Final answer:
Nutrient agar containing vancomycin is a selective medium because it inhibits the growth of non-resistant bacteria, allowing for the isolation of vancomycin-resistant organisms.
Step-by-step explanation:
Nutrient agar that contains vancomycin is selective for the growth of selected microorganisms. Vancomycin is an antibiotic that is effective against gram-positive organisms and can inhibit the growth of many bacteria that are not resistant to it, such as vancomycin-resistant enterococci (VRE) and Staphylococcus aureus.
Therefore, when used in a nutrient agar, it makes the medium selective by supporting the growth of the organism of interest while inhibiting others. This type of media is particularly important in clinical settings to isolate and identify pathogenic bacteria that are resistant to vancomycin.