound organelles in the eukaryotic cell on the right, such as the nucleus, the mitochondria, the golgi, the endoplasmic reticulum. et cetera, they're all missing in bacteria.
But we do have surface structures, as you can see, pili there and flagella. We have a somewhat complex cell envelope, especially in the gram negatives, with multiple layers. And then within the cell, within the cytosol, there are also many structures that we'll go through today.
Okay, so bacteria, to a less extent, do come in a variety of cell shapes. And some of the names for some of the shapes are given there that match the diagrams on the right. So Bedello-Vibrio looks like the organism at the top there in A. And then we have rods, and we have coccoid bacteria, such as the one shown in D.
bacteria with multiple pili, such as you see in the image E there, all extending out from the surface. And we have helical bacteria, such as the one on the bottom. A good example of that would be Treponema pallidum, which is famous for causing syphilis. So we have lots of different shapes, and those shapes are obviously determined by various proteins within the bacteria. So there is a bacterial cytoskeleton.
the cell diameter of bacteria is controlled by a protein called FTSZ, which polymerizes to form a Z ring. So this is a fluorescent FTSZ image where you can see FTSZ forms the septum that actually causes the bacterium to divide. And we find that FTSZ is a bacterial homologue of... of tubulin. You can't tell this by looking at the sequence.
The sequence of FDFZ looks absolutely nothing like tubulin. But if you look at the structure of the two, you see something like what you see in the top right there. So structurally, they have a very similar structure. So that either means they've come to the same solution to the problem by parallel or convergent evolution, or they have a common origin, but they've become so divergent that in terms of the primary sequence that you can no longer spot that just by looking at the sequence, but you can still see the same structure. The elongation of rods or spiral shapes are determined by a different bunch of proteins called the MRE proteins, while crescent-shaped cells are controlled by another protein called CREs.
Okay, so MREB is structurally similar to actin. So just like FDSZ looks like tubulin, MREB looks like actin. And again, we can see that at the structure, and we can see some level of sequence identity here, so almost certainly they are related by a common ancestor, but it's still very low levels of identity. But the fold is the same, as you can see there.
top right. So in E. coli, we can see the MREB forms helical structures around the cell, as you can see in these two panels at the bottom.
So it wraps around the cell in this kind of ribbon-like structure, helping to give that classical rod structure to the E. coli cell. So this is what the envelopes of the two main types of bacteria look like. So you've probably heard of gram-positive and gram-negative bacteria.
That word gram, it's just somebody's name, but he designed a staining technique that's very useful. You'll be doing it yourselves in November when you come into the practical. It's a very useful technique to differentiate bacteria.
So if you want to start classifying an unknown bacteria, the first thing you might do is a gram stain. And gram-positive bacteria stain... black or purple or very dark, while gram-negative bacteria using this particular type of staining technique will stain red or pink, much lighter colored. But there's an underlying reason for that, and it's in this structure.
So if you see on the left, you can see at the bottom, this is a gram-positive bacterial envelope. You can see at the bottom you've got the bacterial plasma membrane. It's the same as plasma membranes in any kind of salient eukaryote mentioned from bacteria orchids.
plants, animals, fungi, you name it. They've all got a phospholipid bilayer surrounding the cytosol. Bacteria are no different. External to that, in a typical gram-positive bacterium, you have a thick layer of what's called peptidoglycan.
And this is analogous to the cellulose cell wall of plant cells. It forms a rigid structure and prevents the bacterium from bursting open under osmotic stress. So I think of it as blowing up a balloon inside a sack.
You can blow up the balloon, but once it fills the sack, you can't blow it up anymore, not unless you've got very strong lungs. And that's because the peptide of glycan holds the bacteria under pressure intact and prevents it from lysing. And then there may be proteins embedded in the various structures. They're shown in green. There may be secreted proteins.
There may be capsular polysaccharide. That's those squiggly lines. on the top there. So that's what a gram-positive envelope looks like, typically.
And then on the right, we have a gram-negative envelope. And again, we have a plasma membrane, but this time the peptide-glycan layer is still present, but it's very much reduced, and it's not necessarily in direct contact with the plasma membrane. Instead, it's in direct contact with another membrane that we call the outer membrane, which we can see on the top there. Now the inner leaflet of the outer membrane is phospholipid, but the outer leaflet of the outer membrane, although it may have patches of phospholipid, as we see towards the right there, it's mostly composed of another molecule called lipopolysaccharide, or sometimes lipooligosaccharide, which has hydrophobic lipid going into the center of the membrane, and that's called lipid A in isolation.
External to that we have those little disks representing... present core oligosaccharide, which is a few sugars stuck together. And that might be all you have, in which case it's lipo-oligosaccharide.
You find that in certain species like Neisseria and Campylobacter and others. But most bacteria, including well-known bacteria such as E. coli and Pseudomonas, have a long polysaccharide side chain.
That's what those black wavy lines are supposed to represent. So if that is present, then the whole molecule is called lipopolysaccharide. External to that you may or may not have additional polysaccharides, that's the green lines again, and there may be secreted proteins, membrane embedded proteins, and so on. So that's the basic structure of the two. There's also a concept that you may come across called diderm and monoderm, meaning two skins and one skin.
