MacConkey agar is a selective medium for Gram-negative bacteria. It contains bile salts and crystal violet that inhibit Gram-positive organisms. It is also differential: lactose fermenters (e.g. E. coli) produce acid, turning colonies pink/red due to neutral red indicator; non-fermenters (e.g. Salmonella, Shigella) remain colourless. While Salmonella and Shigella grow on it, the primary selective property is for Gram-negative organisms.