Correct Answer: C. Selenite F broth
The clinical presentation of fever, loose stools, and blood in stool in a 3-year-old child is classic for Salmonella gastroenteritis, a common enteric pathogen in India. Selenite F broth is the selective enrichment medium of choice for isolating Salmonella from stool samples. Selenite F contains sodium metabisulfite, which inhibits most gram-positive bacteria and coliforms (including E. coli), while allowing Salmonella to preferentially multiply. This selective enrichment dramatically increases the yield of Salmonella isolation from mixed fecal flora, which is critical in pediatric diarrheal disease surveillance in India. The organism is then subcultured onto selective-differential media like XLD (xylose lysine deoxycholate) or HE (Hektoen enteric) agar for identification. In Indian clinical practice, particularly in RNTCP and hospital-based surveillance, Selenite F broth remains the gold standard for Salmonella enrichment from stool samples because it reliably suppresses the overwhelming normal fecal flora while preserving Salmonella viability.
Why the other options are wrong
A. Robertson cooked meat broth — Robertson cooked meat broth is a non-selective enrichment medium used primarily for anaerobic bacteria and Clostridium species isolation. It does not inhibit gram-positive bacteria or coliforms, making it unsuitable for selective isolation of Salmonella from stool. While it maintains anaerobic conditions and preserves organism viability, it lacks the selective property needed to suppress normal fecal flora in a polymicrobial stool sample. B. Brain heart infusion broth — Brain heart infusion (BHI) broth is a general-purpose enrichment medium that supports growth of a wide range of bacteria, including fastidious organisms. It is non-selective and does not inhibit coliforms or gram-positive bacteria, making it unsuitable for isolating Salmonella from stool samples. BHI is used for culturing organisms like Streptococcus and Neisseria, not for selective enteric pathogen isolation from fecal specimens. D. Alkaline peptone water — Alkaline peptone water (APW) is the selective enrichment medium of choice for Vibrio species, not Salmonella. It contains high pH (8.5) that inhibits most bacteria but favors Vibrio growth. While APW is essential for cholera surveillance in India, it is inappropriate for Salmonella isolation. Using APW for Salmonella would result in poor recovery and potential false negatives in a child with bloody diarrhea.
High-Yield Facts
- Selenite F broth is the selective enrichment medium for Salmonella isolation from stool samples; contains sodium metabisulfite that inhibits coliforms and gram-positive bacteria.
- Salmonella gastroenteritis in children presents with fever, loose stools, and blood in stool; common in India due to poor sanitation and food handling practices.
- Alkaline peptone water (pH 8.5) is the selective medium for Vibrio species (cholera), not Salmonella.
- Robertson cooked meat broth is used for anaerobic bacteria and Clostridium isolation, not enteric pathogens.
- XLD and HE agar are the selective-differential media used for subculture after Selenite F enrichment to identify Salmonella colonies.
Mnemonics
SABE for Selective Enrichment Media Selenite F → Salmonella | Alkaline peptone water → Vibrio | Blood agar → Streptococcus | Enriched media (BHI) → Fastidious organisms. Use this to recall which selective medium matches which organism. Selenite F = Salmonella Friend Selenite F broth selectively enriches Salmonella by inhibiting coliforms and gram-positive bacteria. Remember: 'F' in Selenite F stands for 'Fecal pathogen' isolation.
NBE Trap
NBE may pair Robertson cooked meat broth or BHI broth as distractors because they are well-known enrichment media; students unfamiliar with the specific selectivity of Selenite F for Salmonella may incorrectly choose a general-purpose enrichment medium instead of the selective one required for stool culture.
Clinical Pearl
In Indian pediatric practice, particularly in diarrheal disease surveillance programs, Selenite F broth culture of stool is routinely performed for suspected Salmonella cases. A positive Selenite F culture followed by XLD agar confirmation allows rapid identification and antimicrobial susceptibility testing, guiding empiric therapy in a child with bloody diarrhea—critical in settings with high antimicrobial resistance.
_Reference: Jawetz, Melnick & Adelberg's Medical Microbiology Ch. 19 (Salmonella); Koneman's Color Atlas and Textbook of Diagnostic Microbiology Ch. 6_