Bacteria - Chapter 5

- Hello, my name's Lindsay Turnbull and I teach biology at the University of Oxford. In this video, I want to show you how cells can change the tools, machinery, and spare parts that they produce so that they can respond to changes in their environments. This is a bag of tricks that's been perfected by the bacteria, those organisms that we most love to hate, and they form the focus of chapter five of my book "Biology: The Whole Story."

(birds chirping) (frog croaking)

Humans love to eat food, but mostly we prefer not to think about what happens to that food once we've eaten it. But inside our guts, food has to be broken down. And that means breaking down the long molecules into their smaller components. For example, proteins need to be broken down into their amino acids, and that's a job that's carried out by enzymes.

Now, humans produce different kinds of enzymes in order to break down different molecules, but one molecule that they can't really digest is called cellulose. Humans don't produce the enzymes to digest cellulose. And cellulose is a major part of our diet because plants produce loads of it to put in their cell walls. Now, the good news for humans is that there are organisms that can digest cellulose and those are bacteria.

And so in our large intestines, we have huge numbers of bacteria living, and they are helping us to digest cellulose and get nutrition from our food. In fact, there's so many bacteria living in our bodies that for every human cell in our bodies, it's estimated that there's at least one bacterial cell. But this doesn't mean that we're ill. This is entirely normal. In fact, that community of bacterial cells that lives in and on us is called the microbiome.

And it's causing a stir in medical circles as people realise just how important it is for human health.

So a very common bacteria that lives in the guts of humans is called E. Coli. And that's been very well studied so we understand E. coli very well. Now, there are one or two strains of E. coli that can be very damaging for humans and cause public health problems, but the vast majority of strains of E. coli are entirely harmless. Now, living in your gut, E. coli is going to be confronted with many different foodstuffs and it's going to need different tools and machinery to deal with them all.

But it might not want to produce all of those tools and machinery at the same time. It might want to fine-tune what it's doing. So how does it do that? Well, to understand that, let's look inside it. Inside an E. coli cell, we're going to find a genome of course. And in bacteria, that's just one enormous long strand of DNA that forms a circle. And that's made up of thousands of individual instructions and each one's called a gene.

And those genes will have to send out messages if the tools and machinery that they code for are actually going to be made by the cell. So how does that work? Well, in front of each gene, there's a special sequence of letters called a promoter that's like a start here sign. So that marks out where the gene starts, and at the end of the gene, there's a stop sign. Now, if the start here sign is exposed and visible, then that gene will be able to send out messages.

But if that sign is covered up, then the gene won't be able to. And this ability to switch genes on and off is something called gene expression. And it's very, very important as it allows cells to fine tune the machinery and the spare parts and the tools that they produce at any given time.

Okay, let's take a look at how E. coli is going to make a change in response to a change in its environment. E. coli's favourite food is glucose. That's a small sugar molecule and that's what it would like best, but sometimes that's not available. But there might be alternatives. For example, lactose, that's a sugar that's found in milk and dairy products. And lots of humans can't actually digest it and they're called lactose intolerant.

And most of the time, E. coli can't digest it either, but it does contain the genes to digest lactose. It just doesn't turn them on. So normally the way that works is that there's a large repressor protein that's clinging tightly to the start here sign at the start of the genes to process lactose. And that means that those genes are off. But now look what happens when lactose enters the cell.

The lactose molecule combined to that repressor protein and that changes the shape of the repressor protein so that it falls off the start here sign. And now the gene is on and so the cell is going to start making the tools and machinery that it needs to process lactose. So this is a really clever response. It's using the presence of that molecule in the environment to turn on the genes that it needs. And bacteria can do that for all kinds of signals.

So we've seen that E. coli carries genes to process lactose, a sugar that it doesn't really encounter very often. So we might wonder why cells don't just carry thousands of genes to allow them to cope with unlikely situations. Well, the problem is that every time a cell adds a gene to a genome, then that molecule of DNA becomes longer and longer. And when the cell comes to divide, the first thing it has to do is to copy all of that DNA.

And the more of it there is, the longer that's going to take. So that's going to sort of slow an organism down. So it can't just have an infinitely large genome. You know, what it would really like is to be able to have some genes just when it wants them and then if those genes are no longer useful, to be able to dump them again. And amazingly, bacteria do have genes like that. So bacteria have extra bits of DNA, small circles of DNA, and they're called plasmids.

And they can pick them up and they can put them down quite easily. They can even pass them among themselves. And those are just genes for special occasions. And one of the key genes that are carried on plasmids are those for antibiotic resistance. Now, antibiotics are drugs developed by humans to kill bacteria once they're inside human bodies because they just damage bacterial cells but not human cells. So it's not good for us that bacteria are carrying all of these antibiotic-resistance genes.

Why are they? We've said that bacteria will only keep these plasmids if they're actually useful. And that implies that bacteria are often encountering antibiotics in their environments. And the reason they're doing that, well, there's two reasons really. One is that antibiotics are being prescribed to humans a lot even when they don't really need them. And the second reason is that antibiotics are being given to animals even when they're not even ill.

Now, remember, we said it's quite difficult for us to digest a molecule called cellulose, which is found in plant cell walls. But imagine if you're an animal like this, and that's pretty much all you eat. Cows just eat grass, and grass is mostly cellulose, so they have to have a very special system of stomachs in order to allow them to digest it.

And they have a big stomach called a rumen, which is why they're sometimes called ruminants, and that is absolutely stuffed full of bacteria who are all busy digesting cellulose, which then allows the cow to get some nutrition from the grass. There is a bit of a problem with that though, which is that because it's quite low quality food and it takes a lot of digesting, then cows only grow quite slowly. And as humans who eat cows, we might want cows to grow faster.

And in cattle feedlots in the US, they don't feed cows grass. Instead they give them pellets of much easier to digest food and that means that the cows can grow faster. The problem is that the bacteria can also eat that easy-to-digest food. But now the farmers don't really want the bacteria there. They're not helping the cow. They're actually just taking some of the food away.

So what they do is feed the cows antibiotics and they give them those on a routine basis every day to suppress all of the bacteria in their rumens. And that means that that's tonnes of antibiotics going out into the environment all the time. So this is something that we might think is not a terribly good idea. So that's something we all need to decide as a society. How do we use antibiotics safely so that we still have antibiotics that work if one of us gets a nasty infection?

So bacteria might be the organisms that we love to hate, but we really shouldn't hate them. In fact, they're incredibly important to us. For example, they help us to digest our food. And you know what? They have been incredibly successful. They've been around on our planet since 3.8 billion years ago and they're still gonna be around when we are nothing more than a few fossilised traces.

However, there's one thing they haven't been able to do, and that's build a true multicellular being because to do that, you need a very different type of cell, one we're gonna meet in the next episode. Well, I really hope you enjoyed that video

and found it useful. And if you did, please do share with friends and colleagues. There's a lot more information about bacteria in chapter five of this book including other ways that bacteria can respond to their environment. They've got some pretty cool tricks up their sleeves. There's links to buying the book in the description below.

If you also want to watch more videos and there's another one coming, which is all about eukaryotes, which are these kind of monstrous cells that you and I are made from. (birds chirping)