Culture media are built to answer a specific question, and the question determines the recipe. A selective medium contains an inhibitor, such as a dye, an antibiotic, or a high salt concentration, that suppresses organisms you are not looking for while the target organism grows through it. A differential medium contains a substrate and an indicator that change color when a particular biochemical reaction occurs, so colonies of different species become visually distinguishable on the same plate. An enriched medium supplies extra nutrients, most often blood or a blood product, that fastidious organisms cannot synthesize themselves; blood agar is both enriched and differential because it supports growth and reveals hemolysis.
A single medium can carry more than one of these roles. MacConkey agar is selective because bile salts and crystal violet inhibit Gram-positive organisms, and it is differential because lactose fermentation acidifies the medium and turns the colonies pink while non-fermenters stay colorless. The choice is driven by the hypothesis from the smear: a Gram-negative rod from a urinary specimen points toward MacConkey agar, while a Gram-positive coccus in chains from a wound points toward blood agar with an eye toward hemolysis.
Incubation conditions follow the same logic. Most human pathogens grow at 35 to 37 degrees Celsius, but the atmosphere matters: carbon dioxide at roughly 5 percent supports capnophiles such as Haemophilus influenzae and Neisseria species, while obligate anaerobes require oxygen to be excluded entirely, which is achieved in an anaerobic jar or chamber. Setting the wrong atmosphere does not simply slow growth; it can make a pathogen invisible on the plate.