Xylose Lysine Deoxycholate agar.

Ho Ho Ho to all Microbiologists.

Today the topic for discussion is XLD Agar.

Now I don't know about you, but when I was in my earlier Microbiology years I used to never remember the full form of XLD agar. I used to often call it as Xyline lactose decarboxylase agar, or something else which starts with the same initials and has some scientific meaning.

So I kinda manged to remember it this way.

Hope it helps you the way it did to me.

So XLD Agar is a selective and differential medium as it inhibits the growth of other organisms and helps differentiate between Enterobacteriaceae esp. Salmonella and Shigella spp. Sodium deoxycholate is the inhibiting agent which inhibits all the gram positive organisms. Xylose is particularly incorporated in the medium because it is utilized by almost all enterics except Shigella spp which helps in it's differentiation. 

Along with xylose, other sugars in the medium are lactose and sucrose to support the growth of other enterics. Fermentation of the sugar forms acid in the medium thus reducing it's pH causing the color of the medium to change from red to yellow.

Similarly lysine is incorporated to differentiate Salmonella as it has enyzme lyine decarboxylase which converts lysine to cadaverine and hence a reddish color is found around the colonies which decarboxylate lysine. This is because the amnes produced are alkaline in nature and thus give the red color due to increase in pH.

Another differentiating component that is added in to the medium is sodium thiosulpahte and ferric ammonium citrate. This is an H2S system which helps in visualization of hydrogen sulphide produced, resulting in the formation of colonies with black centers. The non-pathogenic H2S producers do not decarboxylase lysine; therefore, the acid reaction produced by them prevents the blackening of the colonies.

I know this is a bit complicated media but that's what Microbiology is about.

Hope this helps you and your viva :D

And to remind you all how amazing are you guys, we will have a high six.

-Staph.