Performance should include plating on media that enables differentiation between Gram-negative and Gram-positive organisms. It should also include an explanation of the media types, examples of each, and their purposes:

• CNA (colistin naladixic acid) agar isolates gram-positive bacteria.
• EMB (eosin methylene blue) agar isolates gram-negative bacteria.
• MacConkey agar isolates gram-negative bacteria.
• Selective vs. differential media

Process/Skill Questions:

• How does CNA agar isolate gram-positive bacteria?
• Which media would be useful in isolating E. coli? What components of the agar make this possible?
• What is the difference between nutritive, enrichment, selective, and differential media?