Growth of vibrio cholerae on TCBS medium produces colonies:
a- Yellow colonies***
b- Blue colonies
c- Green colonies
d- White colonies
e- Red colonies.
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By its acronym in English (Xylose Lysine Deoxycholate agar) is a selective culture medium used in the isolation of Salmonella and Shigella species from clinical and food samples.1 2 It has a pH of approximately 7.4, the medium is characterized by a bright pink color or a red appearance due to the presence of phenol red as an indicator. The fermentation of sugar lowers the pH and the red phenol indicator records it by changing to yellow. Most bacteria present in the intestine, including Salmonella, can ferment xylose sugar to produce acid, so the color of the medium will turn yellow. Shigella colonies cannot ferment xylose and therefore remain red.
After depleting the xylose supply, the Salmonella colonies will decarboxylate the lysine, increasing the pH of the medium again, which will return to its initial red. Salmonella colonies metabolize thiosulfate to produce hydrogen sulphide, which leads to the formation of colonies with black centers, allowing them to be differentiated from Shigella colonies, which will have a similar color after sufficient time.
Other enterobacteria, such as E. coli, will ferment the lactose present in the medium to a point that will prevent pH reversal by decarboxylation and acidify the medium by turning it yellow.
- Salmonella species: red colonies, some with black centers. The agar will turn yellow due to the presence of Salmonella type colonies, although it will return to its initial red once the xylose is depleted.
- Shigella species: red colonies.
- Coliforms: yellow to orange colonies.
- Pseudomonas aeruginosa: pink, flat, rough colonies. This type of colony can easily be confused with Salmonella due to color similarities.
a- Yellow colonies***
b- Blue colonies
c- Green colonies
d- White colonies
e- Red colonies.
-----------------------
By its acronym in English (Xylose Lysine Deoxycholate agar) is a selective culture medium used in the isolation of Salmonella and Shigella species from clinical and food samples.1 2 It has a pH of approximately 7.4, the medium is characterized by a bright pink color or a red appearance due to the presence of phenol red as an indicator. The fermentation of sugar lowers the pH and the red phenol indicator records it by changing to yellow. Most bacteria present in the intestine, including Salmonella, can ferment xylose sugar to produce acid, so the color of the medium will turn yellow. Shigella colonies cannot ferment xylose and therefore remain red.
After depleting the xylose supply, the Salmonella colonies will decarboxylate the lysine, increasing the pH of the medium again, which will return to its initial red. Salmonella colonies metabolize thiosulfate to produce hydrogen sulphide, which leads to the formation of colonies with black centers, allowing them to be differentiated from Shigella colonies, which will have a similar color after sufficient time.
Other enterobacteria, such as E. coli, will ferment the lactose present in the medium to a point that will prevent pH reversal by decarboxylation and acidify the medium by turning it yellow.
- Salmonella species: red colonies, some with black centers. The agar will turn yellow due to the presence of Salmonella type colonies, although it will return to its initial red once the xylose is depleted.
- Shigella species: red colonies.
- Coliforms: yellow to orange colonies.
- Pseudomonas aeruginosa: pink, flat, rough colonies. This type of colony can easily be confused with Salmonella due to color similarities.
