Some bacteriologic culture media are used for both selective and differential purposes. Which of the following media may be considered differential, but not selective
Culture techniques remain a major mechanism in the isolation and identification of specific microorganisms as etiologic agents of disease. While some bacteria may be isolated from a lesion or other clinical specimen in pure culture, more often the pathologic agent will need to be isolated from a mixture of many bacteria (including nonpathogenic normal flora). A selective medium contains an added chemical that will inhibit large numbers of unwanted bacteria (normal flora) and allow specific types to grow. For example, manuitol salt agar selects for Staphylococcus, Thayer-Main agar selects for Neisseria gonorrhoeae and N meningitidis, and bile-esculin agar is selective for enterococci (enteric streptococci). Differential medium will include a system to distinguish specific species within a selected group. The mannitol in mannitol salt agar will distinguish between Staphylococcus epidermidis (mannitol negative-nonpathogenic) and S. aureus (mannitol positive- pathogenic). X and V factors are used to identify species of Haemophilus. Factor X is heme and factor V is nicotinamide (NAD) or other coenzymes. X and V are supplied to the culture system by being placed on paper strips on the agar surface. Blood agar (5% sheep RBC enriched) is an enriched medium, supplying nutrients for a wide variety of microorganisms. It may serve as a base medium for selection or differentiation, but does not perform these functions by itself. For example, it can differentiate between types of hemolysis.
Mannitol salt agar
Blood agar plate
Blood agar containing X and V factors
Thayer-Main agar