Is a prion the smallest microorganism?

From the Guidelines

No, prions are not considered the smallest microorganisms because they do not meet the criteria of being living organisms, as they lack genetic material and the ability to reproduce independently. Prions are misfolded proteins that can cause diseases such as Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy (mad cow disease) in cattle, as discussed in the context of genetic counseling for prion disease 1. The smallest true microorganisms are typically considered to be viruses, with bacteriophages like MS2 measuring about 27 nanometers in diameter. Following viruses, mycoplasmas are the smallest cellular microorganisms, with Mycoplasma genitalium measuring about 200-300 nanometers.

• Key characteristics of microorganisms include:
  • The presence of genetic material (DNA or RNA)
  • The ability to reproduce independently
  • The ability to metabolize and respond to their environment
• Prions do not possess these characteristics, instead, they are infectious agents that induce normal proteins to misfold, leading to a cascade effect that damages tissue, as noted in the updates on best practices for genetic counseling for prion disease 1.
• The classification of prions as non-living infectious agents is crucial for understanding the diseases they cause and for developing effective strategies for diagnosis, treatment, and prevention, including genetic counseling and testing for individuals at risk 1.

From the Research

Characteristics of Prions

• Prions are infectious agents that cause neurodegenerative diseases in humans and animals 2, 3, 4.
• They are resistant to inactivation processes that are effective against conventional viruses, including those that alter nucleic acid structure or function 2.
• Prions have a unique characteristic of being able to modify their protein conformation, which is essential for their infectivity 2, 4.
• The exact nature of prions remains unknown, but it is likely that they consist of protein only 2.

Size and Structure of Prions

• The pathogenic prion protein core consists of 142 amino-acids and has a mass of 15 pikograms 5.
• Prions are filtrable and can survive dry heat at 200 degrees C for 1-2 hours 5.
• Their pathogenetic properties depend on tertiary spatial structure (conformation) which is specific and transmissible in experiment 5.

Comparison with Other Microorganisms

• Prions have biological and physicochemical characteristics that differ significantly from those of other microorganisms 2.
• They are highly susceptible to procedures that modify protein conformation, whereas conventional viruses are susceptible to procedures that alter nucleic acid structure or function 2.
• Prions are extremely resistant to disinfection and sterilization methods used so far, making them a unique challenge in terms of decontamination 5.

Classification as the Smallest Microorganism

• While prions are extremely small and have a unique structure, it is not clear if they can be considered the smallest microorganism 2, 5.
• The definition of a microorganism is typically based on the presence of nucleic acid and the ability to replicate, which prions do not possess in the classical sense 2, 4.
• However, prions are able to induce disease and replicate in a host, which is a characteristic of microorganisms 2, 3, 4.