Extremophiles - Research Projects

Pathogenic Archaea: Do They Exist? The following extract was generated from: Cavicchioli, R., Curmi, P.M.G., Saunders, N. and Thomas, T. 2003. Pathogenic archaea: do they exist? BioEssays 25:1119-1128.

Archaea are microorganisms that are distinct from bacteria and eucaryotes. They are prevalent in extreme environments, and yet found in most ecosystems. They are a natural component of the microbiota of most, if not all, humans and other animals. Despite their ubiquity and close association with humans, animals and plants, no pathogenic archaea have been identified. Because no archaeal pathogens have yet been identified, there is a general assumption that archaeal pathogens do not exist.

We recently examined whether this was a good assumption by investigating the potential for archaea to be or become pathogens. This was achieved by addressing: the diversity of archaea versus known pathogens; opportunities for archaea to demonstrate pathogenicity and be detected as pathogens; reports linking archaea with disease; and immune responses to archaea. In addition, molecular and genomic data were examined for the presence of systems utilised in pathogenesis. Our conclusion was that although archaea can presently be described as non-pathogenic, they have the potential to be (discovered as) pathogens. Moreover, our present optimistic view that there are no archaeal pathogens is tainted by a severe lack of relevant knowledge, which may have important consequences in the future.

The UK Guardian recently reported on this discussion: Sample, I. 2003 (August 21). Biology’s big brainteaser. The Guardian (UK). (http://www.guardian.co.uk/life/feature/story/0,13026,1025851,00.html) Following is an extract from the article:

EducationGuardian.co.uk © Guardian Newspapers Limited 2003

If you hang around the bar at a sufficiently obscure science conference you might overhear talk of one of modern biology's most intriguing puzzles. The conundrum concerns a fifth of all life on the planet, and answering it could help us to treat diseases that have evaded medicine for thousands of years.

At the very least it could point us towards a new generation of drugs. Yet perhaps the biggest puzzle is why modern science has largely ignored it. Have you even heard of the archaea? A growing number of microbiologists have, and they are beginning to turn the study of this third branch of life from bar room chat to scientific reality.

It all started back in 1990. The microbiologist Carl Woese at the University of Illinois found that life could be divided into three distinct groups, each of which had branched from a common, primitive ancestor billions of years ago. He assigned organisms to each category depending, among other things, on what their cells looked like and how their internal machinery worked. Bacteria are one such category. The other two are eukaryotes and archaea. Humans belong to the former, along with animals, plants and fungi. The latter, the archaea, lie somewhere between us and bacteria. Like bacteria, they are tiny, single-celled organisms. Their genetic machinery, however, is much closer to ours.

The greatest outstanding mystery of archaea became apparent when people looked at the organisms known to cause disease in humans and animals. Plenty of bacteria are known culprits and there are numerous disease-causing eukaryotes, mostly fungi. But archaea seemed to be entirely harmless. "What's puzzling is that we have these three kinds of life, but only two bear organisms that cause disease," says Rick Cavicchioli, a molecular biologist at the University of New South Wales. "What is it about these archaea that means they don't cause disease?".

The answer could have huge significance. If there is a fundamental reason why archaea cannot or do not cause disease, then this could be the key to "switching off" pathogenic microbes that make people ill, says Cavicchioli. But there's a problem. What if archaea aren't as benign as we might think? Perhaps they are causing disease, and have been for millennia, but we just haven't realised.

The prospect is looking ever more likely. Archaea are estimated to make up a fifth of all life, meaning they outnumber animals. And they live in all the right places to cause disease. As well as being found in extreme environments on Earth, from hot undersea vents to Antarctica, archaea have been found in the intestines, the mouth and in slimy films of mucous elsewhere in the body. A gram of human faeces can hold 10bn of them. "On the evidence we've got so far, we should be finding more than 30 types of archaea that cause disease," says Cavicchioli.