Selenite F Broth 50 FAQs and 30 MCQs
Selenite F Broth is a selective enrichment medium critical for isolating Salmonella and Shigella from clinical, food, and environmental samples. Its formulation (sodium selenite, peptones, lactose) inhibits gram-positive bacteria and coliforms while promoting pathogen growth. Widely used in diagnostic labs and public health surveillance, it requires precise handling due to selenium toxicity. This guide consolidates 50 FAQs addressing preparation, mechanisms, and troubleshooting, plus 30 MCQs to test your expertise. Ideal for medical microbiologists, food safety auditors, and students mastering ISO 6579, FDA BAM, or CLSI protocols.
How to Use This Mock Test :
1. Timed Simulation: Complete in 40 minutes.
2. Focus Areas:
– Sodium selenite as an inhibitory agent
– Incubation conditions (time/temperature)
– Subculturing techniques to solid media (e.g., XLD Agar)
– Safety handling (selenium toxicity)
3. Scoring: >24/30 = proficiency; review incorrect answers.
Selenite F Broth 50 FAQs:
What is Selenite F Broth?
A selective enrichment medium for isolating Salmonella and some Shigella species from clinical and non-clinical samples.
Who developed Selenite Broth?
Leifson developed it to inhibit coliforms and enhance Salmonella recovery.
What types of samples can be tested with Selenite F Broth?
Feces, urine, water, food, and other sanitary materials.
Why is Selenite F Broth used for Salmonella isolation?
It inhibits competing bacteria (e.g., coliforms, fecal streptococci) while allowing Salmonella to grow.
Can Selenite Broth be used for Vibrio cholerae?
Yes, it can transport Vibrio cholerae strains for 2–5 days.
What are the main ingredients of Selenite F Broth?
Casein hydrolysate, lactose, sodium phosphate, and sodium hydrogen selenite.
What is the role of lactose in Selenite Broth?
It serves as a fermentable carbohydrate to maintain pH.
How does sodium selenite work as a selective agent?
It inhibits Gram-positive and Gram-negative bacteria (e.g., E. coli, Enterococcus).
Why is sodium phosphate added?
It stabilizes pH and reduces selenite toxicity.
It stabilizes pH and reduces selenite toxicity.
7.0–7.2 (varies slightly by manufacturer).
Why should the broth not be autoclaved?
Heat degrades selenite, reducing its effectiveness.
What happens if the pH increases during incubation?
Selenite toxicity decreases, allowing overgrowth of non-target bacteria.
Why is L-cystine sometimes added?
It enhances Salmonella recovery (optional, depending on the manufacturer).
How is Selenite F Broth prepared?
Dissolve in distilled water, heat to boiling, and dispense into tubes.
How should sterilization be done?
Use a boiling water bath or free-flowing steam for 10 minutes (no autoclaving).
What is the recommended depth of the broth in tubes?
At least 5 cm to maintain low oxygen tension.
Can prepared Selenite Broth be stored?
Yes, at 2–8°C away from light for up to 18 months.
What indicates contamination in prepared broth?
A red precipitate (small amounts are acceptable; discard if excessive).
Why is selenite toxic?
It reacts with sulfur/sulfhydryl groups in microbial cells, causing teratogenicity.
What safety precautions are needed when handling selenite?
Avoid skin contact; wash immediately if exposed.
How much stool sample should be added?
1–2 g (or 10–15% by volume).
How should urine samples be processed?
Use double-strength broth and mix with an equal volume of urine.
What is the incubation temperature?
35–37°C for 12–24 hours.
Why shouldn’t incubation exceed 24 hours?
Coliforms may overgrow pathogens after this period.
How is microbial growth detected?
Turbidity in the broth.
What media should be used for subculture?
XLD, Hektoen Enteric, MacConkey, or Bismuth Sulfite Agar.
What indicates a positive result on MacConkey Agar?
Colorless colonies (non-lactose fermenters like Salmonella).
