http://www.sciencedirect.com/topics/immunology-and-microbiology/auxotrophy
http://www.sciencedirect.com/science/article/pii/S0167730608605098
https://www.ncbi.nlm.nih.gov/books/NBK22011/
Cells are usually prototrophic but some can be auxotrophic, meaning they would need that certain metabolite to grow. The "wild type" cell is prototrophic and noted as "+" or "WT" in relation to the metabolite while the auxotrophic version is noted with a "-" in relation to the metabolite. For example, a cell can be noted as "his-" if it needs histidine or "his+" if it does not.
DIAGRAM
Consider this diagram of two colonies of bacteria place on different media. If it is assumed that colony A is prototrophic and colony B is auxotrophic to metabolite "1", then A can grown on any minimal media. Colony B, however, can only grow if there is at least minimal media and metabolite "1".
Peer Review (Kristen)
I thought your article was really good and was simplified enough to understand. Also, it was well organized and expanded well on the topic. Some suggestions: