12.6.2: XLD Agar
- Page ID
- 123449
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)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. \(\PageIndex{1}\)).
![E. coli on XLD agar](https://bio.libretexts.org/@api/deki/files/83376/xldecoli.gif?revision=1)
- 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. \(\PageIndex{2\)).
![Shigella on XLD agar](https://bio.libretexts.org/@api/deki/files/83378/xldshig2.gif?revision=1)
- 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. \(\PageIndex{3}\)).
![Photograph of <EM> Enterobacter cloacae</EM>
growing on XLD agar and showing acid produced from the fermentation of lactose and/or sucrose, thus lowering the pH and turning the phenol
red from red (alkaline) to yellow (acid). In addition, breakdown of the amino
acid lysine has produced alkaline end products which raises the pH and has causing some
of the agar to turn a deeper red. No hydrogen sulfide production is seen.](https://cwoer.ccbcmd.edu/science/microbiology/lab%20manual/lab12/images/xldentc.gif)
- 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.
![Photograph of <EM>Salmonella</EM>
growing on XLD agar and showing neither lactose nor sucrose being fermented so that no acid is produced and the phenol red remains red. The amino acid
lysine has been broken down (decarboxylated) producing alkaline end products which has caused the phenol red in the agar to turn a deeper red. In addition, hydrogen sulfide production has caused some of the colonies to appear black.](https://cwoer.ccbcmd.edu/science/microbiology/lab%20manual/lab12/images/XLD%20agar_Salmonella_Nathan%20Reading_Halesowen_UK.jpg)
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)