Germ-free animals are animals that have no microorganisms living in or on it. Such animals are raised within germ-free isolators in order to control their exposure to viral, bacterial or parasitic agents. When known strains of bacteria or microbiota are introduced to a germ-free animal, it usually referred to as a gnotobiotic animal

Gnotobiotic animal

A gnotobiotic animal is an animal in which only certain known strains of bacteria and other microorganisms are present. Technically, the term also includes germ-free animals, as the status of their microbial communities is also known...

, however technically speaking, germ-free animals are also gnotibiotic because the status of their microbial community is known.

Germ-free animals are used in the study of probiotic

Probiotic

Probiotics are live microorganisms thought to be beneficial to the host organism. According to the currently adopted definition by FAO/WHO, probiotics are: "Live microorganisms which when administered in adequate amounts confer a health benefit on the host"...

research and other animal research requiring careful control of outside contaminants that can affect the experiment.