Xylose

Xylose

Xylose is a sugar first isolated from wood, and named for it. Xylose is classified as a monosaccharide of the aldopentose type, which means that it contains five carbon atoms and includes an aldehyde functional group. It is the precursor to hemicellulose, one of the main constituents of biomass...

 lysine

Lysine

Lysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG....

 deoxycholate agar

Agar

Agar or agar-agar is a gelatinous substance derived from a polysaccharide that accumulates in the cell walls of agarophyte red algae. Throughout history into modern times, agar has been chiefly used as an ingredient in desserts throughout Asia and also as a solid substrate to contain culture medium...

(XLD agar) is a selective growth medium

Growth medium

A growth medium or culture medium is a liquid or gel designed to support the growth of microorganisms or cells, or small plants like the moss Physcomitrella patens.There are different types of media for growing different types of cells....

 used in the isolation of Salmonella

Salmonella

Salmonella is a genus of rod-shaped, Gram-negative, non-spore-forming, predominantly motile enterobacteria with diameters around 0.7 to 1.5 µm, lengths from 2 to 5 µm, and flagella which grade in all directions . They are chemoorganotrophs, obtaining their energy from oxidation and reduction...

and Shigella

Shigella

Shigella is a genus of Gram-negative, nonspore forming, non-motile, rod-shaped bacteria closely related to Escherichia coli and Salmonella. The causative agent of human shigellosis, Shigella causes disease in primates, but not in other mammals. It is only naturally found in humans and apes. During...

species from clinical samples and from food. It has a pH

PH

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 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

Hydrogen sulfide

Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

, 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. 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
  Pseudomonas aeruginosa
  Pseudomonas aeruginosa is a common bacterium that can cause disease in animals, including humans. It is found in soil, water, skin flora, and most man-made environments throughout the world. It thrives not only in normal atmospheres, but also in hypoxic atmospheres, and has, thus, colonized many...
  
  : pink, flat, rough colonies. This type of colony can be easily mistaken for Salmonella due to the colour similarities.

XLD agar contains:

Yeast extract Yeast extract Yeast extract is the common name for various forms of processed yeast products made by extracting the cell contents ; they are used as food additives or flavourings, or as nutrients for bacterial culture media. They are often used to create savory flavors and umami taste sensations. Monosodium...	3g/l
L-Lysine Lysine Lysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG....	5g/l
Xylose Xylose Xylose is a sugar first isolated from wood, and named for it. Xylose is classified as a monosaccharide of the aldopentose type, which means that it contains five carbon atoms and includes an aldehyde functional group. It is the precursor to hemicellulose, one of the main constituents of biomass...	3.75g/l
Lactose Lactose Lactose is a disaccharide sugar that is found most notably in milk and is formed from galactose and glucose. Lactose makes up around 2~8% of milk , although the amount varies among species and individuals. It is extracted from sweet or sour whey. The name comes from or , the Latin word for milk,...	7.5g/l
Sucrose Sucrose Sucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...	7.5g/l
Sodium deoxycholate	1g/l
Sodium chloride Sodium chloride Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...	5g/l
Sodium thiosulfate Sodium thiosulfate Sodium thiosulfate , also spelled sodium thiosulphate, is a colorless crystalline compound that is more familiar as the pentahydrate, Na2S2O3•5H2O, an efflorescent, monoclinic crystalline substance also called sodium hyposulfite or “hypo.”...	6.8g/l
Ferric Ferric Ferric refers to iron-containing materials or compounds. In chemistry the term is reserved for iron with an oxidation number of +3, also denoted iron or Fe3+. On the other hand, ferrous refers to iron with oxidation number of +2, denoted iron or Fe2+...  ammonium Ammonium The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...  citrate Citrate A citrate can refer either to the conjugate base of citric acid, , or to the esters of citric acid. An example of the former, a salt is trisodium citrate; an ester is triethyl citrate.-Other citric acid ions:...	0.8 mg/l
Phenol red Phenol red Phenol red is a pH indicator that is frequently used in cell biology laboratories.-Chemical structure and properties:...	0.08g/l
Agar Agar Agar or agar-agar is a gelatinous substance derived from a polysaccharide that accumulates in the cell walls of agarophyte red algae. Throughout history into modern times, agar has been chiefly used as an ingredient in desserts throughout Asia and also as a solid substrate to contain culture medium...	12.5g/l