Single tube screen for enterics


Hardy Diagnostics EnteroScreen 4™ is recommended for use as a single tube screen for non-lactose-fermenting, oxidase-negative, enteric pathogens isolated from stool specimens. It is specifically designed to aid in the detection of Yersinia enterocolitica, Edwardsiella, Salmonella and Shigella species.


Three media, Triple Sugar Iron (TSI) Agar, Lysine Iron Agar (LIA), and Urea Agar, are used to screen non-lactose-fermentative cultures isolated from stool specimens. This battery of tests is helpful in differentiating Salmonella and Shigella from other members of the Enterobacteriaceae family.

Hardy Diagnostics EnteroScreen 4™ contains a unique media combination, which has been demonstrated to be less redundant and labor intensive, yet equivalent in performance, to the traditional three tube set. It is recommended that EnteroScreen 4™ be used in conjunction with the developed algorithm (see Figure 2 or Figure 3) for organism identification.

The EnteroScreen 4™ system consists of four layers in a single tube; three agar layers and a petrolatum layer. The tube contains, in order from bottom to top, Urea Agar, Petrolatum Plug, Modified Lysine Iron Agar, and Lysine Iron Agar. With this method, urease activity, lysine-decarboxylase, lysine-deaminase, gas production, and hydrogen sulfide production reactions can be simultaneously evaluated.

Urea Agar, the bottom agar layer, contains urea and phenol red as a pH indicator. Organisms capable of hydrolyzing urea form ammonia as a by-product, thus turning the medium alkaline. The pH indicator turns from pale orange to pink-red in color under these conditions. The reduced buffer content and peptone in this medium promote more rapid growth and reaction time for many members of the Enterobacteriaceae. Dextrose is included in the formulation to stimulate urease activity in organisms that hydrolyze urea slowly, and to exclude false-negative reactions.Urea Agar is added to the tube to differentiate other non-lactose-fermenters, i.e. Proteus, Morganella and Providencia species, from Yersinia enterocolitica, lactose-negative E. coli, Edwardsiella, Salmonella, and Shigella species. These latter organisms, all potential stool pathogens, can be separated from non-pathogenic organisms by the inability to hydrolyze urea. However, some strains of Yersinia enterocolitica may hydrolyze urea and can be differentiated from non-pathogenic organisms by the inability to deaminate lysine, lack of H2S production, and the absence of gas. The Urea Agar, is separated from the next agar layer by the addition of a sterile, hydrophobic petrolatum plug.

Modified Lysine Agar comprises the agar layer above the petrolatum plug and is used to detect by-products of dextrose-fermentation and the presence of lysine-decarboxylase. The media formulation is similar to traditional Lysine Iron Agar (LIA), however, hydrogen sulfide indicators are absent, to prevent excessive hydrogen sulfide production from masking the lysine-decarboxylase reaction. The media contains 0.1% percent dextrose as a carbohydrate source. Enteric organisms that are capable of fermenting dextrose will produce acid and sometimes gas, seen as cracks and bubbles in the medium.

Modified Lysine Iron Agar is especially useful for differentiating organisms from the genus Salmonella and Edwardsiella from Shigella spp. and Yersinia enterocolitica, as the lysine-decarboxylase reaction is significantly easier to interpret than the traditional LIA reaction. Salmonella and Edwardsiella species possess lysine-decarboxylase enzymes, which remove a molecule of carbon dioxide from lysine to form an alkaline reacting amine. The pH indicator in the media, bromcresol purple, turns the agar purple in the presence of these alkaline compounds. A yellow color, the result of dextrose-fermentation, is observed in the agar when lysine-decarboxylation does not occur, as is the case with Shigella and Yersinia enterocolitica species.

The top layer (the slanted portion) of EnteroScreen 4™ contains Lysine Iron Agar (LIA) which is used to detect hydrogen sulfide production as well as the deamination of lysine. LIA contains sodium thiosulfate and ferrous sulfate, which are added to detect organisms that have the ability to liberate sulfur from sulfur containing compounds in the form of hydrogen sulfide. Production of hydrogen sulfide generates an insoluble, heavy metal sulfide, which produces a black precipitate in the medium. Another advantage of LIA is that certain enteric organisms, like Proteus and Providencia species, possess enzymes responsible for the oxidative-deamination of lysine. If lysine is deaminated, in the presence of oxygen, a red color is observed on the slant. A negative lysine-deamination reaction, observed when the slant is purple in color, is indicative of Salmonella, Edwardsiella, Yersinia enterocolitica, and Shigella species. Furthermore, organisms can be isolated from the slant, and used to perform additional biochemical and serologic testing.

20 tubes ......................... Cat. no. HDL225


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