Intro To Biochemical Testing II: Differential And Selective Media Flashcards
What are the ways selection can be carried out through?
- Control of ingredients of the medium
- Alteration of atmospheric conditions
- Adjustment of the incubation temperature
Define selective medium
Permits the growth of certain organisms while preventing or retarding the growth of others
Define differential medium
Causes certain colonies to develop differently from other organisms present
What media type is the MacConkey Agar?
Selective and differential
Components and selective agents of MacConkey Agar are:
Bile salts, crystal violet, lactose, and neutral red.
Bile salts and crystal violet inhibit growth of gram + while lactose and neutral red help to differentiate lactose fermenters (pink colonies) and non-fermenters (clear colonies)
Organisms encourages to grow in MacConkey Agar:
Differentiates among gram - while inhibiting gram +
What media type is the Hektoen Enteric Agar?
Selective and differential
What components or selective agents are in Hektoen Enteric Agar?
Bile salts and sugars. Bile salts inhibit growth of gram + while the sugars help to show fermenters (yellow or pink colonies) and non-fermenters (blue or green with black centers)
Organisms encouraged to grow on Hektoen Enteric Agar?
Gram - rods, particularly Enterobacteriaceae family bacteria from fecal material
Hektoen Enteric Agar also detects:
Rapid sugar-fermenting organisms in fecal specimens and hydrogen sulfide producers
What media type is the Eosin Methylene Blue (EMB)?
Selective and differential
What components or selective agents are in Eosin Methylene Blue?
Eosin, methylene blue, lactose: eosin and methylene blue inhibit growth of gram + bacteria while lactose fermenters appear purple-pink (low acid) or metallic green (high acid) and have a dark center. Non-lactose fermenters appear colorless
Organisms encouraged to grow on Eosin Methylene Blue
Gram - rods, particularly Enterobacteriaceae family bacteria from fecal material
What media type is Mannitol-Salt Agar (MSA)?
Selective and differential
What components or selective agents are in Mannitol-Salt Agar?
Mannitol and sodium chloride: mannitol differentiates. If mannitol fermentation occurs, lactic acid produced, pH drops making plate yellow. Salt portion selective for halophiles (orgs that survive high salt conc.), orgs that can’t withstand high salt content will not grow.
