Xylose Lysine Deoxycholate agar (XLD agar) is a selective growth medium used in the isolation of Salmonella and Shigella species from clinical samples and from food.[1][2] The agar was developed by Welton Taylor in 1965.[3] 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 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. The agar itself will turn red due to the presence of Salmonella type colonies.
Shigella species: red colonies.
Coliforms: yellow to orange colonies.
Pseudomonas aeruginosa: pink, flat, rough colonies. This type of colony can be easily mistaken for Salmonella due to the color similarities.
XLD agar contains:
Yeast extract
3g/l
L-Lysine
5g/l
Xylose
3.75g/l
Lactose
7.5g/l
Sucrose
7.5g/l
Sodium deoxycholate
1g/l
Sodium chloride
5g/l
Sodium thiosulfate
6.8g/l
Ferric ammonium citrate
0.8g/l
Phenol red
0.08g/l
Agar
12.5g/l
^Zajc-Satler J, Gragas AZ (1977). "Xylose lysine deoxycholate agar for the isolation of Salmonella and Shigella from clinical specimens". Zentralbl Bakteriol [Orig A]. 237 (2–3): 196–200. PMID 848209.
^Nye KJ, Fallon D, Frodsham D, et al. (April 2002). "An evaluation of the performance of XLD, DCA, MLCB, and ABC agars as direct plating media for the isolation of Salmonella enterica from faeces". J. Clin. Pathol. 55 (4): 286–8. doi:10.1136/jcp.55.4.286. PMC 1769632. PMID 11919214.
^Taylor, Welton I. (1965). "Isolation of shigellae. I. Xylose lysine agars; new media for isolation of enteric pathogens". American Journal of Clinical Pathology. 44 (4): 471–475. doi:10.1093/ajcp/44.4_ts.471. PMID 5839918.
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