However, rare strains of Salmonella are capable of lactose fermentation, which will appear in the same way. Those that are may be identified as red or yellow colonies with a black centre, indicating that they are fermenting sugar and probably not Salmonella. Most of these are inhibited on HEA plates by the inclusion of bile salts, so encountering a black colony that is not Salmonella is unusual, although not unheard of. The pH of XLD agar is 7.4 and it contains the pH indicator methyl red, which gives a red or pink colour. It has a pH of approximately 7.4, leaving it with a bright pink or red appearance due to the indicator Phenol red. įew sulfur-reducing bacteria exist other than Salmonella, which can be isolated from the intestines. Applications: Xylose lysine deoxycholate agar (XLD agar) is a selective growth medium, which is used for isolation of Salmonella spp. 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. The presence of thiosulfate or ferric ammonium citrate in the medium produces a black precipitate in the presence of H 2S, allowing Shigella – which does not produce H 2S, and appears as green colonies – to be distinguished from Salmonella – which does produce hydrogen sulfide and appears as black colonies.

Peptone metabolism by Shigella and Salmonella alkalises the medium, turning a pH indicator blue. Since most bacteria can use the sugars in preference to peptone, these 'uninteresting' bacteria acidify the medium and turn a pH indicator yellow or red. What color are Shigella colonies on Salmonella Shigella Agar colorless Result Interpretation on Salmonella Shigella Agar The resulting bacterial colonies will appear colorless with black centers. The plates contain various sugar sources ( lactose, sucrose, and salicin), none of which can be used by either Shigella or Salmonella, but the medium also includes peptone which can be used as a carbon source. Some Proteus strains will give black-centered colonies on XLD Agar. Use of these plates assumes that the user is not interested in other enteric pathogens such as Klebsiella or Escherichia. 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. Effectively, HEA uses a metabolic assay to divide colonies into ' Salmonella and Shigella' and 'everything else'. However, while the other bacteria may be clinically relevant, the assay does not discriminate among them. The definitive use of HEA is to discriminate between Shigella and Salmonella, although many other species may grow on these plates. It is named after the Hektoen Institute in Chicago, where researchers developed the agar. HEA contains indicators of lactose fermentation and hydrogen sulfide production as well as inhibitors to prevent the growth of Gram-positive bacteria. Hektoen enteric agar ( HEK, HE or HEA) is a selective and differential agar primarily used to recover Salmonella and Shigella from patient specimens. Gram negative Shigella sonnei bacteria which spent 48 hours cultured on Hektoen enteric agar (HEK).

Shigella xld agar