XLT4 Agar

XLT4 Agar is used with Tergitol 4 for the isolation of non-typhi Salmonella spp and is not intended for use in the diagnosis of disease or other conditions in humans. XLT4 Agar is a selective differential isolation medium for the specific detection of Salmonella spp. from environmental and food samples. Due to its highly selective nature, XLT4 Agar is particularly effective when used with samples where overgrowth of contaminating flora is expected, for example, fecallycontaminated agricultural samples. Developed to perform as per Miller & Tate in 1990, this medium was found to improve the recovery of non- typhi Salmonella from chicken and farm environmental samples. Dusch and Altwegg further established the application of XLT4 Agar to salmonellae detection in clinical samples, with the notable exceptions of Salmonella typhi and Salmonella paratyphi. The presence of peptone and yeast extract provides sufficient nutrients to allow the optimal growth of Salmonella spp. Selectivity is provided by the anionic surfactant Niaproof® 4 (formerly known as Tergitol-4 / sodium tetradecylsulfate). This compound acts as an effective selective agent which is active against Grampositive and many Gram-negative organisms, including Proteus spp. Differentiation is based on fermentation of the sugars xylose, lactose and sucrose in addition to the decarboxylation of lysine. The inclusion of the pH indicator, phenol red, provides visual evidence of a pH decrease (yellow) or increase (red) in the medium. Ammonium iron (III) citrate is present to distinguish hydrogen-sulfide (H2S) producing from non-H2S producing organisms. Most enteric organisms, except Shigella, will ferment xylose to produce acid. However, the salmonellae will also decarboxylate the lysine to keep the pH neutral to alkali, thus maintaining red coloration. At nearneutral pH Salmonella can produce H2S from the reduction of ammonium iron (III) citrate and thiosulphate ions producing black or black-centered colonies. Non H2S-producing salmonellae will be red without a black center. Other Enterobacteriaceae (non-salmonellae) which are not inhibited by Niaproof-4, will ferment xylose, lactose and/or sucrose but will not decarboxylate lysine. This fermentation activity causes a decrease in pH, resulting in a color change within the colonies from red to yellow.