Xylose Lysine Deoxycholate (XLD) agar 50 FAQs and 30 MCQs
Master Xylose Lysine Deoxycholate (XLD) Agar with this definitive resource for clinical microbiologists and lab professionals. Our expertly curated 50 FAQs and 30 MCQs cover everything from basic principles to advanced troubleshooting for enteric pathogen isolation.
📌 Key Learning Objectives:
✅ 50 Comprehensive FAQs
- Selective & Differential Mechanisms:
- Xylose fermentation (yellow → red colonies)
- Lysine decarboxylation (black centers in Salmonella)
- Deoxycholate inhibition of Gram-positives
- Colony Morphology Mastery:
- Salmonella: Red with black centers
- Shigella: Red/pink (non-H₂S producers)
- Proteus: Yellow (swarming inhibition)
- Protocol Optimization:
- Inoculation techniques for stool/rectal swabs
- Incubation conditions (35-37°C, 18-24h)
- QC strains (Salmonella Typhi vs. E. coli)
- Clinical Pitfalls:
- Overgrowth by Pseudomonas
- False positives from Citrobacter
✅ 30 Challenging MCQs
◼️ Differentiating Salmonella from Arizona species
◼️ Interpreting weak H₂S production
◼️ CLSI M35 guidelines for stool cultures
◼️ Comparison to Hektoen/SS agar
🎯 Perfect For:
- Medical lab scientists (MLS/MLT certification prep)
- Public health outbreak investigators
- Microbiology students (USMLE, ASCP exams)
✨ Bonus Tools:
✔ Color Atlas of XLD Results
✔ Flowchart: “XLD vs. MAC for Bloody Diarrhea”
✔ Downloadable QC Checklist
Xylose Lysine Deoxycholate (XLD) agar 50 FAQs
What is XLD Agar?
A selective and differential medium for isolating Salmonella and Shigella from clinical, food, and environmental samples.
Who developed XLD Agar?
Developed by Taylor for isolating Shigella from stool specimens.
What is the pH of XLD Agar?
Approximately 7.4 ± 0.2.
Why does XLD Agar appear pink/red?
Due to the pH indicator phenol red.
Is XLD Agar selective or differential?
Both selective (inhibits Gram-positive bacteria) and differential (distinguishes pathogens by biochemical reactions).
What are the key ingredients in XLD Agar?
Yeast extract, xylose, lactose, sucrose, lysine, sodium deoxycholate, sodium thiosulfate, ferric ammonium citrate, and phenol red.
Why is sodium deoxycholate added?
Acts as a selective agent to inhibit Gram-positive bacteria.
What is the role of xylose in XLD Agar?
Fermented by most enterics (except Shigella), aiding differentiation.
Why is lysine included?
Helps differentiate Salmonella (lysine decarboxylase-positive) from non-pathogens.
How is XLD Agar prepared?
Suspend 55–56.7 g in 1L water, heat to boil (do not autoclave), cool to 50°C, and pour into plates.
Can XLD Agar be autoclaved?
No, overheating destroys selective agents (e.g., deoxycholate).
What happens if XLD Agar is overheated?
Forms a precipitate, reducing efficacy.
How does XLD Agar differentiate Salmonella and Shigella?
Salmonella ferments xylose (turns yellow), then decarboxylates lysine (reverts to red); Shigella remains red.
Why do Salmonella colonies have black centers?
H₂S production (from sodium thiosulfate) reacts with ferric ammonium citrate.
How does phenol red indicate fermentation?
Turns yellow at acidic pH (sugar fermentation) and red at alkaline pH (lysine decarboxylation).
Why are lactose and sucrose added?
Prevent false alkalinization by non-pathogens (e.g., E. coli).
Why might Proteus give false positives?
Some strains produce H₂S and black centers but are non-pathogenic.
What do Salmonella colonies look like on XLD?
Red with black centers (H₂S-positive) or red (H₂S-negative).
How does Shigella appear on XLD?
Red colonies (no H₂S or xylose fermentation).
What color are E. coli colonies?
Large, flat, yellow (ferments lactose/sucrose).
How do Klebsiella/Enterobacter appear?
Mucoid yellow colonies.
What indicates H₂S production?
Black centers in colonies.
What samples is XLD Agar used for?
Stool, food, water, dairy, and clinical specimens.
Is XLD Agar used in food testing?
Yes, per ISO 6579 (Salmonella) and ISO 21567 (Shigella).
Can XLD detect Pseudomonas?
Yes, but they appear as red colonies (non-target).
What are limitations of XLD Agar?
False positives (Proteus, Pseudomonas), no growth of S. Paratyphi A, overgrowth of E. coli.
Why might S. Paratyphi A be missed?
Forms red colonies without H₂S, resembling Shigella.
How long should XLD plates be incubated?
24–48 hours (longer incubation causes false positives).
Can XLD Agar replace biochemical tests?
No, further tests (e.g., serology, PCR) are needed for confirmation.
Why might Citrobacter confuse results?
Some strains ferment lactose (yellow) but also produce H₂S (black centers).
How should XLD Agar be stored?
Dehydrated powder: dry, 10–30°C; prepared plates: 2–8°C, protected from light.
What QC organisms are used for XLD?
Salmonella Typhimurium (black centers), Shigella flexneri (red), E. coli (inhibited/yellow).
What happens if XLD Agar expires?
Reduced selectivity/differentiation; discard if contaminated.
Why does Salmonella revert to red after initial yellow?
Lysine decarboxylation raises pH.
How does XLD compare to MacConkey Agar?
XLD is more selective for Salmonella/Shigella; MacConkey detects lactose fermenters.
Can XLD Agar detect Vibrio?
No, it’s not optimized for Vibrio.
Why is yeast extract included?
Provides B vitamins and nitrogen for growth.
What happens if NaCl is omitted?
Osmotic imbalance may affect bacterial growth.
Can XLD Agar be used for urine cultures?
Not standard; designed for enteric pathogens.
Why do Shigella colonies stay red?
They cannot ferment xylose or produce H₂S.
What if XLD Agar turns yellow before inoculation?
Contamination or improper storage; discard.
Why are some colonies partially black?
Weak H₂S production or mixed cultures.
How to avoid precipitate formation?
Avoid prolonged heating; prepare small batches.
Can XLD Agar be used for Yersinia?
No, use CIN Agar for Yersinia.
Why is Enterococcus inhibited on XLD?
Sodium deoxycholate targets Gram-negatives.
Is XLD Agar FDA-approved?
Yes, for food and clinical testing (USP standards).
Does XLD meet ISO requirements?
Yes, per ISO 6579 (Salmonella) and ISO 21567 (Shigella).
Can XLD replace enrichment broths?
No, enrichment (e.g., Selenite broth) improves sensitivity.
Why is ferric ammonium citrate added?
Reacts with H₂S to form black ferrous sulfide.
Can XLD detect Aeromonas?
Yes, but they appear as yellow colonies (non-target).
Xylose Lysine Deoxycholate (XLD) agar 30 MCQs
- What is the primary purpose of XLD Agar?
a) Isolate Gram-positive cocci
b) Differentiate Salmonella and Shigella✔
c) Culture anaerobic bacteria
d) Identify fungal pathogens - XLD Agar is:
a) Only selective
b) Only differential
c) Both selective and differential✔
d) Neither selective nor differential - Who developed XLD Agar?
a) Robert Koch
b) Louis Pasteur
c) Taylor✔
d) Hans Christian Gram - The pH of XLD Agar is approximately:
a) 5.4
b) 7.4✔
c) 8.4
d) 9.0
- Which ingredient inhibits Gram-positive bacteria?
a) Xylose
b) Sodium deoxycholate✔
c) Phenol red
d) Ferric ammonium citrate - Why is xylose included in XLD Agar?
