<Li> Hektoen enteric agar is designed to isolate and recover fecal bacteria belonging to the Enterobacteriaceae family . It is particularly useful in isolating Salmonella and Shigella . </Li> <Li> MacConkey agar is a selective and differential medium used to differentiate between Gram - negative bacteria while inhibiting the growth of Gram - positive bacteria . The addition of bile salts and crystal violet to the agar inhibits the growth of most Gram - positive bacteria, making MacConkey agar selective . Lactose and neutral red are added to differentiate the lactose fermenters, which form pink colonies, from lactose nonfermenters that form clear colonies . An alternative medium, eosin methylene blue serves a similar purpose . </Li> <Li> Mannitol salt agar is also a selective and differential medium . The mannitol indicates organisms that ferment mannitol: mannitol fermentation produces lactic acid, lowering the pH and turning the plate yellow . The salt is to select for halophiles; organisms that cannot withstand a high salt content are unable to grow well . </Li> <Li> Mueller - Hinton agar contains beef infusion, peptone, and starch, and is used primarily for antibiotic susceptibility testing . It can be in a form of blood agar . </Li>

When would blood agar media be used and why