Which of the following types of animals are considered gnotobiotes?

• A. SCID mice
• B. C57/BL6 mice
• C. Defined flora mice
• D. Nude mice

Answer: (C)

Gnotobiotic animals are those whose microbiology is completely known—their microbiota is either entirely absent (germ-free) or composed of a defined, known set of microorganisms. The defining feature is a known microbial status, not just a particular genetic background or immune deficiency.

Defined flora mice fit this description because they are raised with a specific, defined community of microorganisms. Researchers know exactly which species are present, so these animals are truly gnotobiotes. The other options describe strains with particular genetic or immune characteristics, but they do not by default have a defined microbial population; their gut flora is typically unknown or variable, so they aren’t considered gnotobiotes unless specifically managed to meet that status.