Yersinia Selective Agar 50 FAQs and 30 MCQs:
Yersinia Selective Agar 50 FAQs:
What is Yersinia Selective Agar?
A selective and differential medium for isolating Yersinia enterocolitica and other Yersinia species.
Who developed Yersinia Selective Agar?
It was first described by Schiemann as an alternative to MacConkey Agar.
What pathogens does Yersinia Selective Agar target?
Primarily Yersinia enterocolitica and Yersinia pseudotuberculosis.
What diseases does Yersinia enterocolitica cause?
Gastroenteritis, diarrhea, lymphadenopathy, pneumonia, and spontaneous abortions in animals.
Is Yersinia Selective Agar used for clinical or food testing?
Both; it is used for clinical specimens, food, and water samples.
What is the pH of Yersinia Selective Agar?
7.4 ± 0.2 at 25°C.
What sugar is used for differentiation in this medium?
Mannitol (fermentation turns colonies red).
What indicates mannitol fermentation on Yersinia Selective Agar?
Colonies appear dark pink/red with a bulls-eye appearance.
What inhibits Gram-positive bacteria in this medium?
Sodium deoxycholate and crystal violet.
Why are antibiotics added to Yersinia Selective Agar?
To increase selectivity (e.g., cefsulodin, novobiocin).
What is the role of sodium pyruvate in the medium?
Enhances bacterial recovery and growth stimulation.
What is the purpose of neutral red in the agar?
Acts as a pH indicator, turning red when mannitol is fermented.
Why is magnesium sulfate included?
Supports bacterial growth.
What is the solidifying agent in Yersinia Selective Agar?
Agar (12.5 g/L).
How is Yersinia Selective Agar prepared?
Suspend 29.02 g in 500 mL water, autoclave, then add antibiotics.
What sterilization method is used?
Autoclaving at 121°C for 15 minutes.
At what temperature should the medium be cooled before adding antibiotics?
45-50°C.
What supplements are added to Yersinia Selective Agar?
Cefsulodin (4 mg) and novobiocin (2.5 mg).
How should samples be streaked on the agar?
Streak for isolation or roll swabs on a small area before streaking.
What are the incubation conditions?
22-32°C for 24-48 hours or cold enrichment at 4°C for up to 21 days.
Why is cold enrichment sometimes used?
Enhances recovery of Yersinia from food/feces samples.
How often should subculturing be done during cold enrichment?
Periodically (e.g., weekly) onto fresh Yersinia Selective Agar.
How do Y. enterocolitica colonies appear?
Translucent with dark pink centers (bulls-eye) and a transparent border.
What distinguishes Y. pseudotuberculosis colonies?
Dark pink centers but no transparent zone around colonies.
What indicates mannitol-negative organisms?
Colorless, translucent colonies.
What does a zone of precipitated bile indicate?
Likely Yersinia growth due to pH change from mannitol fermentation.
Which bacteria may resemble Yersinia on this agar?
Aeromonas, Serratia liquefaciens, Citrobacter freundii, Enterobacter agglomerans.
How can Yersinia be confirmed if other bacteria grow?
Biochemical tests (e.g., oxidase, urease, motility).
What types of samples can be tested with Yersinia Selective Agar?
Clinical (stool), food (pork, dairy), water, and environmental samples.
Can this agar detect Yersinia pestis?
No, it is optimized for Y. enterocolitica and Y. pseudotuberculosis.
Why is Y. enterocolitica a concern in food safety?
It spreads via contaminated pork, milk, and water, causing foodborne outbreaks.
Does this medium inhibit E. coli and Proteus?
Yes, due to sodium deoxycholate and antibiotics.
What is the ISO standard for Yersinia detection using this agar?
ISO/DIS 10273:1994 recommends this method.
What if no growth appears after incubation?
Check cold enrichment, antibiotic concentration, or sample quality.
Why might non-Yersinia bacteria grow on the agar?
Overgrown samples or insufficient antibiotic supplementation.
Can this medium expire?
Yes, store below 30°C and use before the expiry date.
Is Yersinia Selective Agar safe to handle?
Follow lab safety protocols; consult the Material Safety Data Sheet (MSDS).
Can this medium be used for antibiotic susceptibility testing?
No, it is only for isolation and enumeration.
How does Yersinia Selective Agar differ from MacConkey Agar?
More selective for Yersinia due to antibiotics and bile salts.
Is CIN Agar the same as Yersinia Selective Agar?
Similar, but CIN Agar is an earlier formulation by Schiemann.
What alternatives exist for Yersinia isolation?
Salmonella-Shigella Agar (SSA) or MacConkey Agar, but less selective.
How should prepared plates be stored?
2-8°C, protected from light.
How long can prepared plates be stored?
Typically 1-2 weeks if sealed properly.
Can the powder form be stored at room temperature?
Yes, below 30°C in a dry place.
Why does Yersinia grow better at lower temperatures?
It is psychrotrophic, thriving at 4°C (cold enrichment improves detection).
What biochemical tests confirm Y. enterocolitica?
Urease-positive, oxidase-negative, and motility at 25°C (but not 37°C).
Does Y. pseudotuberculosis cause diarrhea?
No, it typically causes abdominal pain and fever without diarrhea.
How does Yersinia evade the immune system?
Via virulence factors (Yops) that inhibit phagocytosis.
