XLD agar

Xylose lysine deoxycholate agar (XLD agar) is a growth medium used in the isolation of "Salmonella" and "Shigella" species from clinical samples and from food. [cite journal |author=Zajc-Satler J, Gragas AZ |title=Xylose lysine deoxycholate agar for the isolation of Salmonella and Shigella from clinical specimens |journal=Zentralbl Bakteriol [Orig A] |volume=237 |issue=2-3 |pages=196–200 |year=1977 |pmid=848209] [cite journal |author=Nye KJ, Fallon D, Frodsham D, "et al" |title=An evaluation of the performance of XLD, DCA, MLCB, and ABC agars as direct plating media for the isolation of Salmonella enterica from faeces |journal=J. Clin. Pathol. |volume=55 |issue=4 |pages=286–8 |year=2002 |month=April |pmid=11919214 |pmc=1769632 |url=http://jcp.bmj.com/cgi/pmidlookup?view=long&pmid=11919214] It has a pH of approximately 7.4, leaving it with a bright pink or red appearance due to the indicator phenol red. Sugar fermentation lowers the pH and the phenol red indicator registers this by changing to yellow. Most gut bacteria, including "Salmonella", can ferment the sugar xylose to produce acid; "Shigella" colonies cannot do this and therefore remain red. After exhausting the xylose supply "Salmonella" colonies will decarboxylate lysine, increasing the pH once again to alkaline and mimicking the red "Shigella" colonies. Salmonellae metabolise thiosulfate to produce hydrogen sulfide, which leads to the formation of colonies with black centers and allows them to be differentiated from the similarly coloured "Shigella" colonies.

Other Enterobacteria such as "E. coli" will ferment the lactose and sucrose present in the medium to an extent that will prevent pH reversion by decarboxylation and acidify the medium turning it yellow.

*"Salmonella" species: red colonies, some with black centers.
*"Shigella" species: red colonies.
*Coliforms: yellow to orange colonies.
*"Pseudomonas aeruginosa": pink, flat, rough colonies.

XLD agar contains:

ee also

*R2a agar
*MRS agar
*agar plate

References

External links

* [http://www.hpa-standardmethods.org.uk/documents/msop/pdf/msop14.pdf Standard Operating Procedure for Xylose lysine desoxycholate (XLD). agar]