Selective and Differential Media
  • Selective: agar has ingredients that prevent growth of some bacteria, permit only certain kinds of bacteria to grow.

ex) Eosin and methlyne blue prevent gram positive bacterial growth

pseudomonsas = gram negative (growing
streptococcus= gram positive (not growing)

ex) Manitol salt agar: has salt in VERY high concentration

bacteria can not grow easily because salt concentration is too high
staph grows on skin {grew on plate}
strep grows in throat {didn't grow on plate}

  • Differential: has ingredients that allow you to see metabolic differences between kinds of bacteria that you grow on it

ex) Manitol salt sugar: has very high salt concentration AND Manitol
manitol is a sugar that some bacteria can ferment plus methylred: dye that changes from red to yellow at low pH
ex) EMB
< Eosin: pH indicator (changes from pink to green at low pH levels)
< Lactose: can be fermented by some bacteria; producing an acid byproduct, but not by other bacteria.

Kassie Hein

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