Feature Fungus toxins
Mushroom
roulette
© science photo library
Can you tell the difference between a tasty
paddy straw mushroom and a toxic death cap?
Emma Shiells discovers the deadly chemistry
hidden in those gills
26 | Education in Chemistry | September 2013 | www.rsc.org/eic
Feature Fungus toxins
© geoffrey kibby
Death by name, death by nature
‘Death cap is still the commonest cause
of [mushroom] poisoning in Europe,’ says
Newcombe. A notorious fungus, it is part of
the Amanita genus that contains around 600
species, some of which are the most highly
toxic in the world. It is believed that this genus
alone is responsible for approximately 95% of
all mushroom poisonings, with 75% of fatal
fungal poisonings attributed to death caps
(Amanita phalloides) and the related destroying
angel (A. virosa).1
The toxicity of the Amanita species is due
to the presence of two groups of toxins called
amatoxins and phallotoxins, both multicyclic
peptides. It is believed that the death cap
contains six related phallotoxins and five or
more amatoxins. 2 Geoffrey Kibby, a research
associate at the Royal Botanic Gardens at Kew
in the UK says that death cap ‘smells like goneoff, sickly sweet honey and they apparently
taste really good. But you only need one cap to
kill an adult; they are pretty nasty things to eat.’
After someone has eaten the deadly
mushrooms, ‘there’s usually a delay of
between six and 30 hours from ingestion
before the symptoms start to present
themselves,’ explains Kibby. ‘Typically the
person will experience food poisoning-type
symptoms: severe abdominal pain, sickness
and diarrhoea, briefly followed by a day or so
of apparent recovery,’ he says. ‘However, the
patient will then relapse and deteriorate quite
rapidly due to severe renal and liver failure’
often leading to death.
The less-harmful phallotoxins are
responsible for the initial gastrointestinal
symptoms, but it is the amatoxins that cause
the most damage internally. In particular,
a-amanitin has a high specificity for RNA
polymerase II in the liver. By inhibiting this
enzyme it prevents the formation of mRNA
and stops protein synthesis, resulting in cell
death and subsequent liver failure. When
filtered through the kidneys, the toxin can
then be reabsorbed into the bloodstream
and recirculated around the body, causing
repeated liver and kidney damage.3
The death rate has certainly decreased
over the years, thanks to advances in
treatments, according to Kibby. ‘In the US,
they have more cases than we do [in the
UK], so they’ve had more opportunity to try
different treatments. Standard treatments
usually involve large doses of penicillin and
vitamin C, as well as keeping the liver and
kidneys going,’ he says. Blood can be filtered
using carbon-column haemodialysis units or
attempting organ transplants in severe cases.
A drug called silibilin is currently in trials,
according to Kibby, and seems to be very
effective. Research shows that the drug works
by preventing the uptake of the amatoxins
by the liver cells and thereby protecting
undamaged liver tissue. It also stimulates
DNA-dependent RNA polymerases, resulting
in an increase in RNA synthesis.4
© martin newcombe
On a damp and drizzly autumnal morning
you may think there are better places to
be than foraging in the undergrowth of an
orchard, but amateur mushroom hunters are
sure to disagree with you. Martin Newcombe,
an ecologist and fungi enthusiast, is one of
those hooked.
‘The fact that fungi can grow so quickly
makes them fascinating,’ says Newcombe.
‘They typically grow in two to three days
depending on the environmental parameters.’
For some species, such as the stinkhorn, ‘they
can take just a matter of hours,’ he says. The
fungal kingdom consists of a wide range of
organisms that differ in their size, shape and
colour, and in their means of reproduction
and spore distribution. You could hold up two
mushrooms that appear identical to the naked
eye, but on closer inspection find that they are
two different species entirely – one edible,
the other poisonous. It is only when you take
a closer look at the spores using a microscope
that you can identify the species accurately.
‘The spores are uniquely coloured, patterned
and sculpted for each species, while some of
them also undergo specific chemical reactions
as well,’ explains Newcombe. ‘In the field, there
is often a limit to what you can do in terms
of identification, as sometimes you can only
recognise the genus until further tests can be
carried out back in the lab. Only once you’ve
looked at the spores in more detail and carried
out some chemical tests, can you truly identify
the species and determine its edibility,’ he adds.
No one really knows what mushroom
colours are for, as they are not necessarily
warnings. Contrary to popular belief, not
all poisonous toadstools are bright red with
white spots or warn you with lurid yellow and
green colours. Often poisonous mushrooms
look like the edible varieties you might buy
in a shop and therein lies the danger for the
inexperienced forager.
