The microorganism most resistant to ionizing radiation is:

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

Answer: (A)

Ionizing radiation resistance tracks closely with how well a microorganism protects its DNA and maintains dehydration and structural integrity. Bacterial spores have thick protective coats, high levels of dipicolinic acid, small acid-soluble proteins that stabilize DNA, and very low water content, all of which help shield the genetic material from radiation damage. Among the options, Clostridium botulinum forms spores with an especially robust set of protections, making them the most resistant to ionizing radiation in this group. Vegetative cells like Salmonella spp. are far less resistant because they don’t have a spore core and its protective features. Bacillus anthracis spores are also highly resistant, but in this context Clostridium botulinum spores are considered the tougher, higher-resistance example. The PA 3697 spores are less well characterized and don’t match the level of resistance seen with C. botulinum spores.