Isolement sélectif et différenciation des entérobactéries.

Summary and Explanation
MacConkey Agar is based on the bile salt-neutral red-lactose agar of MacConkey.
The original MacConkey medium was used to differentiate strains of Salmonella typhosa from members of the coliform group. Formula modifications improved the growth of Shigella and Salmonella strains. These modifications included the addition of 0.5% sodium chloride, decreased agar content, and altered bile salts and neutral red concentrations. The formula improvements gave improved differential reactions between these enteric pathogens and the coliform group.

MacConkey Agar contains crystal violet and bile salts that inhibit gram-positive organisms and allow gram-negative organisms to grow. Isolated colonies of coliform bacteria are brick red in color and may be surrounded by a zone of precipitated bile. This bile precipitate is due to a local pH drop around the colony due to lactose fermentation. Colonies that do not ferment lactose (such as typhoid, paratyphoid and dysentery bacilli) remain colorless. When lactose nonfermenters grow in proximity to coliform colonies, the surrounding medium appears as cleared areas. MacConkey Agar is listed as one of the recommended media for the isolation of E. coli from nonsterile pharmaceutical products.

MacConkey Agar Base is prepared without added carbohydrates, which permits their addition either individually or in combination. It is recommended that carbohydrates such as sucrose or lactose be added in a concentration of 1% to the basal medium.

MacConkey Agar without Crystal Violet is a differential medium that is less selective than MacConkey Agar. The lack of Crystal violet permits the growth of Staphylococcus and Enterococcus.

Staphylococci produce pale pink to red colonies and enterococci produce compact tiny red colonies either on or beneath the surface of the medium. The medium is used also to separate Mycobacterium fortuitum and M. chelonae from other rapidly growing mycobacteria.
MacConkey Agar without Crystal Violet or Salt and MacConkey Agar without Salt (which also lacks crystal violet) are differential media used for isolating and cultivating gram-negative enteric organisms and gram-positive cocci from waters, feces and other sources suspected of containing these organisms, as well as limiting the swarming of Proteus species.

Principles of the Procedure
Peptones are sources of nitrogen and other nutrients. Yeast extract is a source of trace elements, vitamins, amino acids and carbon. Lactose is a fermentable carbohydrate. When lactose is fermented, a local pH drop around the colony causes a color change in the pH indicator (neutral red) and bile precipitation. Bile salts, bile salts no. 3, oxgall and crystal violet are selective agents that inhibit growth of gram-positive organisms. Sodium chloride maintains osmotic balance in the medium. Magnesium sulfate is a source of divalent cations. Agar is the solidifying agent.

Formule :
Difco™ MacConkey Agar
Approximate Formula* Per Liter :
 - Pancreatic Digest of Gelatin : 17.0 g
 - Peptones (meat and casein) : 3.0 g
 - Lactose : 10.0 g
 - Bile Salts No. 3 : 1.5 g
 - Sodium Chloride : 5.0 g
 - Agar : 13.5 g
 - Neutral Red : 0.03 g
 - Crystal Violet : 1.0 mg

Poids	0,8 Kg   (800 g)
Volume	0,0056 m³   (5600 cm³)
Largeur	0,15 m   (15 cm)
Longueur	0,15 m   (15 cm)
Hauteur	0,25 m   (25 cm)

Les dimensions, le poids et le volume sont bruts (emballage inclus)