Final answer:
Blood agar is useful as a primary isolation medium mainly because it offers a nutrient-rich environment for various microorganisms and serves as a differential medium, allowing the observation of hemolytic versus non-hemolytic bacteria by the clear zones they produce.
Step-by-step explanation:
Blood agar is useful as a primary isolation medium because it provides a nutrient-rich environment for various microorganisms. This supports the growth of a large number of bacteria, making it an ideal initial medium for culturing samples with diverse bacterial flora. Furthermore, blood agar is also a differential medium, which means it allows for the differentiation of bacteria according to their hemolytic properties. Hemolysis refers to the breakdown of red blood cells, a process that can be visually observed on blood agar; hemolytic bacteria produce a clear zone in the agar where the red blood cells have been destroyed, such as with hemolytic Streptococcus, while non-hemolytic bacteria, like Staphylococcus, leave the red blood cells intact and no clear zone is formed.