Final answer:
Columbia CNA agar is both selective, inhibiting gram-negative bacteria, and differential, distinguishing bacterial species based on hemolysis. It helps to differentiate species like Streptococcus pyogenes or Staphylococcus aureus from non-hemolytic colonies.
Step-by-step explanation:
When observing bacterial colonies on various media, the appearance of the colonies can indicate the possible types of bacteria present in a sample. Columbia CNA (Colistin-Nalidixic Acid) agar is a type of agar that can be classified as both selective and differential.
It is selective because it contains antibiotics that inhibit the growth of gram-negative bacteria, allowing for the isolation of gram-positive bacteria. It is also differential because it contains blood, which allows for the observation of hemolytic activity.
This hemolytic activity can help differentiate between species of gram-positive bacteria based on their ability to break down red blood cells. For instance, colonies that display a clear zone of hemolysis are likely to be hemolytic strains, which could be Streptococcus pyogenes or Staphylococcus aureus, whereas non-hemolytic colonies would not have this clear zone around them.