XLD AGAR, USP

Cat. no. G65 XLD Agar, USP, 15x100mm Plate, 18ml 10 plates/bag

INTENDED USE

Hardy Diagnostics XLD Agar, USP is recommended for use as a selective and differential medium for the isolation of gram-negative enteric pathogens. The U.S. Pharmacopeia National Formulary (USP) describes the use of XLD Agar for growth promotion and indication of Salmonella in the microbiological examination of nonsterile products.(2)

SUMMARY

Xylose Lysine Deoxycholate (XLD) Agar was developed by Taylor for the differentiation, isolation, and identification of enteric pathogens, and to support the growth of more fastidious enteric organisms. XLD Agar was especially designed to allow the growth of Shigella species, and is a proven medium for the isolation of this organism. It has also been found to be an excellent medium for isolating Salmonella species as well.

The selective agent in XLD Agar is sodium deoxycholate, which inhibits the growth of gram-positive organisms. The carbohydrate source is xylose which is fermented by most enterics, except for Shigella species; therefore, these colonies appear red on this medium. A second differential mechanism for Salmonella is employed by the addition of lysine. Lysine decarboxylation reverts the pH of the medium to an alkaline condition. To avoid this reversal to a Shigella reaction, lactose and sucrose are added in excess. The addition of sodium thiosulfate and ferric ammonium citrate as a sulfur source and indicator, respectively, allow hydrogen sulfide forming organisms to produce colonies with black centers under alkaline conditions. Organisms that ferment xylose are lysine decarboxylase-negative and do not ferment lactose or sucrose; therefore, these organisms cause an acid pH shift in the medium and form yellow colonies. Examples of such organisms are Citrobacter spp., Proteus spp., and Escherichia coli.

FORMULA

Ingredients per liter of deionized water:*

Lactose 7.5gm
Sucrose 7.5gm
Sodium Thiosulfate 6.8gm
L-Lysine 5.0gm
Sodium Chloride 5.0gm
Xylose 3.5gm
Yeast Extract 3.0gm
Sodium Deoxycholate 2.5gm
Ferric Ammonium Citrate 0.8gm
Phenol Red 0.08gm
Agar 13.5gm

Final pH 7.4 +/- 0.2 at 25ºC.

* Adjusted and/or supplemented as required to meet performance criteria.

STORAGE AND SHELF LIFE

Storage: Upon receipt store at 2-8ºC. away from direct light. Media should not be used if there are any signs of deterioration (shrinking, cracking, or discoloration), contamination, or if the expiration date has passed. Product is light and temperature sensitive; protect from light, excessive heat, moisture, and freezing.

PRECAUTIONS

PROCEDURE

Specimen Collection: Consult listed references for information on sample preparation and pre-enrichment.(1,2)

Method of Use: Allow plates to warm to room temperature and the agar surface to dry before inoculating. Subculture 0.1ml of the pre-enriched sample from Rappaport Vassiliadis Enrichment Broth (Cat. no. K246) and streak the sample with a sterile loop to obtain isolated colonies. Incubate plates aerobically at 30-35ºC. for 18-48 hours. Examine growth for typical colony morphology and perform appropriate biochemical or confirmatory tests as outlined in the references.(1,2)

INTERPRETATION OF RESULTS

Salmonella spp. appear as red colonies with or without black centers. Lysine-positive organisms appear red. Shigella spp. also appear red. Other lysine-negative fermenters, such as E. coli, Citrobacter and Proteus spp. appear yellow. Consult listed references for further biochemical and confirmatory tests required for identification.(1,2)

LIMITATIONS

MATERIALS REQUIRED BUT NOT PROVIDED

Standard microbiological supplies and equipment such as loops, slides, staining supplies, other culture media (Cat. no. K246), microscopes, incinerators, and incubators, etc., as well as serological and biochemical reagents, are not provided.

QUALITY CONTROL

Test Organisms
Inoculation Method*
Incubation
Results
Time
Temperature
Atmosphere
Salmonella enterica
ATCC® 14028**
J 18-24hr 30-35°C Aerobic Growth; red colonies with black centers
Shigella flexneri
ATCC® 12022**
A 24hr 35°C Aerobic Growth; red to pink colonies
Enterococcus faecalis
ATCC® 29212**
B 24hr 35°C Aerobic Partial to complete inhibition; small clear colonies
Escherichia coli
ATCC® 25922**
B 24hr 35°C Aerobic Partial to complete inhibition; yellow to yellow-red colonies

** Tested in accordance with USP <62>.(2)

User Quality Control

PHYSICAL APPEARANCE

XLD Agar, USP should appear clear, and red in color.

REFERENCES

1. The Official Compendia of Standards. USP-NF. United States Pharmacopeial Convention, Rockville, MD.

2. The Official Compendia of Standards. USP General Chapter <62> Microbiological Examination of Nonsterile Products: Testsfor Specified Microorganisms. USP-NF. United States Pharmacopeial Convention Inc., Rockville, MD.

ATCC is a registered trademark of the American Type Culture Collection.

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