What term describes animals that are free from all microorganisms?

The term "axenic" specifically refers to organisms or environments that are free from all microorganisms, including bacteria, viruses, fungi, and protozoa. In laboratory settings, axenic animals provide a controlled environment for research, allowing scientists to study biological processes without the confounding effects of microbial inhabitants. This level of sterility is crucial for various experimental protocols, especially in immunology and pharmacology, where the presence of microorganisms could skew results.

In contrast, the other terms do not imply the same level of sterility. "Contaminated" indicates the presence of unwanted microorganisms, while "infected" specifically denotes the presence of pathogenic microorganisms that negatively affect the health of the animal. "Defined flora" refers to animals that harbor a known composition of microorganisms, not necessarily free from all microbes, thus differentiating it from the axenic classification. Therefore, "axenic" is the most accurate term to describe animals completely free from microorganisms.