And you can see here that the gram negatives would come under the title diderm and the gram positives monoderm. But there are also some other things that you may come across. some special cases so for example if you look at the cell wall of mycobacteria such as mycobacterium tuberculosis they're essentially a gram-positive bacterium but there are membrane there are there are lipids embedded in the peptide of glycan that make it more or less functionally like a pseudomembrane so they're also called died arms so there are some exceptions there so that's how the the two main structures of the bacteria differ by the way when we come to do the staining You will stain them with Gram's iodine and crystal violet, and these two stains will stain the peptide of glycan irreversibly.
You then wash it with... Grams acetone, which is basically just acetone, that will wash away those stains unless it's been firmly bound by the peptide of glycan. The small amount of peptide of glycan in the gram-negative binds much less, and also it's protected by the presence of the outer membrane.
They don't tend to stain with those stains, and then you counter-stain them with a lighter stain, which allows you to see those that have not been stained with the gram-xyladine. So these guys would look pink, and those guys would look black or purple. And as I say, you'll be doing that for yourselves in November. In ARCI, there are very diverse structures.
So these are just some examples. I wouldn't expect you to remember them, but some look very much like gram-negatives. For example, this Igneo Coccus Hospitalis has a cytoplasmic membrane and an outer membrane. It looks very similar to a gram-negative.
Others look like gram-positives, but they have lots of different structures and different... types of chemical make-up of their envelopes and some of the molecules I mentioned there in the key at the bottom so more diverse than bacteria So that's what they look like. This is a gram stain which contains some gram negative rods.
I think they're E. coli. I could be wrong. That's what E. coli would pretty much look like anyway.
And some gram positive cocci. Again, I think they're staphylococcus aureus. But anyway, that's what gram-positive cocci look like.
So you get a very clear distinction between both the structures of those cells and also the way they stain with gram stain. So that's what peptidoglycan looks like. So you have a tetrapeptide linked to these two sugar molecules, N-acetylglucosamine and N-acetylmuramic acid, and that forms this sort of sheet-like structure that gives that rigidity.
One important thing to know about peptidoglycan is it's fairly porous, so it forms that rigid structure, but most molecules, even proteins, can kind of float through it more or less without any impedance. So it doesn't really form a permeability barrier, it's just there as a structural barrier. And many bacteria have capsule as well. So this is on the outside, this is an organism that you can see staining in green there. We have the capsule, it's a particular chemical reaction that you can use that allows you to see polysaccharide capsules.
This is a Streptococcus pneumoniae, and you can see the capsule there. One of the important aspects of the capsule is it can sort of hide other proteins or other surface molecules. antibodies to a particular molecule might not be able to reach that molecule on the surface because it's covered in polysaccharide. On the other hand, polysaccharide capsules themselves can be targets for immunity.
The other thing they do is they can inhibit interaction with phagocytic cells, such as macrophages, and therefore protect the bacterium in that way. And they may provide resistance to desiccation, so they may allow the bacterium to survive in a dry environment. where they would otherwise desiccate and die off.
And then there are something called S-layers. This might look like a compound eye of an insect, but it's not. It's the surface of a bacterium in an electron micrograph.
And S-layers are actually... crystalline lattice of particular proteins and they again form a protective layer on the surface of certain bacteria and they're found in both gram negatives and in gram positives as well so that's another type of envelope component you find in some bacteria So capsules vary greatly in their chemistry. So here's just a few examples. So among the gram-positives, bacillus anthracis there, obviously causes anthrax, has a polypeptide capsule. with structural subunits of D-glutamic acid.
There are polysaccharides, such as the one you just saw in the image that I showed you a couple of slides ago, the streptococcus pneumoniae. There are other polysaccharides. such as in Streptococcus pyogenes has highly aronic acid. And then among the gram-negative bacteria, many have polysaccharide capsules.
The example given here is Pseudomonas aeruginosa. E. coli have different...
Polysaccharides, different types of E. coli. One example here is colonic acid, and you can see the structural subunits there.
Neisseria meningitidis, again, a pathogen causes human disease. That has lots of different types of polysaccharide capsule, depending on the particular serogroup. But one example there is polysialic acid, or polyanacetylmanosamine 1-phosphate. So there are a couple of different examples there. So what I want you to take from that slide is that there are lots of different chemistries, polysaccharide and peptide in the surface layers of the capsules.
So at the end of this first part of the lecture, I hope that you now understand that bacteria have complex envelopes containing one or more membranes. So sometimes there are two membranes. Most bacteria have a peptidoglycan.
layer and that provides that rigid succulous structure. I think I did in the last lecture give you some exceptions to that. So mycoplasmas would be one example.
And these are organisms that live inside cells, so they don't need that. rigidity. Many bacteria have external capsules or S-layers composed of either polysaccharides or proteins. And bacteria do have cytoskeletal proteins that are very similar to the cytoskeletal proteins that you find in eukaryotes, and these are involved in giving the cells their characteristic shape.