Brilliant Green Agar 50 FAQs and 30 MCQs
Brilliant Green Agar (BGA) is a highly selective culture medium used in clinical microbiology for the isolation of Salmonella species from clinical, food, and environmental samples. The medium contains brilliant green dye along with selective agents such as phenol red and bile salts that inhibit the growth of Gram-positive bacteria and most Gram-negative organisms other than Salmonella.
Brilliant Green Agar is especially useful in stool culture and food safety testing, where it helps in suppressing normal intestinal flora while allowing Salmonella colonies to grow as characteristic pink to red colonies against a reddish background. Due to its high selectivity, it is often used in combination with other selective media for accurate diagnosis.
This article provides a complete set of 50 FAQs and 30 MCQs covering the composition, principle, preparation, colony characteristics, and clinical applications of Brilliant Green Agar in microbiology.
Brilliant Green Agar 50 FAQs
What is Brilliant Green Agar (BGA) used for?
BGA is a selective and differential medium for isolating Salmonella species (excluding S. Typhi and S. Paratyphi) from clinical, food, and environmental samples.
Why is BGA considered a selective medium?
It contains brilliant green dye, which inhibits Gram-positive bacteria and most Gram-negative bacteria, allowing selective growth of Salmonella.
Can BGA detect Salmonella Typhi?
No. BGA inhibits S. Typhi; Bismuth Sulphite Agar is recommended for its isolation.
Which organizations recommend BGA?
APHA (American Public Health Association), FDA, and USP (United States Pharmacopeia).
Why use BGA alongside other media like Hektoen Enteric Agar?
To increase recovery rates, as BGA’s high selectivity may inhibit some pathogens.
What are the key ingredients in BGA?
Peptones, lactose, sucrose, phenol red (indicator), brilliant green (inhibitor), and agar.
Why are lactose and sucrose included?
They act as fermentable carbohydrates to differentiate lactose/sucrose fermenters (e.g., E. coli) from non-fermenters (e.g., Salmonella).
What is the role of phenol red?
It indicates acid production: yellow zones (fermenters) vs. red/pink zones (non-fermenters).
Why is sodium chloride added?
To maintain osmotic balance for bacterial growth.
How much brilliant green dye is in BGA?
0.0125 g/L, sufficient to inhibit contaminants without affecting Salmonella.
How do you prepare BGA?
Suspend 58.09 g in 1L water, boil, autoclave (121°C for 15 mins), cool to 45–50°C, and pour plates.
Why avoid overheating during preparation?
Excessive heat may degrade brilliant green, reducing selectivity.
What is the final pH of BGA?
6.9 ± 0.2 at 25°C.
How should prepared plates be stored?
At 10–25°C, away from light; use within 6 months.
What is the shelf life of dehydrated BGA powder?
4 years if stored dry and sealed.
How are fecal samples tested with BGA?
Heavily inoculate BGA plates and incubate at 35°C for 18–24 hours alongside enrichment broths.
What enrichment broths are used with BGA?
Selenite Cystine Broth or Tetrathionate Broth for food/environmental samples.
Why incubate food samples at 43°C?
To enhance Salmonella growth while suppressing competitors.
How long are BGA plates incubated?
18–24 hours at 35°C for clinical samples; 24–48 hours for food samples.
Should BGA be used aerobically or anaerobically?
Aerobically, as Salmonella are facultative anaerobes.
What do Salmonella colonies look like on BGA?
White to pink/red opaque colonies with red zones (from alkaline byproducts).
How does E. coli appear on BGA?
Inhibited or as yellow-green colonies with yellow halos (lactose fermentation).
Can Proteus mimic Salmonella on BGA?
Yes; Proteus may form small red colonies but lacks red zones.
What color are Pseudomonas colonies?
Pink to red, resembling Salmonella but often smaller.
Why might Enterococcus faecalis grow on BGA?
Rarely, it shows light growth as yellowish colonies (partial inhibition).
Why is BGA unsuitable for Shigella?
Shigella is inhibited by brilliant green.
Can BGA fully replace other media?
No—use with less selective media (e.g., MacConkey Agar) for better recovery.
What bacteria can interfere with BGA results?
Citrobacter, Proteus, and Pseudomonas may produce false-positive-like colonies.