Mushrooms from
the Amanita genus,
such as death cap
(A. phalloides, top)
or destroying angel
(A.virosa, bottom), can
be poisonous
The amatoxin
a-amanitin has a high
specificity for RNA
polymerase II in the
liver and causes death
through liver failure
Education in Chemistry | September 2013 | www.rsc.org/eic | 27
Feature Fungus toxins
© geoffrey kibby
Paddy straw
mushrooms
(V. volvacea, left)
are commonly eaten
in Asian cuisines.
Although they do not
grow in Australia, the
related V. speciosa
(below) do and can
easily be mistaken for an
Amanita and vice versa.
Mistaken identity
Almost all poisoning cases that Kibby has been
involved in have been due to misidentification,
where immigrants have picked what they
thought they recognised from their native
country, or based on knowledge passed down
from older generations. Recently, Australian
news channels reported a case where four
people in Canberra accidently poisoned
themselves after consuming death cap at a
New Year’s Eve party. Two of them later died.
They were of Asian descent, and it is believed
that they may have
mistaken the death
‘Great care
caps for edible paddy
should be
straw mushrooms
(Volvariella volvacea),
taken in
a
common ingredient
identification
used in Asian cuisine.
to avoid
Brett Summerell,
confusion’
a fungi expert from
the Royal Botanic
Gardens in Sydney, Australia, told ABC News
shortly after the deaths that ‘mushrooms
that are poisonous in Australia are very close
relatives to those that are quite safe to eat and
have a very similar appearance to the straw
mushroom’. Newcombe agrees: ‘They’ve
28 | Education in Chemistry | September 2013 | www.rsc.org/eic
come from a culture where mushrooms like
that are eaten and so probably thought they
recognised them, rather than identified them.’
Since these deaths, the Australian
government and the Australian National
Botanical Gardens have released a factsheet
on death caps.6 It provides detailed
descriptions and photographs, as well as
possible locations in southern Australia and
what symptoms to expect. They emphasise
the importance of not confusing death caps
with the edible straw mushrooms that are
grown and eaten in Asia. Even though the
straw mushrooms are not native to Australia,
a related species V. speciosa is and looks very
similar to straw mushrooms. They have even
been found to grow side by side with the
death caps in some areas of Australia. Each
species in the Volvariella genus has a volva
at the base of the stem and gills that do not
reach the stem – just like the death cap. As the
guidebook3 says, ‘great care should be taken
in identification to avoid confusion with the
deadly white-spored Amanitas’.
‘The greenish form of death cap does look
like the paddy straw mushroom,’ says Kibby.
‘Even the bulbous bag at the base of the stem
can be present on the paddy straw mushroom
so that wouldn’t have alerted them initially.’
The only way to tell them apart is by looking at
their spores, according to Kibby: ‘The paddy
straw mushrooms have pink spores whereas
the death cap has white spores.’ There are also
chemical tests that can be carried out: adding
sulfuric acid to the mushroom’s stem or gills
will turn black if it is a death cap. The Meixner
test uses concentrated hydrochloric acid and
newspaper to determine the presence of the
deadly toxin a-amanitin.7
As an experienced mycologist, Kibby is often
© Accurimbo / wikimedia commons
But treatment is not always successful and
it very much depends on how quickly people
are diagnosed after ingestion. Victims who
are hospitalised and given treatment almost
immediately after ingestion have a mortality
rate of around 10%, whereas those admitted 60
hours or more after ingestion have a 50–90%
mortality rate.5 In spite of years of detailed
research into its toxins, death cap is still the
most deadly fungus known, and there is still no
known antidote for humans.
called upon to help with mushroom poisonings
via the National Poisons Information Service
(NPIS), which is a clinical toxicology service
available for healthcare professionals in the
UK. In 2011, the NPIS saw 257 poisoning cases
linked to eating mushrooms in the UK, but
without any deaths in over four years. That
was until last year, when Kibby’s expertise
was urgently needed as a married couple
were hospitalised after eating mushrooms
suspected to be death caps. ‘[NPIS] got the
son to go down to the bottom of the garden
and photograph the mushrooms for me, they
also packaged some up and sent them to me in
London that same afternoon,’ explains Kibby.
‘As soon as I opened the package I could smell
that gone-off honey smell that is so typical for
A. phalloides [death cap].’
Sadly the woman died from multiple organ
failure, but her husband survived having only
consumed a very small amount of the fungi.
‘She was doubly unlucky, as it was the end of
the year so they shouldn’t have been there in
the first place, and they were albino in colour;
Feature Fungus toxins
Metal recycling
Unfortunately, even non-poisonous fungi
can be pretty bad for you, because they
are very good at recycling. They are nonphotosynthetic organisms, gaining energy
and nutrients for their biosynthetic pathways
by degrading plants and other matter, which
means they can easily absorb trace elements
from their local environment.
