A pathogen is an agent,
especially a microorganism,
that's able to cause disease.
Bacteria and viruses include
the most well known pathogens,
but fungi, protozoa,
misfolded proteins called prions
and even algae can infect a host
and cause disease too.
But why?
Pathogenesis is a bad idea right,
because what you're doing
is making the organism
that's supporting you ill.
What you don't want when you've found
that nice place to live
is destroy it so you can't
live there anymore.
This is James Chong, a microbiologist from
the University of York.
Pathogenesis happens by accident.
This is a process that's gone wrong,
the organism is taking food
but it takes too much,
or it's producing something
that is toxic,
and so that's causing its host to get sick.
But there's one group of microbes
that's conspicuously missing
from this list of bad guys.
In fact, it's not just a group,
it's an entire domain of life – the archaea.
Until the 1970s, we thought that
all life on earth
was made up of just two domains –
the bacteria and the eurkaryotes,
which includes
plants animals and fungi.
But actually, some of these so-called bacteria
were nothing of the sort,
they were archaea.
More closely related to animals and plants,
and forming their own separate domain of life.
What doesn't make sense at the moment is
why we haven't managed to find
any solid examples
of archaeal pathogens.
There are pathogens in all the other
microbes that we know about –
bacteria, viruses, fungi –
all of those things make host organisms ill.
But at the moment, there are no concrete
examples of archaea doing that kind of thing,
which is... strange.
But, there's some evidence starting to appear
that things like Methanobrevibacter smithii
might cause disease.
So for example, you see a much
higher frequency of that organism
in the stools of patients with diverticulosis,
so this is a gut lining condition.
This same species of archaea
has been found to cause obesity
in germ-free mice, so that's mice
that don't have a normal gut flora.
A related species,
Methanobrevibacter oralis,
is found far more frequently
around the gums
of people with periodontal disease
than in healthy cases.
And we see the same organism
appearing in about 40%
of human brain abscesses,
at least in the studies
that have been performed so far
So it might not be that archaea
are a special case at all,
but we just don't have
the right evidence yet.
We've only realised that archaea exist
as a separate group of organisms
for the last 40 years.
So it may just be that
we haven't had enough time
to catch these organisms
in the act of pathogenesis
and demonstrate
that they cause disease.
