Determinants of pathogenicity of Clostridium botulinum

• Botulism literally stands for sausage (botulus, Latin- sausage).
• The food poisoning in early days was generally caused by ingestion of poorly cooked sausage.
• Clostridium botulinum is worldwide in distribution.
• Its spores are found in wide evironment like soil, animal manure, vegetables, fruits, leaves and sea mud.
• It is a gram-positive, large, non-capsulated bacillus.
• Generally measures 4-8µm x 0.8-1.2µm.
• Peritrichous flagella are present due to which it is motile.
• It shows sub-terminal, oval and bulging spore.
• It grows in nutrient agar and blood agar media in anaerobic condition and in Robertson’s cooked meat medium in 24-48 hours at 35⁰C . 
• Hemolysis is noticed around the colonies in blood agar medium.
• The spores are highly resistant and also can withstand moist heat at 100⁰C for 4 to 5 hours.
• But autoclaving can easily destroy them.
• Type-A spores are more resistant as compared to other types of spores of C. botulinum.
• There are various determinants which reflect the pathogenicity of the organisms.
• They are as follows:

i) Botulinum neurotoxin (BONT) which is a potent lethal poison is formed by C. botulinum along with other unique strains of C. butyricum, C. baratii and C. argentinense.

ii) Eight antigenic toxin types have been found that are distinguished on the basis of their antigenically distinct toxins.

Image source: sciencedirect

iii) They are the toxins from C. botulinum (A, B, C1, C2, D, E, F,G).

1. All the toxins produced by different strains have identical pharmacological action.
2. An active (A) region and a binding (B) region are present in all toxins.
3. A single strain produces a single or only one toxin.

iv) Among all toxins, type-A toxin is the most potent and toxic which minimum dose i.e.3.3 x 10^-8 mg can kill a mice and 1 mg toxin can kill more than 200 millions of mice.

v) can also kill other laboratory animals like rabbit and guinea pig.

vi) Types A, B, E and F are the main reasons for botulism of humans.

• Toxins A through F are neurotoxins that interfere with the transmission at the peripheral cholinergic synapses.
• This prevents the release of acetyl choline leading to flaccid paralysis.
• Types C and D are associated primarily with botulism in birds and mammals.
• Type C is divided into C1 and C2 where C1 is a neurotoxin and C2 is not though it produces vascular permeability and has enterotoxic activity.
• Toxin G has not been clearly indicated in botulism which is now known as C. argentinense commonly found in the soil in Argentina.

Image source: microbeonline

vii) Botulism toxins are considered as exotoxins but they are released generally after autolysis in the surrounding environment.

viii) Some of the botulinum toxins require partial digestion by proteolytic enzymes to become active.

1. Botulin is sensitive to heat.
2. But on boiling for 20 minutes the toxin is inactivated.
3. The toxigenecity is under the control of a temperate phage.

References: 

i) https://microbewiki.kenyon.edu/index.php/Botulism

ii) http://www.pathwaymedicine.org/clostridium-botulinum

Determinants of pathogenicity of Clostridium botulinum