Microbial Toxins Flashcards
define toxins:
substances of natural origin that have the potential to cause harm
Why have microogranisms evolved an impressive arsenal of chemical weapons ?
they have a vast arsenal of chemical toxins due to evolutionary pressures
- some are very selective
e. g penicillin - highly toxic to many bacteria but not to humans
what has the discovery of the mechanisms of action of these highly toxic substances allowed ?
- greatly added to our knowledge of cell functions
- provided useful tools for exploring cellular events
- and some have use for therapeutic gain
what are most toxins?
mainly large molecular weight proteins and enzymes- they have a catalytic ability with extraordinary potency and only a few of their molecules are needed to enter a cell and cause disruption of biology causing disastrous effects
what are clostridia and where do they normally live ?
clostridia species are anaerobic spore forming bacteria
they normally inhabit the soil- the spores are very resilient and can reside in soil for many years
what does clostridia species produce?
produce paralytic neurotoxins
what does clostridia botulinum do ?
it releases the neurotoxin botulin
clostridia botulinum is a gram +ve anaerobic bacterium
what does clostridia tetani do ?
it releases tetanospasmin which causes tetanus - this is very similar to botulin toxin but instead of targetting peripheral nerves it targets the nerves in the spinal cord
what does clostridium perfringens do ?
it releases an alpha toxin - a cytotoxic lecthinase which causes cell lysis
what are the 3 methods in which infection and exposure of a neurotoxin can occur ?
food borne
infant botulinism
wound botulinism
when is botulinism caused by food borne infection ?
this is a classic type of botulinism
caused by ingestion of contaminated food with preformed neurotoxin
it was first recognised in germany at the end of 18th century - discovered to be due to consumption of contaminated sausages - sausage disease
why has botulinism occurred ?
due to our desire to store food
what is infant botulinism ?
most common form of the disease
occurs when children ingest food containing bacterial spores - germinate in the infants gastrointestinal tract producing the neurotoxin
what is wound botulinism ?
spores can contaminate wounds and then they germinate and the neurotoxin released is absorbed into the body
- intravenous heroin users are at risk of this type of botulinism because injected heroin may be contaminated with bacterial spores
what is botulins target ?
peripheral nerves - mainly the motor nerves- it prevents the release of ACh and this causes flaccid paralysis of skeletal muscle
- this has a characteristic descending pattern - paralysis starts at the top of the body and descends down
how long does it take for symptoms to arise after ingestion of botulin and how long for spores ?
occur after a few hours after ingestion
can take several days to develop if spores have contaminated food or a wound
what are the typical first symptoms of botulinism poisoning ?
weakness of the muscles innervated by the cranial nerves - patient finds it difficult to speak, swallow and their face becomes expressionless
- vision is blurred, eyelids drop (ptosis), blinking is inhibited (stare)
what is the major dangerous symptom caused by botulinum poisoning ?
paralysis of the chest - there is a progressive weakness of the muscles supporting breathing and may lead to respiratory failure and death - may have to support the patients respiration for weeks
how is botulinum poisoning treated ?
treatment varies depending on the type of the disease
if the neurotoxin was ingested- activated charcoal is administered to prevent absorption
antibacterial drugs can be given to treat wounds or gastrointestinal infections
antitoxin or human botulinism immune globulin can be given to inactivate absorbed neurotoxin
main priority is to maintain respiration
what is the mortality rate for botulinum toxin ?
25% even with intensive care
what is botulinum toxin described as ?
natures most potent chemical weapon
what are the types of different toxins and what is the structure of botulinum toxin?
toxins A-G - A and B are the most lethal
it is a 150kDa single polypeptide precursor- active toxin consists of 50kDa light chain= enzyme component that has a zinc dependent protease. 100kDA heavy chain joined to the light chain by a disulphide bond and this is responsible for targetting the toxin
botulinum toxin is a large molecular weight protein, therefore how does it enter cells ?
uses trafficking system in epithelial cells to enter them
- undergoes transcytosis= endocytosis + exocytosis
for an 80kg adult how much botulinum toxin is lethal by intravenous injection ?
80ng
0.1 to 1ng per kg of body weight
how much neurotoxin is required to kill an 80kg adult by inhalation ?
800ng
what does the c-terminal portion of the botulinum toxin do ?
responsible for nerve recognition
what is the mechanism of action of intoxication by botulinum toxin ?
