Strains grown separately in complete medium strain A Strain B met bio thrt leu thi met bio thr leu thi Mix A and B in complete medium: incubate overnight Plat n Plate on minimal medium minimal medium and incubate and incubate No growth A and B No growth (no prototrophs) met bio thr leu thi (no prototrophs) and mett bjo thr ley thi Plate on minimal medium and incubate Colonies of prototrophs