Which microorganism spores are most resistant to ionizing radiation?

• A. Clostridium botulinum spores
• B. Bacillus anthracis spores
• C. PA 3697 spores
• D. Salmonella spp.

Answer: (A)

Clostridium botulinum spores stand out for their extreme resistance to ionizing radiation. Endospores protect their DNA with a thick protective coat and cortex, a dehydrated core, high levels of dipicolinic acid, and DNA-binding small acid-soluble proteins. These features work together to limit radiation-induced damage and enhance repair after exposure, so a higher dose is needed to achieve the same level of inactivation compared with other spores.

Among the options, this organism’s spores are known to withstand ionizing radiation more than the others listed. Bacillus anthracis also forms resistant spores, but not to the same extreme level in this context. The remaining choices either are not spore-formers or are not relevant to spore resistance, so they don’t fit as well.