1- binding to target tissue 2- internalisation 3- membrane translocation 4- enzymatic modification of molecular target
1) heavy chain recognises polysialogangliosides which are prevalent in neuronal membrane particularly in nerve terminals
2) it highjacks recycling of neurotransmitter vesicles enabling it to internalise into the cell within the vesicle
3) vesicles undergo acidification - protons are pumped into them and this allows botulinus toxin to undergo conformational change and produce a pore in the vesicle so the light chain can enter into the cytosol - this means the the reduction of the disulphide bond must occur
4) the light chain is now in the cytosol and can target snare proteins involved in capturing vesicles with NT and exocytosis of them- the light chain chops up the snare proteins preventing exocytosis
how do they know that botulinum toxin interacts with polysialogangliosides?
because preincubation of botulinus toxin with gangliosides prevents botulinus induced blockade of neuromuscular function
and
pretreatment of cells with polysialogangliosides increases their sensitivity to botulinus toxin
and
treatment of the membrane with neuramidase which removes sialic acid residues from gangliosides decreases membrane binding by botulinus toxin
what is a use of botulinus toxin but it is not medicinal?
for cosmetic purposes to cause muscle paralysis- botox
when is botulinus toxin used for therapeutic gain ?
in conditions with abnormal levels of muscle contraction or spasm = dystonias
what is the most commonly used botulinus toxin and how is it used ?
A
it is injected in carefully controlled doses into the affected muscles
what was the first dystonias that botulinus toxin was used for and what others used it ?
strabismus- sever squint eyes
also now used for nystagmus - inability to fix gaze
also used for hyperhidrosis- excessively sweaty palms due to excessive activity of the autonomic nervous system
what are mycotoxins?
diverse groups of chemicals produced by filamentous fungi
e.g ergot from fungus claviceps purpurea
what is the function of mycotoxins ?
uncleat but some are toxic to competing micro organisms
many are toxic to mammals
25% of worlds food crops are contaminated with them - especially in developing countries
do fungi or bacteria produce more chemically diverse toxins ?
fungi but few are proteins
what does ergot alkaloid do ?
it grows on a variety of cereal grains especially rye
warm, damp summers encourage its infestation
it grows on individual cereal grains and replaces grain with a curved purple structure called SCLEROTIUM - this contains lots of pharmacologically active substances= ergot alkaloids
what do the ergot alkaloids cause?
hallucinations- not really hallucinations but distorts sensory perceptions
what are the symptoms of ergotism ?
nausea vomiting drowsiness itching twitching muscles high blood pressure - severe and long lasting vasoconstriction- this can cause an intense feeling of burning and pain hallucinations gangrene - tissue in hands, arms, feet and legs become anaerobic and this can cause bacteria like cl.perfringens to infect and cause gangrene abortion
why is ergotism referred to as st.anthonys fire or holy fire ?
st anthony lived in eygpt (250-350AD) and he spent long times in the desert fasting and resisting satan and is said to have experienced hallucinations and visions that were similar to ergotism symptoms
what substance are ergot alkaloids chemically related to ?
LSD
when was the first pure and pharmacologically active alkaloids isolated from ergot ?
1920s and 1930s
what are the most important active alkaloids present in ergot ?
ergotamine and ergometrine - potent vasoconstrictor agents and stimulants of uterine smooth muscle
- ergotamine used to treat migraines
- ergometrine- promotes parturition and stops post partum bleeding - constricts blood vessels in the uterus
what are amatoxins ?
heat stable bicyclic octapeptides
- LD50 of 0.1mg/kg
where are amatoxins found ?
amanita mushrooms - some have enough in them to be fatal after consumption of a single mushroom
how do amatoxins cause their damage ?
absorbed from the gut and transported into hepatocytes and renal tubule cells
- it inhibits RNA polymerase 2 - prevent gene transcription and halt protein synthesis
- GIT, liver and kidney with high rates of protein synthesis are target organs and death results from liver or kidney failure several days after exposure
what is the treatment for amatoxin poisoning ?
no antidote
remove part or undigested mushrooms from stomach by gastric lavage
restrict toxin absorption by activated charcoal and correctly fluids
liver/kidney transplant
what synthesises aflatoxins and what do they mainly contaminate ?
aspergillus species
maize, peanuts and cottonseed
which aflatoxin are humans mainly exposed to and what produces it ?
aflatoxin B1
aspergillus flavus or aspergillus parasiticus
what are the 4 major aflatoxins ?
B1, B2, G1 and G2
what does consumption of aflatoxin B1 in high doses over a short period cause ?
acute aflatoxicosis - acute liver damage leading to haemorrhage , jaundice and sometimes liver death
what does consumption of low doses of aflatoxin B1 for long periods ?
chronic consumption causes cirrhosis and liver cancer
- malnutrition and exposure to hepatitis vviruses may promote tumour formation
what happens to aflatoxin B1 in the body ?
it is oxidatively biotransformed in the liver by cytochromes P-450 to an aflatoxin b1 -8,9 expoxide
- this metabolite forms covalent adducts with DNA, major one is with N7- nitrogen atoms guanine - this is thought to be the important step in carcinogenicity
