Selective and Differential Media

Selective media..

allow certain types of organisms to grow and inhibit the growth of other organisms (a particular selective agent can inhibit gram-negative bacteria while promoting the growth of gram-positive bacteria)

Differential media..

are used to differentiate closely related organisms or groups of organisms (useful in differentiating bacteria according to some biochemical characteristics)

Examples of selective media include…

eosin methylene blue agar, mannitol salt sugar, macconkey agar, and blood agar

Examples of differential media include…

Eosin Methylene blue agar, Mannitol salt sugar, MacConkey Agar, and Blood Agar

Phenylethyl Alcohol (PEA) Agar

Selective medium for the isolation of gram-positive organisms and inhibits growth of gram-negative organisms

How does it inhibit growth gram -ve bacteria?

Phenylethyl alcohol acts to disrupt the lipid structure in the membrane of gram-negyative allowing influx of otherwise blocket molecules and allowing leackage of cellular potassium, thus interfering with DNA synthesis

Plate A

PEA Agar with growth of Gram +ve organism (left) and Gram –ve organism (right) with inhibited growth.

Columbia CNA with 5% Sheep Blood Agar

Differential media of gram-positive organisms based on hemolytic reaction. Selective media against gram-negative organisms due to antibiotics colistin and nalidixic (CNA)

Hemolysis…

the destruction of red blood cells (erythrocytes)

Beta (B) hemolysis is…

complete breakdown of the RBCs and is recognized by clearing around the colonies

Gamma (y) hemolysis…

causes no damage to RBCs and no change in the medium

Mannitol Salt Agar (MSA)

Selective media for halophilic bacteria (staphylococci). 7.5% NaCl, MSA contains high concentrations of salt. Differential media with respect to mannitol fermentation. Contains the sugar mannitol and the pH indicator phenol red

Mannitol Salt Agar (MSA)

Yellow indicates mannitol positive (+). No color change indicates mannitol negative (-)

Eosin Methylene Blue (EMB) Agar

Selective media for gram-negative species and inhibit gram-positive organisms. Differential media with respect to lactose fermentation.