Viruses and their threats: In conversation with Professor Dorothy Crawford
In the mid-1980s, microbiology hit the headlines in a big way with the rise of the AIDS epidemic. Since then, microbiological illnesses have rarely been out of the news: salmonella, Legionnaires’ disease, Ebola fever, necrotising fasciitis, anthrax, smallpox, foot-and-mouth disease, West Nile virus, MRSA and SARS have all been front page stories in the last twenty years. However, despite this coverage, there is still much confusion amongst the general public about the nature of the microbes which cause these illnesses. Particularly, many people are unaware that viruses and bacteria are very different things.
‘This is a bit of a hobby horse of mine,’ says Dorothy Crawford, Robert Irvine Professor of Medical Microbiology at Edinburgh University. ‘It is really very irritating that the media tend to label every nasty illness a virus. Viruses are actually very different from bacteria. In fact, about the only thing which they have in common, apart from the fact that they cause illness, is that they’re very small. But even here, viruses are in fact much smaller than bacteria; for example, you can’t see viruses without an electron microscope, whereas you can see bacteria with an ordinary light microscope. Indeed, until the electron microscope was invented, people weren’t at all clear about what viruses were; they thought that they were probably just very small bacteria, but in fact they’re very different.
‘Bacteria are single celled organisms, which have the ability to exist independently of other organisms,’ Crawford explains. ‘They are able to metabolise and to reproduce on their own. Viruses, on the other hand, are obligate parasites; they are only able to reproduce within a host organism. They can’t do anything until they get inside a living cell. Once they’re there, they have mechanisms which enable them to take over the cell, and to use the cell’s organelles in order to reproduce themselves. They have nucleic acid – DNA or RNA – so they have inherited characteristics; but apart from that there isn’t really much to them.’
The fact that viruses do not produce their own energy and cannot reproduce without hijacking the cells of a living host raises the question as to whether they are alive or not. Does Crawford have a view about this?
‘Well, it’s a question which I sometimes ask my students,’ she replies. ‘It’s a good debate, but I’m not sure how much it matters. Ultimately it hinges on how life is defined. If life is about reproduction, then viruses are alive; but if it is about the ability to metabolise, then they’re not. I guess, if pushed, I’d come down on the side of viruses not being alive. They just don’t seem to have life-like characteristics.’