What indicates a negative result on MacConkey Agar?
Pink colonies (lactose fermenters like E. coli).
What QC strains are used for Selenite Broth?
Salmonella Typhimurium (ATCC 14028) → Good growth
E. coli (ATCC 25922) → InhibitedWhy is Selenite Broth not a standalone diagnostic tool?
It must be paired with selective plating media for accuracy.
What bacteria are resistant to selenite?
Proteus and Pseudomonas (non-lactose fermenters).
When should the broth be discarded?
If a large red precipitate forms or after the expiry date.
What reduces selenite’s inhibitory effect?
Prolonged incubation (>12–24 hours).
Can nutritional variations affect bacterial growth?
Yes, some strains may grow poorly.
What if no growth occurs after incubation?
Check sample quality, incubation time, and broth preparation.
Why is there overgrowth of contaminants?
Incubation was too long (>24 hours) or pH shifted.
How does Selenite Broth compare to Tetrathionate Broth?
Both enrich Salmonella, but selenite is less harsh on Shigella.
Can Selenite Broth recover injured Salmonella?
Less effectively than non-selective broths (e.g., Buffered Peptone Water).
Is selenite broth suitable for Shigella?
Yes, but not as effective as for Salmonella.
What are alternative enrichment broths?
GN Broth (for Shigella) or Rappaport-Vassiliadis Broth (for Salmonella).
Is Sodium Selenite hazardous?
Yes, toxic, corrosive, and teratogenic.
How should spills be handled?
Clean with water and follow hazardous waste protocols.
Can selenite broth be disposed of in regular waste?
No, follow local biohazard disposal regulations
What PPE is recommended?
Gloves, lab coat, and eye protection.
How should dehydrated powder be stored?
10–30°C in a dry, tightly sealed container.
What is the shelf life of dehydrated medium?
4 years (check expiry date).
How long can prepared broth be stored?
Up to 1 year at 2–8°C (if uncontaminated).
Can Selenite Broth be used for food testing?
Yes, per APHA (American Public Health Association) guidelines.
What references support its use?
Manual of Clinical Microbiology (ASM)
CLSI M22-A3 (QC standards)
Leifson’s original 1939 paperWhy is it called “Selenite F Broth”?
The “F” refers to Leifson’s modified formula for better selectivity.
Selenite F Broth 30 MCQs:
- What is the primary purpose of Selenite F Broth?
a) Isolate E. coli
b) Enrich Salmonella and Shigella✔
c) Culture Staphylococcus
d) Detect viruses - Who developed Selenite Broth?
a) Robert Koch
b) Louis Pasteur
c) Leifson✔
d) Joseph Lister - Which component makes Selenite Broth selective?
a) Lactose
b) Sodium selenite✔
c) Casein hydrolysate
d) Sodium phosphate - Selenite F Broth is NOT recommended for:
a) Stool samples
b) Urine samples
c) Blood cultures✔
d) Food samples - What is the final pH of Selenite F Broth?
a) 5.2 ± 0.2
b) 7.0 ± 0.2✔
c) 8.5 ± 0.2
d) 6.0 ± 0.2
- Which ingredient maintains pH in Selenite Broth?
a) Lactose
b) Sodium phosphate✔
c) Sodium selenite
d) Casein hydrolysate - Lactose in Selenite Broth serves to:
a) Inhibit Gram-positive bacteria
b) Provide fermentable carbohydrates
c) Act as a buffer
d) Both b and c✔ - Sodium selenite primarily inhibits:
a) Salmonella
b) E. coli and fecal streptococci✔
c) Pseudomonas
d) Mycobacterium - Why should Selenite Broth NOT be autoclaved?
a) It becomes too selective
b) Heat destroys selenite’s inhibitory properties✔
c) Lactose caramelizes
d) Casein precipitates - What indicates microbial growth in Selenite Broth?
a) Color change to pink
b) Formation of gas bubbles
c) Turbidity✔
d) Red precipitate
- How is Selenite Broth sterilized?
