Selective and differential medias

Culture media

Chemically defined - exact composition unknown.

Agar-agar

Plant based gelatine derived from seaweed used as a gelling agent in cultative media.

Advantages of using agar agar

Inert, does not change in any way. Not metabolised by microorganisms. Does not melt or soften at incubator temp. Forms a solid gel at a low concentration.

Define selective media

Inhibits the growth of some bacteria whilst selecting for the growth of others.

Brilliant green agar

Inhibits the growth of G+ve bacteria, selects for G-ve bacteria.

Eosin methylene blue

Dyes inhibit g+ve bacteria, selects for g-ve bacteria. Differentiates bacteria that ferment lactose (e.g., e. coli).

Organisms that ferment lactose appear as pink and mucoid.

Differential media

Differentiates between different organisms growing on the same plate

Blood agar (differential)

TSA (Tryptic Soy Agar) with 5% sheep blood, differentiates types of streptococci.

Mannitol salt agar.

Used to differentiate between staphylococcus aureus and epidermis. High salt conc. of 7.5% which inhibits most bacteria.

MacConkeys agar

Used to identify salmonella, contains bile salts/crystal violet inhibiting G-ve.

General purpose media

Generally used for performing microbial counts. Contains nutrients to allow the growth of most bacteria and fungi. A total count of all growth is taken using this agar.

Barid parker agar

Both selective and differential. Selective isolation of G+ve staphylococci species.

Colonies show up with black convex colonies.

Rose Bengal chloramphenicol Agar

Selective and differential for fungi, contains chloramphenicol to supress bacterial growth. Differentiates yeast and mould.

Rose Bengal dye selectively absorbed by yeast. Yeast appears pink and mould appears black.

Malt extract agar

Both selective and differential. Acidic culture media cultivate fungi including mould and yeast whilst inhibiting bacterial growth.

High conc. of maltose makes it suitable for the growth of yeast and mould.

Yeast appear yellow/pink uniform round colonies. Mould appear filamentous.

Brilliance salmonella agar

Selective growth medium used in the isolation of salmonella and shigella species. Sugar fermentation will turn the pH from red to yellow. Purple colonies with black centres.

Brilliance Bacillus Cereus Agar

Selective growth medium used in the isolation of Bacillus Cereus. Appear as blue-green colonies.

Pseudomonas Selective Agar

Selective medium used for isolating pseudomonas spp. Colonies appear straw coloured with green pigmentation.

Chromogenic Listeria Agar

Selective medium used for isolating listeria monocytogenes. Colonies are blue green in colour with surrounding clearing zones. Listeria innocula show growth with no halo.

MacConkey broth

Differential medium containing neutral red for the detection of coliform organisms. Positive reaction to E. coli turns turbid, acid production and gas evolved, collected i the inverted tube.

Colour of broth changes from purple to yellow.

Sugar mannitol agar

pH agar turns yellow in the presence of acid. Aureus yellow and epidermis does not produce acid so no change.