Fungi collected near former smelters,
landfill sites and land treated with sewage
sludge can accumulate significant quantities
of metal ions, such as cadmium, mercury, lead,
copper and chromium. Even worse, high levels
of radioactive caesium-137 have been found in
mushrooms. ‘It was only a few years ago that
the British government stopped monitoring
toadstools generally from the fallout at
Chernobyl,’ says Newcombe. Kibby believes
that there are still some areas of far eastern
Scandinavia where picking mushrooms is
not advisable, due to the radioactive nuclides
found in them.
Newcombe once harvested mushrooms
growing near a disused factory that was an
old iron smelter. ‘You could taste the iron
in them,’ he says. ‘I sent samples off to be
tested and, sure enough, there was more iron
in them than there
should have been.’ And
‘Problems
mushrooms that grow
arise when
near roadside edges
people try to
can absorb metals
identify edible from passing vehicles.
The mushrooms
mushrooms
themselves might be
using only a
edible, but once they
have absorbed metals
guidebook’
from their surrounding
environment they can become poisonous
due to the high concentration of heavy metals
found inside them.
Reader beware
In the UK, collecting mushrooms is more of an
occasional pastime, whereas in other cultures
people pick them for everyday consumption.
© f1online digitale bildagentur gmbh / alamy
the least typical colour form [for death caps],’
says Kibby. ‘Apparently, she had identified
them as edible mushrooms using a field guide,’
he adds.
Boletus edulis are
edible and hard to
mistake for poisonous
varieties
Hail Caesar!
Not all mushrooms from the Amanita
genus are bad for your health; some of
its species are edible, like the glorious
Amanita caesarea with its orange cap and
yellow stem and gills. Known in English
as Caesar’s mushroom, it has been prized
since Roman times and is not easily
confused with other Amanita species,
thanks to its vibrant colouring. It has yet to
be found in the UK, but grows in warmer
climates like southern Europe and North
Africa. It was said to be a favourite of the
Roman emperor Claudius and has been
found growing along old Roman roads.
Many ancient historians believe that
Claudius died from mushroom poisoning,
murdered by his own wife Agrippina in
AD54. It has been widely speculated that
death caps were used as the poison, but it
is now considered to be unlikely given the
short timeframe reported between him
getting sick and dying.8
Kibby has tried some edible Amanitas,
‘but even then my heart was in my mouth,’
he adds. He has eaten A. rubescens,
which although edible after cooking,
are poisonous if eaten raw. ‘It did taste
delicious although I couldn’t help but
watch for symptoms over the next few
hours,’ he says.
In mainland Europe, pharmacists can be asked
to help identify them. ‘Problems arise when
people try to identify edible mushrooms
using only a guidebook. If you want to eat wild
mushrooms, you’ve got to go on organised
forays with someone that knows what they are
doing,’ says Kibby. ‘No matter how good the
field guide is, it can’t possibly cover all colour
variations and eventualities. It seems silly to risk
your life when you can go to your local deli and
buy exotic mushrooms there,’ he says.
Kibby says that, contrary to what you might
expect, most poisonings happen not from
picking really weird-looking mushroom, but from
the most normal looking ones, such as paddy
straw mushrooms. ‘If only people would stick to
really weird and wonderful mushrooms like some
Boletus,’ he says. ‘Then you can’t go wrong as you
can’t mistake them for anything else.’
© science photo library
Emma Shiells is picture editor at the Royal
Society of Chemistry
Prized by the Romans, the brightly coloured A. caesarea is one of the edible Amanita varieties
References
1 J R Hanson, The chemistry of fungi. Cambridge, UK: RSC
Publishing, 2008
2 G Kibby, Guide to mushrooms of Britain and Europe. London,
UK: Octopus Publishing, 2006
3 R Phillip, Mushrooms and other fungi of Great Britain and
Europe. London, UK: Pan, 1981
4 K Hruby et al, Hum. Toxicol., 1983, 2, 183
(DOI: 10.1177/ 096032718300200203)
5 Bad bug book (2nd ed), p219. Silver Spring, US: US Food and
Drug Administration, 2012, http://1.usa.gov/XO6DF6
6 Australian National Botanical Gardens http://bit.ly/10gSeRl
7 M Beuhler, D C Lee and R Gerkin, Ann. Emerg. Med., 2004, 44,
114 (DOI: 10.1016/j.annemergmed.2004.03.017)
8 V J Marmion and T E J Wiedemann, J. R. Soc. Med., 2002, 95, 260
(DOI: 10.1258/jrsm.95.5.260)
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