The growth and maintenance of a single species in isolation, free from foreign or contaminating species. Isolation is usually achieved by growing the species in an environment that was previously sterilized, and was thereby rid of contaminating organisms. Since, from a practical viewpoint, the contaminating organisms usually encountered are microorganisms, axenic cultures, whether of invertebrates (for example, nematodes) or vertebrates (for example, rodents), are often referred to as germ-free. Indeed, the terms axenic and germ-free are occasionally used interchangeably. Gnotobiotic is also often used interchangeably with axenic; however, in common practice gnotobiotic specifically refers to germ-free conditions.