XLT4 Supplement

Description

They reported the isolation of non-typhi Salmonella from chicken and farm environmental drag-swab samples from heavily contaminated samples. XLT4 Agar can be used clinically to screen stool samples for non-typhoid Salmonella.

This medium allows the optimum growth of Salmonella. Differentiation of Salmonella from other organisms in this medium is based on the fermentation of carbohydrates (lactose, xylose, sucrose) with the resulting production of hydrogen sulfide. H₂S production is detected by the reaction of the iron salt, colonies appearing black or black-centered. Sodium thiosulfate and ferric ammonium citrate are the H₂S indicators. The bacteria that decarboxylate the L-lysine to cadaverine are identified by the presence of a purple-red color around the colonies due to the elevation of the pH. Phenol red is the pH indicator. Sodium thiosulfate is also added as a source of inorganic sulfur. Yeast extract and peptone are a nitrogen and amino acids source. Bacteriological agar is the solidifying agent.

Typical Salmonella colonies (H₂S-positive) appear black or black-centered with a yellow halo after 18-48 hours of incubation at a temperature of 35 ± 2 °C. Upon continued incubation, the colonies become entirely black or pink to red with black centers. Colonies of H₂S-negative Salmonella strains appear pink-yellow.

Most Citrobacter colonies are yellow without evidence of blackening. The growth of Enterobacter aerogenes and Escherichia coli is markedly inhibited; colonies that grow in this medium appear yellow without evidence of blackening. The growth of Proteus, Pseudomonas and Yersinia enterocolitica is markedly to completely inhibited. Shigella species are partially inhibited and colonies appear red.

Technique

Add 4.6 mL of XLT4 supplement to 1 L of XLT4 Agar Base (LC-1159). Mix well and heat with frequent agitation until completely dissolved. Boil for one minute. AVOID OVERHEATING. DO NOT AUTOCLAVE. Distribute into sterile containers.

Instructions for use

• Inoculate the sample in a pre-enrichment medium, such as Tetrathionate Broth (LC-1114).
• Incubate at 35 ± 2 °C for 18-24 hours.
• Spread or streak the sample from the enrichment medium on the surface of the XLT4 Agar Base.
• Incubate aerobically at a temperature of 35 ± 2 °C for 18-48 hours.

Physical/chemical control

Microbiological control

Distribute the complete medium, cooled to 50 °C, in plates.

Inoculate: practical range 100 ± 20 CFU, min. 50 CFU (productivity) / 10⁴-10⁶ (selectivity).

Analytical methodology according to ISO 11133:2014/Amd 1:2018; A2:2020.

Aerobiosis. Incubation at 35 ± 2 °C, reading at 18-48 h.

Sterility control

Incubation 48 hours at 30-35 °C and 48 hours at 20-25 °C: NO GROWTH.

Check at 7 days after incubation in same conditions.

• Miller, R. G., and C. R. Tate. 1990. XLT4: A highly selective plating medium for the isolation of Salmonella. The Maryland Poultryman, April:2-7.
• Tate, C. R., R. G. Miller, and E. T. Mallinson. 1992. Evaluation of two isolation and two non-isolation methods for detecting naturally occurring salmonellae from broiler flock environmental drag-swab samples. J. Food Prot. 55:964-967.
• Dusch, H., and M. Altwegg. 1995. Evaluation of five new plating media for the isolation of Salmonella species. J. Clin. Microbiol. 33:802-804.