a) To inhibit Shigella
b) To differentiate Shigella (non-fermenter) from other enterics✔
c) To enhance Gram-positive growth
d) To act as a pH stabilizer - What happens if XLD Agar is autoclaved?
a) Enhances selectivity
b) Destroys sodium deoxycholate✔
c) Improves lactose fermentation
d) Turns the medium blue - Which component detects H₂S production?
a) Phenol red
b) Sodium thiosulfate + ferric ammonium citrate✔
c) Lactose
d) Yeast extract
- How do Salmonella colonies typically appear on XLD Agar?
a) Yellow with yellow zones
b) Red with black centers✔
c) Blue with halos
d) Colorless - What color are Shigella colonies on XLD?
a) Yellow
b) Red✔
c) Black
d) Green - Which organism produces large, flat, yellow colonies?
a) Salmonella
b) Shigella
c) E. coli✔
d) Pseudomonas - Black centers in colonies indicate:
a) Lactose fermentation
b) H₂S production✔
c) Gram-positive contamination
d) Sucrose utilization
- Phenol red turns yellow when the pH is:
a) Alkaline
b) Neutral
c) Acidic✔
d) Variable - Why does Salmonella initially turn the medium yellow?
a) Lactose fermentation
b) Xylose fermentation✔
c) Lysine decarboxylation
d) H₂S production - What causes the medium to revert to red after xylose exhaustion?
a) Sucrose fermentation
b) Lysine decarboxylation✔
c) Sodium chloride breakdown
d) Yeast extract metabolism
- Which organism may cause false-positive black centers?
a) Shigella
b) Proteus✔
c) E. coli
d) Enterococcus - Incubation beyond 48 hours may lead to:
a) Enhanced Shigella growth
b) False-positive reactions✔
c) Medium hardening
d) Loss of H₂S production - Which Salmonella species may resemble Shigella on XLD?
a) S. Typhi
b) S. Paratyphi A✔
c) S. Typhimurium
d) S. Enteritidis
- XLD Agar is recommended by which ISO standard for Salmonella?
a) ISO 6579✔
b) ISO 21567
c) ISO 10932
d) ISO 17025 - Which sample type is NOT typically tested with XLD?
a) Stool
b) Dairy products
c) Blood cultures✔
d) Water - What additional test is required after XLD Agar?
a) Gram staining
b) Biochemical/serological confirmation✔
c) Antibiotic susceptibility
d) PCR for viruses
- Which sugar prevents false alkalinization by coliforms?
a) Xylose
b) Lactose✔
c) Glucose
d) Maltose - What is the role of ferric ammonium citrate?
a) Nutrient source
b) H₂S indicator✔
c) Gram-positive inhibitor
d) pH stabilizer - Why is Enterococcus inhibited on XLD?
a) Due to sodium deoxycholate✔
b) Lack of vitamins
c) High pH
d) Oxygen sensitivity
- If a colony is red without a black center, it is likely:
a) Salmonella H₂S-negative
b) Shigella✔
c) E. coli
d) Proteus - Yellow colonies with yellow zones suggest:
a) Salmonella
b) Klebsiella✔
c) Pseudomonas
d) Staphylococcus - A mucoid yellow colony is characteristic of:
a) Salmonella
b) Enterobacter✔
c) Shigella
d) Vibrio
- QC strain for Salmonella on XLD:
a) E. coli ATCC 25922
b) Salmonella Typhimurium ATCC 14028✔
c) Staphylococcus aureus ATCC 25923
d) Pseudomonas aeruginosa ATCC 27853 - How should XLD Agar be stored?
a) Frozen at -20°C
b) Room temperature in light
c) 2–8°C, protected from light✔
d) In a desiccator at 50°C - Expired XLD Agar may lead to:
a) Faster results
b) Loss of selectivity✔
c) Enhanced H₂S production
d) Blue colonies