Why confirm BGA results with biochemical tests?
To rule out mimics and ensure accurate Salmonella identification
Does BGA work for urine samples?
Yes, but enrichment may be needed for low-pathogen concentrations.
How is BGA used in food testing?
Pre-enrich food samples in Buffered Peptone Water, then subculture to BGA after Selenite Broth enrichment.
Why use BGA with Bismuth Sulphite Agar for food?
Bismuth Sulphite Agar improves Salmonella Typhi detection, complementing BGA.
What food samples require BGA?
Dairy, meat, and processed foods prone to Salmonella contamination.
How long is food enrichment before plating?
24 hours in Selenite Cystine Broth at 43°C.
Can BGA detect Salmonella in environmental swabs?
Yes, but enrichment is recommended for low-bioburden samples.
What QC strains validate BGA performance?
Salmonella Typhimurium (ATCC 14028), E. coli (ATCC 25922), and inhibited strains like Staphylococcus aureus.
What indicates acceptable QC for BGA?
Salmonella shows good growth with red zones; E. coli is inhibited or shows poor growth.
How is selectivity tested in BGA?
Inoculate 10⁴–10⁶ CFU of non-target organisms (e.g., S. aureus)—growth should be inhibited.
What if Shigella grows on BGA?
The batch is faulty, as Shigella should be inhibited.
How often should BGA undergo QC testing?
With each new batch and periodically during use (per lab protocols).
Why are colonies faint or poorly colored?
Overheating during preparation or expired ingredients may degrade dyes.
What if the agar appears discolored?
Discard—light exposure or moisture may degrade brilliant green.
Is BGA hazardous?
No dangerous substances, but follow standard lab safety protocols.
Can BGA expire?
Yes: 4 years (powder), 2 years (bottled agar), 6 months (prepared plates).
Why avoid freezing BGA?
Freezing disrupts agar structure and may inactivate selective agents.
Can BGA detect Salmonella in pharmaceuticals?
Yes, as part of microbial limits testing (with novobiocin for enhanced selectivity).
How does BGA compare to XLD Agar?
Both are selective for Salmonella, but XLD includes lysine to differentiate Shigella.
Why add novobiocin to BGA?
To inhibit Proteus and other contaminants in food/pharmaceutical testing.
What pH changes occur with lactose fermentation?
Fermentation lowers pH, turning phenol red yellow; non-fermenters raise pH (red).
Can BGA be used for water testing?
Yes, after filtration and enrichment, but competing flora may require stronger inhibition.
Brilliant Green Agar 30 MCQs
Q1. What is the primary use of Brilliant Green Agar?
A) Isolate Staphylococcus aureus
B) Detect lactose-fermenting bacteria
C) Selectively isolate Salmonella (excluding S. Typhi)✔
D) Grow Pseudomonas species
Q2. Which organism is NOT typically isolated using BGA?
A) Salmonella Typhimurium
B) Salmonella Typhi✔
C) Salmonella Enteritidis
D) Salmonella Abony
Q3. Brilliant Green Agar inhibits growth of:
A) Gram-positive bacteria✔
B) Salmonella species
C) Non-lactose fermenters
D) Yeasts
Q4. Which medium is recommended for Salmonella Typhi isolation?
A) Hektoen Enteric Agar
B) Bismuth Sulphite Agar✔
C) MacConkey Agar
D) XLD Agar
Q5. Which ingredient acts as the pH indicator in BGA?
A) Brilliant green
B) Phenol red✔
C) Lactose
D) Sodium chloride
Q6. What is the concentration of brilliant green dye in BGA?
A) 0.08 g/L
B) 0.0125 g/L✔
C) 5.0 g/L
D) 10.0 g/L
Q7. Sodium chloride in BGA primarily:
A) Inhibits Gram-positive bacteria
B) Maintains osmotic balance✔
C) Acts as a carbon source
D) Enhances lactose fermentation
Q8. Which carbohydrates are included in BGA?
A) Glucose and sucrose
B) Lactose and sucrose✔
C) Mannitol and lactose
D) Xylose and lactose
Q9. How much BGA powder is needed to prepare 1 liter of medium?
