Final answer:
To isolate gram-negative bacteria from gram-positive bacteria without distinguishing biochemical differences, a researcher should use a selective medium like MacConkey agar, which inhibits the growth of gram-positive bacteria and supports gram-negative bacteria.
Step-by-step explanation:
A researcher who wants to isolate gram-negative bacteria from gram-positive bacteria should use a selective medium. An ideal choice is MacConkey agar, which contains bile salts and crystal violet, inhibiting the growth of many gram-positive organisms and therefore favoring the growth of gram-negative bacteria, especially enterics that are adapted to the presence of bile salts. The selective medium would allow the researcher to grow gram-negative bacteria without the need to distinguish them biochemically from other gram-negative bacteria.