Final answer:
An organism is determined to be an auxotroph by using replica plating to expose it to different media, one that is nutritionally complete and others that lack specific nutrients.
Step-by-step explanation:
To determine if an organism is an auxotroph, scientists can use a method known as replica plating. This technique involves exposing a population of bacterial cells to a mutagen, allowing them to form colonies on a nutritionally complete medium. After colonies form, cells from these colonies are transferred to both a complete medium and media lacking specific nutrients, such as amino acids, to determine nutritional requirements.
For example, a histidine auxotroph can be identified by comparing growth on plates that contain histidine to those that lack it. If a mutant grows on the medium containing histidine but shows no growth on the medium lacking histidine, it is classified as a histidine auxotroph. Cells that are unable to grow in the absence of an essential nutrient, which the wild-type strain can synthesize, are considered to be auxotrophic for that nutrient.
Therefore, by conducting replica plating and observing the growth patterns on different media, researchers can pinpoint exactly which nutrient an organism can no longer synthesize and thus requires from its environment.