prototrophic cells (also referred to as the 'wild type') are self sufficient producers of all required metabolites (e.g. amino acids, lipids, cofactors), while auxotrophs require to be on medium with the metabolite that they cannot produce. for example saying a cell is methionine auxotrophic means that it would need to be on a medium containing methionine or else it would not be able to replicate. in this example this is because it is unable to produce its own methionine (methionine auxotroph). however, a prototroph or a methionine prototrophic cell would be able to function and replicate on a medium with or without methionine.