12.6.2: XLD Agar

Xylose Lysine Desoxycholate (XLD) agar is used for isolating and differentiating Gram-negative enteric bacteria, especially intestinal pathogens such as Salmonella and Shigella. XLD agar contains sodium desoxycholate, which inhibits the growth of Gram-positive bacteria but permits the growth of Gram-negatives. It also contains the sugars lactose and sucrose, the amino acid L-lysine, sodium thiosulfate, and the pH indicator phenol red. Results can be interpreted as follows:

• If the Gram-negative bacterium ferments lactose and/or sucrose, acid end products will be produced and cause the colonies and the phenol red in the agar around the colonies to turn yellow (see Fig. \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): Escherichia coli Growing on XLD Agar. Acid from fermentation lowers the pH and turns the phenol red from red (alkaline) to yellow (acid). No hydrogen sulfide is produced. ( Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0 )

• If lactose and sucrose are not fermented by the bacterium but the amino acid lysine is decarboxylated, ammonia, an alkaline end product will cause the phenol red in the agar around the colonies to turn a deeper red (see Fig. \(\PageIndex{2\)).

Figure \(\PageIndex{2\): Shigella Growing on XLD Agar Neither lactose nor sucrose are fermented so no acid is produced. The amino acid lysine is not broken down (decarboxylated). No hydrogen sulfide is produced. ( Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0 )

• Sometimes the bacterium ferments the sugars producing acid end products and breaks down lysine producing alkaline end products. In this case some of the colonies and part of the agar turns yellow and some of the colonies and part of the agar turns a deeper red (see Fig. \(\PageIndex{3}\)).

Figure \(\PageIndex{3}\): Enterobacter cloacae Growing on XLD Agar. Acid from fermentation of lactose and/or sucrose lowers the pH and turns the phenol red from red (alkaline) to yellow (acid). In addition, breakdown of the amino acid lysine produces alkaline end products which raises the pH and causes some of the agar to turn a deeper red. No hydrogen sulfide is produced. ( By Nathan Reading from Halesowen, UK (Salmonella species on X.L.D. agar.) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons )

• If hydrogen sulfide is produced by the bacterium as a result of thiosulfate reduction, part or all of the colony will appear black (see Fig. \(\PageIndex{4}\)). Well-isolated colonies are usually needed for good results.

Figure \(\PageIndex{4}\): Salmonella Growing on XLD Agar. Neither lactose nor sucrose are fermented so no acid is produced. The amino acid lysine is broken down (decarboxylated) producing alkaline end products which cause the phenol red in the agar to turn a deeper red. Hydrogen sulfide production causes some of the colonies to appear black. ( Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0 )

Typical colony morphology on XLD agar is as follows:

1. Escherichia coli: flat yellow colonies; some strains may be inhibited.

2. Enterobacter and Klebsiella: mucoid yellow colonies.

3. Proteus: red to yellow colonies; may have black centers.

4. Salmonella: usually red colonies with black centers.

5. Shigella, Serratia, and Pseudomonas: red colonies without black centers

Keep in mind, however, that some species and subspecies do not show typical reactions.

Contributors and Attributions

• Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)