What animals are common carriers of Yersinia?
Pigs are a major reservoir for Y. enterocolitica.
Can this agar detect antibiotic-resistant Yersinia?
No, additional antimicrobial susceptibility testing (AST) is needed.
Yersinia Selective Agar 30 MCQs:
- What is the primary purpose of Yersinia Selective Agar?
a) Isolate Salmonella
b) Isolate Yersinia enterocolitica and Y. pseudotuberculosis ✔
c) Detect E. coli O157:H7
d) Culture Staphylococcus aureus - Who developed Yersinia Selective Agar?
a) Robert Koch
b) Louis Pasteur
c) Schiemann ✔
d) Joseph Lister - Which disease is NOT caused by Yersinia enterocolitica?
a) Gastroenteritis
b) Bubonic plague (Y. pestis causes plague)✔
c) Pneumonia
d) Lymphadenopathy - What type of medium is Yersinia Selective Agar?
a) Enriched only
b) Selective and differential ✔
c) Transport medium
d) Chromogenic - Which of the following samples can be tested using Yersinia Selective Agar?
a) Blood only
b) Stool, food, and water ✔
c) Urine only
d) Sputum only
- What is the pH of Yersinia Selective Agar?
a) 6.2 ± 0.2
b) 7.4 ± 0.2 ✔
c) 8.0 ± 0.2
d) 5.5 ± 0.2 - Which sugar is used as a fermentable substrate in this medium?
a) Glucose
b) Lactose
c) Mannitol ✔
d) Sucrose - What indicates mannitol fermentation on Yersinia Selective Agar?
a) Yellow colonies
b) Dark pink/red colonies with a bulls-eye appearance ✔
c) Blue-green colonies
d) Black colonies - Which component inhibits Gram-positive bacteria?
a) Sodium chloride
b) Sodium deoxycholate and crystal violet ✔
c) Magnesium sulfate
d) Neutral red - Why are cefsulodin and novobiocin added?
a) To enhance pigment production
b) To increase selectivity for Yersinia ✔
c) To solidify the medium
d) To neutralize toxins - What is the role of neutral red in the agar?
a) Nutrient source
b) pH indicator (red at pH <6.8) ✔
c) Antibiotic
d) Bile salt - Which ingredient stimulates Yersinia growth?
a) Sodium pyruvate ✔
b) Crystal violet
c) Agar
d) Sodium chloride
- How is Yersinia Selective Agar sterilized?
a) Filtration
b) Autoclaving at 121°C for 15 minutes ✔
c) Boiling for 5 minutes
d) UV radiation - At what temperature should antibiotics be added?
a) 70°C
b) 37°C
c) 45-50°C ✔
d) Room temperature - What is the recommended incubation temperature?
a) 37°C only
b) 22-32°C ✔
c) 42°C
d) 15°C - Why is cold enrichment (4°C) used?
a) To kill other bacteria
b) To enhance Yersinia recovery ✔
c) To solidify the agar
d) To reduce contamination - How long can cold enrichment be performed?
a) 2 days
b) 1 week
c) Up to 21 days ✔
d) 48 hours
- How do Y. enterocolitica colonies appear?
a) Green metallic sheen
b) Dark pink centers with translucent borders (bulls-eye) ✔
c) Black with H₂S production
d) Mucoid and yellow - Which organism grows with dark pink centers but no translucent zone?
a) Y. pestis
b) Y. pseudotuberculosis ✔
c) E. coli
d) Proteus mirabilis - Mannitol-negative organisms produce:
a) Red colonies
b) Colorless, translucent colonies ✔
c) Blue colonies
d) Swarming growth - Which bacteria may resemble Yersinia on this agar?
a) Aeromonas and Citrobacter ✔
b) Streptococcus pyogenes
c) Neisseria gonorrhoeae
d) Clostridium difficile
- Which ISO standard recommends Yersinia Selective Agar?
a) ISO 6579 (Salmonella)
b) ISO 10273 (Yersinia) ✔
c) ISO 16654 (E. coli)
d) ISO 6888 (Staphylococcus) - What is a limitation of this medium?
a) Cannot detect Y. pestis ✔
b) Inhibits all Gram-negatives
c) Requires anaerobic incubation
d) Turns blue on contamination - If non-Yersinia bacteria grow, what could be the issue?
a) Incorrect pH
b) Antibiotics not added ✔
c) Too much agar
d) Overheating - How should prepared plates be stored?
a) 2-8°C ✔
b) -20°C
c) Room temperature
d) 37°C
- Which virulence factor helps Yersinia evade immunity?
a) Coagulase
b) Yops (Yersinia outer proteins) ✔
c) Enterotoxin
d) Capsule - What is a key biochemical test for Y. enterocolitica?
a) Oxidase-positive
b) Urease-positive ✔
c) Indole-positive
d) Citrate-positive - Which animal is a major reservoir for Y. enterocolitica?
a) Cows
b) Pigs ✔
c) Chickens
d) Fish - Cold enrichment works because Yersinia is:
a) Thermophilic
b) Psychrotrophic✔
c) Acidophilic
d) Halophilic - What confirms Yersinia beyond colony morphology?
a) Gram staining (Gram-positive rods)
b) Biochemical tests (e.g., urease, motility) ✔
c) Hemolysis on blood agar
d) Spore staining
