CRITERION™ XLD AGAR

Cat. no. C7320 CRITERION™ XLD Agar 114gm
Cat. no. C7321 CRITERION™ XLD Agar 500gm
Cat. no. C7322 CRITERION™ XLD Agar 2kg
Cat. no. C7323 CRITERION™ XLD Agar 10kg
Cat. no. C7324 CRITERION™ XLD Agar 50kg

INTENDED USE

Hardy Diagnostics CRITERION™ XLD Agar is recommended for use as a selective and differential medium for the isolation of gram-negative enteric bacilli such as Salmonella and Shigella.

This dehydrated culture medium is a raw material intended to be used in the making of prepared media products, which will require further processing, additional ingredients, or supplements.

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.(5) 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, and these colonies appear red on this medium as a result. 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, allows hydrogen sulfide forming organisms to produce colonies with black centers, under alkaline conditions. Organisms which ferment xylose, are lysine- decarboxylase-negative, and do not ferment lactose or sucrose cause an acid pH in the medium, and form yellow colonies. Examples of such organisms are Citrobacter spp., Proteus spp., and Escherichia coli .

FORMULA

Gram weight per liter: 57.0gm/L
Lactose 7.5gm
Sucrose 7.5gm
Sodium Thiosulfate 6.8gm
Sodium Chloride 5.0gm
L-Lysine 5.0gm
Xylose 3.75gm
Yeast Extract 3.0gm
Sodium Deoxycholate 2.5gm
Ferric Ammonium Citrate 0.8gm
Phenol Red 0.08gm
Agar 15.0gm

Final pH 7.4 +/- 0.2 at 25ºC.

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

STORAGE AND SHELF LIFE

Store the sealed bottle(s) containing dehydrated culture medium at 2-30ºC. Dehydrated culture medium is very hygroscopic. Keep lid tightly sealed. Protect dehydrated culture media from moisture and light. The dehydrated culture media should be discarded if it is not free-flowing or if the color has changed from its original pink.

Store the prepared culture media at 2-8ºC.

PRECAUTIONS

METHOD OF PREPARATION FOR DEHYDRATED CULTURE MEDIA

1. Suspend 57.0gm of the dehydrated culture media in 1 liter of distilled or deionized water.

2. Heat to boiling and mix to dissolve completely. Avoid overheating.

3. Cool to 45-50ºC. and dispense as desired.

PROCEDURE AND INTERPRETATION OF RESULTS

For information on procedures and interpretation of results, consult listed references or refer to the prepared media Instructions for Use (IFU) for Cat. No. G65.

LIMITATIONS

As some species of Salmonella may form red colonies without a black center, which resemble Shigella colonies. In addition, a few species of Shigella ferment lactose, and Salmonella that fail to decarboxylate lysine would not be detected on this medium.

Red, false-positive colonies may occur with some Proteus and Pseudomonas spp.

Processing delays of over 2-3 hours of unpreserved stool specimens greatly jeopardizes the recovery of many enteric pathogens, as these organisms are very susceptible to the acidic changes that occur with a temperature drop of the feces.

Longer incubation may result in false-positive results.

MATERIALS REQUIRED BUT NOT PROVIDED

Standard microbiological supplies and equipment such as autoclaves, incinerators, and incubators, etc., are not provided.

QUALITY CONTROL

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

User Quality Control

PHYSICAL APPEARANCE

CRITERION™ XLD Agar powder should appear homogeneous, free-flowing, pink in color. The prepared media should appear clear, and red in color.

REFERENCES

1. Tille, P., et al. Bailey and Scott's Diagnostic Microbiology, C.V. Mosby Company, St. Louis, MO.

2. Isenberg, H.D. Clinical Microbiology Procedures Handbook, Vol. I, II & III. American Society for Microbiology, Washington, D.C.

3. MacFaddin, J.F. 1985. Media for Isolation, Cultivation, Identification, Maintenance of Bacteria, Vol. I. Williams & Wilkins, Baltimore, MD.

4. Jorgensen., et al. Manual of Clinical Microbiology, American Society for Microbiology, Washington, D.C.

6. Quality Assurance for Commercially Prepared Microbiological Culture Media, M22. Clinical and Laboratory Standards Institute (CLSI - formerly NCCLS), Wayne, PA.

8. Atlas, R.M. 1997. Handbook of Microbiological Media, 2nd ed. CRC Press, Boca Raton, FL.


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

062916gr