a) Autoclaving at 121°C
b) Boiling water bath for 10 minutes✔
c) Filtration
d) Dry heat sterilization - The recommended depth of broth in tubes is:
a) 1 cm
b) 3 cm
c) 5 cm✔
d) 10 cm - What should you do if a red precipitate forms in the broth?
a) Shake vigorously
b) Discard the medium✔
c) Autoclave again
d) Add more lactose - Selenite Broth must be stored at:
a) 2–8°C✔
b) 25–30°C
c) –20°C
d) 37°C - Sodium selenite is hazardous because it is:
a) Radioactive
b) Corrosive and teratogenic✔
c) Flammable
d) Explosive
- How much stool sample should be added to Selenite Broth?
a) 0.1 g
b) 1–2 g✔
c) 5 g
d) 10 g - For urine samples, Selenite Broth should be:
a) Mixed 1:1 with urine
b) Used at double strength✔
c) Avoided
d) Autoclaved first - Optimal incubation time for Selenite Broth is:
a) 6 hours
b) 12–24 hours✔
c) 48 hours
d) 72 hours - After incubation, subculture should be done on:
a) Blood agar
b) MacConkey or XLD agar✔
c) Chocolate agar
d) Sabouraud dextrose agar - On MacConkey Agar, Salmonella appears as:
a) Pink colonies
b) Colorless colonies✔
c) Green colonies
d) Black colonies
- Selenite Broth should NOT be used as the sole medium because:
a) It lacks nutrients
b) It may miss low-pathogen loads✔
c) It inhibits Salmonella
d) It is too expensive - Which bacteria are naturally resistant to selenite?
a) E. coli
b) Proteus and Pseudomonas✔
c) Streptococcus
d) Clostridium - The inhibitory effect of selenite diminishes after:
a) 1 hour
b) 6–12 hours✔
c) 24–48 hours
d) 1 week - If coliforms overgrow in Selenite Broth, the likely cause is:
a) Incubation <12 hours
b) Incubation >24 hours✔
c) Too much lactose
d) Autoclaving the broth - Which supplement may improve Salmonella recovery?
a) L-cystine✔
b) Hemin
c) Vitamin K
d) Sucrose
- Which organization recommends Selenite Broth for food testing?
a) WHO
b) CDC
c) APHA✔
d) FDA - Selenite Broth can preserve Vibrio cholerae for:
a) 2–5 days✔
b) 1 week
c) 1 hour
d) 24 hours - Dehydrated Selenite Broth powder should be stored:
a) Frozen
b) At 10–30°C in a dry place✔
c) In sunlight
d) With desiccants - Quality control for Selenite Broth includes testing with:
a) Salmonella Typhimurium (ATCC 14028)
b) E. coli (ATCC 25922)
c) Both a and b✔
d) Candida albicans - The “F” in Selenite F Broth stands for:
a) Fast-growing
b) Leifson’s modified formula✔
c) Fermentative
d) Fecal
Key Topics Covered
Composition: Peptone, lactose, sodium selenite.
Mechanism: Selenite disrupts cellular respiration in non-target bacteria.
Protocols: 1:10 sample-to-broth ratio, 37°C incubation, subculture to selective agars.
Limitations: Not for *Shigella*; short shelf-life; toxic to lab personnel.
Applications: Stool cultures, food testing (ISO 6579), outbreak investigations.
Conclusion
Selenite F Broth remains indispensable for Salmonella detection despite emerging alternatives. These 50 FAQs demystify its selectivity, safety protocols, and compliance with global standards (ISO, FDA, CLSI), while 30 MCQs validate competency in application and interpretation. Mastery ensures accurate outbreak investigations and regulatory compliance—cornerstones of public health microbiology.
Pro Tip:
Always pair Selenite F Broth with a secondary selective agar (e.g., Hektoen or SS Agar) for optimal *Salmonella* recovery. Handle under fume hoods due to selenium volatility!
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