• MacConkey agar
  • Contains lactose, bile salts, sodium chloride, and a pH indicator
  • Lactose fermenters (e.g., E. coli and Klebsiella)
  • Pink colonies (indicator turns pink when pH is lowered due to fermentation of lactose to acidic hydrogen sulfide)
• Hektoen enteric agar
  • Composed of proteose peptone, various sugars (e.g., lactose, sucrose, salicin), sodium thiosulfate, and iron (III) ammonium citrate
  • Enteric bacteria (e.g., Salmonella and Shigella)
    • Salmonella: black colonies
      • Salmonella produces hydrogen sulfide (H₂S) using thiosulfate, which results in black colony formation, that can be differentiated from Shigella colonies that do not produce H₂S.
    • Shigella: green colonies