Which of the following types of animals are completely free of all detectable microorganisms?

• A. Axenic
• B. Specific pathogen free
• C. Cesarean-derived
• D. Barrier-reared

Answer: (A)

Germ-free status defines axenic animals. They are raised in completely sterile conditions, typically inside isolators with sterilized air, water, feed, and equipment, and with strict aseptic handling by personnel. The goal is to prevent any live microorganisms from entering the environment or the animals, and animals are continuously monitored for contamination with microbiological tests. Because of these rigorous controls, axenic animals have no detectable microorganisms.

Specific pathogen free animals, by contrast, are bred and maintained to be free from particular pathogens, but they can still harbor other non-target microorganisms in their normal flora. Cesarean-derived animals are produced to avoid exposure to maternal vaginal microbes at birth, but they soon acquire environmental microbes and are not germ-free. Barrier-reared animals are kept in protective facilities to reduce exposure to microbes, yet they are not guaranteed to be completely free of all microorganisms.