Clostridium tetani
Tetanus toxin is an extremely potent neurotoxin produced by the vegetative cell of "Clostridium tetani" in anaerobic conditions, causing tetanus. It has no known function for clostridia in the soil environment where they are normally encountered. It is also called spasmogenic toxin, or TeNT. The LD50 of this toxin has been measured to be approximately 2.5-3 ng/kg, making it second only to the related botulinum toxin (LD50 2 ng/kg) as the deadliest toxin in the world. However, these tests are conducted solely on mice, which may react to the toxin differently from humans and other animals.