selective, differential and enriched media
DESCRIPTION
SELECTIVE, DIFFERENTIAL AND ENRICHED MEDIA. Selective media: used to select (isolate) specific groups of bacteria; chemical substances in the media inhibit the growth of one type of bacteria while permitting growth of another (MSA, EMB, MacConkey) - PowerPoint PPT PresentationTRANSCRIPT
SELECTIVE, DIFFERENTIAL AND ENRICHED MEDIA
• Selective media: used to select (isolate) specific groups of bacteria;
• chemical substances in the media inhibit the growth of one type of bacteria while permitting growth of another (MSA, EMB, MacConkey)
• Differential media: distinguishes among morphologically and biochemically related groups of organisms;
• chemical compounds (following inoculation and incubation) produce a characteristic change in the appearance of bacterial growth and/or the medium surrounding the colonies (MSA, EMB, MacConkey)
• Enriched media: supplemented with highly nutritious materials, such as blood, serum, or yeast extracts, for the cultivation of fastidious organisms
Mannitol Salt Agar(MSA)
• Mannitol salt agar is both selective and differential
• Selective: It favors organisms capable of tolerating high salt concentrations (7.5 % NaCl)
• Differential: It distinguishes bacteria based on their ability to ferment mannitol
• Differentiates Staphylococcus species, by mannitol fermentation
• (S. aureus ferments, S. epidermidis does not)
• Phenol red is the pH indicator
Neutral - Basic pHred at 7.4 to 8.4
Acidic pH yellow below 6.8
RESULTS:
• Positive Results:The development of “yellow halos” around the bacterial growth means mannitol has been fermented and acid end products have been produced (S. aureus)
• Negative Results:No color change in the medium is a negative result (S. epidermidis)
• No growth on the medium indicates a Gram- organism (E. coli)
PROCEDURE:
On an MSA Plate:
E.coli
(No growth)
S.epidermidis
(growth, red)
S. aureus (growth, yellow)
• Eosin methylene blue agar is a selective and differential medium
• Selective: EMB contains the dyes methylene blue and eosin which inhibit Gram + bacteria, thus favoring growth of Gram –
• Differential: EMB contains lactose, thus allowing for the distinction between lactose fermenters and nonferments
Eosin Methylene Blue Agar (EMB)
Typically used for the family Enterobacteriaceae—enteric (gut) bacteria (Gram- rods)
They may be divided into those that produce acid from lactose fermentation (coliforms) and those that do not
Coliforms include Escherichia coli and Enterobacter aerogenes
• Large amounts of acid from lactose fermentation cause the dyes to precipitate on the colony surface, producing a black center or a “green metallic sheen” (E. coli)
• Smaller amounts of acid production result in pink coloration of the growth (E. aerogenes)
• Nonfermenting enterics do not produce acid so their colonies remain colorless or take on the color of the media (P. vulgaris)
• No growth indicates a Gram + organism (S.aureus)
Results
•Large amounts of acid from lactose fermentation cause the dyes to precipitate on the colony surface, producing a black center or a “green metallic sheen” (E. coli)
PROCEDURE:
On an EMB Plate:
P. vulgaris
(growth,
no color)
S. aureus
(no growth)
E. aerogenes
black colonies or green metallic sheen
E.coli
black colonies or green metallic sheen
MacConkey Agar(MAC)
• A selective and differential medium used to isolate members of the Enterobacteriaceae
• Contains nutrients, including lactose, as well as bile salts, neutral red and crystal violet
• Bile salts and crystal violet inhibit growth of G+ organisms (selective)
• Neutral red is a pH indicator that is colorless, but yellow above pH 8 and red at pH less than 6.8(differential)
• Acid accumulating from lactose fermentation turns the colorless neutral red to a red color—therefore coliforms produce a red “halo” on the medium (E.coli, E.aerogenes)
• Lactose nonfermenters will grow, but don’t produce acid. Therefore, the neutral red remains colorless (P. vulgaris)
• No growth indicates a Gram + organism (S.aureus)
Macconkey agar with lactose(left) and non-lactose(right) fermenters
PROCEDURE:
• On a MacConkey Agar Plate:
P.vulgaris (colorless) S.aureus (no growth)
E.coli (Red)E.aerogenes(Red)