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selective and differential media- part 3
Written on: Tuesday, July 28, 2009
Time: 9:14 AM
selective and differential media
Hektoen Enteric Agar (HE) is a selective and differential medium designed to isolate and differentiate members of the species Salmonella and Shigella from other Enterobacteriaceae. Bile salts and the dyes bromthymol blue and acid fuchsin inihibit the growth of most Gram positive organisms. Lactose, sucrose, and salicin provide fermentable carbohydrates to encourage the growth and differentiation of enterics.
Sodium thiosulfate provides a source of sulfur. Ferric ammonium citrate provides a source of iron to allow production of hydrogen sulfide from sodium thiosulfate, which provides a source of sulfur. Ferric ammonium citrate also allows the visualiztion of hydrogen sulfide production by reacting with hydrogen sulfide gas to form a black precipitate.
Enterics that ferment one or more of the carbohydrates will produce yellow to salmon-colored colonies. Non-fermenters will produce blue-green colonies. Organisms that reduce sulfur to hydrogen sulfide will produce black colonies or blue-green colonies with a black center.
•MacConkey Agar (MAC) is a selective and differential medium designed to isolate and differentiate enterics based on their ability to ferment lactose. Bile salts and crystal violet inhibit the growth of Gram positive organisms. Lactose provides a source of fermentable carbohydrate, allowing for differentiation. Neutral red is a pH indicator that turns red at a pH below 6.8 and is colorless at any pH greater than 6.8.
Organisms that ferment lactose and thereby produce an acidic environment will appear pink because of the neutral red turning red. Bile salts may also precipitate out of the media surrounding the growth of fermenters because of the change in pH. Non-fermenters will produce normally-colored or colorless colonies.
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selective and differential media- part 3
Written on: Tuesday, July 28, 2009
Time: 9:14 AM
selective and differential media
Hektoen Enteric Agar (HE) is a selective and differential medium designed to isolate and differentiate members of the species Salmonella and Shigella from other Enterobacteriaceae. Bile salts and the dyes bromthymol blue and acid fuchsin inihibit the growth of most Gram positive organisms. Lactose, sucrose, and salicin provide fermentable carbohydrates to encourage the growth and differentiation of enterics.
Sodium thiosulfate provides a source of sulfur. Ferric ammonium citrate provides a source of iron to allow production of hydrogen sulfide from sodium thiosulfate, which provides a source of sulfur. Ferric ammonium citrate also allows the visualiztion of hydrogen sulfide production by reacting with hydrogen sulfide gas to form a black precipitate.
Enterics that ferment one or more of the carbohydrates will produce yellow to salmon-colored colonies. Non-fermenters will produce blue-green colonies. Organisms that reduce sulfur to hydrogen sulfide will produce black colonies or blue-green colonies with a black center.
•MacConkey Agar (MAC) is a selective and differential medium designed to isolate and differentiate enterics based on their ability to ferment lactose. Bile salts and crystal violet inhibit the growth of Gram positive organisms. Lactose provides a source of fermentable carbohydrate, allowing for differentiation. Neutral red is a pH indicator that turns red at a pH below 6.8 and is colorless at any pH greater than 6.8.
Organisms that ferment lactose and thereby produce an acidic environment will appear pink because of the neutral red turning red. Bile salts may also precipitate out of the media surrounding the growth of fermenters because of the change in pH. Non-fermenters will produce normally-colored or colorless colonies.
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