A) 20.0 g
B) 58.09 g✔
C) 10.0 g
D) 5.0 g
Q10. What is the sterilization method for BGA?
A) Filtration
B) Autoclaving at 121°C for 15 minutes✔
C) Boiling for 10 minutes
D) UV irradiation
Q11. Prepared BGA plates should be stored:
A) Frozen at -20°C
B) At 10–25°C away from light✔
C) In a CO₂-rich environment
D) Submerged in water
Q12. What is the shelf life of dehydrated BGA powder?
A) 1 year
B) 4 years✔
C) 6 months
D) 2 years
Q13. Salmonella colonies on BGA typically appear as:
A) Yellow colonies with green halos
B) White to pink colonies with red zones✔
C) Blue translucent colonies
D) Metallic green sheen
Q14. Escherichia coli on BGA shows:
A) Red colonies with red zones
B) Yellow-green colonies with yellow halos
C) Inhibited growth
D) Both B and C✔
Q15. Which organism may mimic Salmonella on BGA?
A) Staphylococcus aureus
B) Proteus vulgaris✔
C) Enterococcus faecalis
D) Bacillus subtilis
Q16. Pseudomonas species on BGA form:
A) Pink to red colonies✔
B) Blue colonies
C) Yellow zones
D) No growth
Q17. Why is BGA unsuitable for Shigella isolation?
A) Promotes Shigella overgrowth
B) Contains inhibitors for Shigella✔
C) Lacks nutrients for Shigella
D) Requires anaerobic incubation
Q18. Which test is ESSENTIAL after isolating colonies on BGA?
A) Gram staining
B) Biochemical/serological confirmation✔
C) Antibiotic susceptibility
D) Motility testing
Q19. Which medium should be used alongside BGA for better recovery?
A) Blood agar
B) Less selective media (e.g., MacConkey)✔
C) Chocolate agar
D) Sabouraud agar
Q20. Why might Salmonella Typhi show poor growth on BGA?
A) It ferments lactose
B) It is inhibited by brilliant green✔
C) Requires higher pH
D) Needs anaerobic conditions
Q21. Which enrichment broth is used for food samples before BGA plating?
A) Nutrient broth
B) Selenite Cystine Broth✔
C) Tryptic Soy Broth
D) Thioglycollate broth
Q22. Food samples are pre-enriched in:
A) Buffered Peptone Water✔
B) Saline solution
C) LB broth
D) Mannitol Salt Broth
Q23. Incubation temperature for food enrichment in Selenite Broth:
A) 25°C
B) 35°C
C) 43°C✔
D) 55°C
Q24. Which QC strain confirms BGA selectivity?
A) Salmonella Typhimurium ATCC 14028
B) Escherichia coli ATCC 25922
C) Staphylococcus aureus ATCC 25923
D) All of the above✔
Q25. Shigella flexneri ATCC 12022 on BGA should show:
A) Good growth
B) Inhibited growth✔
C) Yellow colonies
D) Red zones
Q26. Overheating BGA during preparation may:
A) Enhance brilliant green activity
B) Degrade selective agents✔
C) Promote lactose fermentation
D) Increase agar solidification
Q27. BGA is classified as:
A) A hazardous medium
B) Non-dangerous under standard use✔
C) Radioactive
D) Carcinogenic
Q28. BGA is used in pharmaceutical testing with:
A) Novobiocin✔
B) Ampicillin
C) Vancomycin
D) Tetracycline
Q29. Which pH change occurs with lactose fermentation?
A) Alkaline (red)
B) Acidic (yellow)✔
C) Neutral (pink)
D) No change
Q30. For water testing, BGA requires:
A) Direct plating without enrichment
B) Filtration and enrichment✔
C) Anaerobic incubation
D) Freezing samples first
Brilliant Green Agar plays a vital role in the selective isolation of Salmonella species from complex specimens such as stool, food, and environmental samples. Its strong inhibitory properties against competing flora make it a valuable medium in diagnostic and public health microbiology.
A clear understanding of Brilliant Green Agar is essential for microbiology students, laboratory technologists, and professionals involved in enteric pathogen identification and food safety testing. The FAQs and MCQs provided in this article serve as an effective tool for revision, exam preparation, and laboratory practice.
