This blog has been created partly as a companion to Chemistry for the Biosciences, the textbook that I co-author with Tony Bradshaw, and to act as an archive of posts I write for other sites (particularly the OUPblog). Like the book itself, it explores how life on the scale of atoms and molecules has an impact on biology - at the scale of cells, tissues, and organisms - and seeks to demystify a range of biological and chemical concepts.

The blog's name takes as its inspiration the cover of the first edition of Chemistry for the Biosciences, which depicts a gecko seemingly clinging to its surface. To find out what links geckos to chemistry, read this.

Monday, 12 September 2011

How are bacteria and viruses different?

In my last post I discussed how our bodies protect us from the threat of attack from our surroundings. Two of these threats take the form of bacteria and viruses. But is there any major difference between these two invaders? In short, yes – and we’ll explore what this difference is during the rest of this post.

Let’s start by thinking about bacteria, and how they differ from the cells from which our bodies are made. By contrast with the sprawling metropolis of cells that form our body, a bacterium (we talk of a single bacterium, but many bacteria) is just a single cell. And unlike the cells of our body, which rely on each other for survival, a bacterium is a self-contained living entity. Inside that single cell is all the machinery needed for the bacterium to survive, along with that all-important element of life: a genome; the genome contains all the information needed to instruct the formation of a new bacterium, allowing the continuation of life. A bacterium feeds, grows, reproduces and dies, mirroring our own existence, but at the level of a single cell. However, bacterial cells have distinct differences from other cells, the cells of our bodies included. For example, the building materials used to construct a bacterial cell wall are quite different from the materials that encapsulate each of our cells. These differences are important, as we’ll see a little later.

So what of a virus? In short, a virus is nothing more than a neatly-packaged instruction manual. A bacterium contains its own instruction manual – its genome – but also all the molecular machinery needed to read and make use of that instruction manual: it can use its genome to maintain its own existence (and does so very effectively). By contrast, a virus simply contains a set of instructions – its own genome – but none of the machinery to bring those instructions to life. So how does it survive? Well, in the absence of its own set of molecular machinery, it steals a set from elsewhere – namely, from a living organism like us. When a virus infects us, it hijacks the molecular machinery found in each of our cells; this machinery stops reading the information stored in our own genome, and starts to read the viral genome instead.

When a viral genome is ‘read’ by a cell, two things happen: the cell makes more copies of the viral genome, and more packaging to wrap it in. The virus is basically hijacking a living cell and using the cell as a photocopier to generate multiple copies of itself. So why is this such a problem? In some cases, it isn’t: some viruses infect us without there being any adverse effects – and, indeed, without us ever being aware that the infection has happened. Others, however, are much less forgiving. In some instances, once the hijacked cell has made copies of the viral genome and new packaging to wrap it in, the cell bursts, releasing its newly-built viral cargo, but also killing the cell in the process.

By contrast with viruses, bacteria seem almost to be tame, and certainly don’t hijack our cells to drive forward their existence in the same ruthless way that some viruses do. Indeed, some bacteria are entirely harmless to humans: we have 1.5 kilos of bacteria in our gut alone, which carry out a range of useful functions. Other bacteria are less accommodating, however, and release toxins, substances that our bodies find poisonous. It is these toxins that cause our bodies to react in ways we associate with ‘being ill’ – for example, the bouts of diarrhoea and vomiting that we could all live without.

So how do we defend ourselves against these particularly harmful invaders? The key issue for our own wellbeing is that we want to eliminate the alien intruders without damaging our own cells. A bottle of bleach might well do serious damage to some invading bacteria, but it would do just as much damage to our own cells too. Not an ideal solution. The problem is compounded when it comes to viruses: the ideal solution would be to switch off their means of reproducing – but as that means of reproducing is us – our own cells – we’re left having to find other solutions. The key is to find a way of discriminating between ‘them’ and ‘us’. So what options are there?

I mentioned earlier how bacteria are encased by cell walls that are made from different building materials than those that are used to build our own cells. This is just the kind of difference that can be exploited to our benefit. Some antibiotics – naturally-occurring or man-made chemicals that destroy bacteria – can selectively target and destroy bacterial cell walls (having much the same effect as a sledgehammer on a brick wall) while leaving our own cells intact.

Viruses are somewhat trickier beasts to overcome; as I mention above, if we were to target the machinery they use to reproduce, we’d be targeting our own cells. Instead, we need to find ways of stopping the virus getting into the cell in the first place. Typically, the first step in a virus attacking a cell is its attachment to the outside of that cell. If we can prevent the virus from clinging on to the cell surface, we can stop it from subsequently getting inside to wreak its usual havoc. And this is exactly what some vaccines do: they stimulate the production of antibodies, which essentially act as cellular bodyguards, getting in the way of the virus and its intended target.

We don’t have antibiotics and vaccines to protect us against infection by all bacteria and viruses, but only those that have the potential to do us particular harm, and which could overwhelm our immune system before it had chance to respond. Other bacteria and viruses – the ones that cause us merely some annoyance and discomfort (such as the viruses that cause the common cold) – are removed through the protective action of our immune system, as explained in my previous post.

The eagle-eyed amongst you will have noted an important point above: antibiotics and vaccines attack bacteria and viruses in different ways. This is why there’s no point taking antibiotics to treat a viral infection: the antibiotic just won’t work; they’re designed to do a completely different job. You wouldn’t eat a peppermint and expect it to relieve a headache; equally, you shouldn’t take an antibiotic and expect it to cure a viral infection. So, your GP diagnoses you with a viral infection, don’t think they’re neglecting you if they don’t immediately write you a prescription for an antibiotic; to do so will be of no benefit to you at all. In fact, taking antibiotics unnecessarily can do more harm than good, as I’ll explain in a future post.