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Answer of long question c
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Q.2 Long Answer Question (c) - Full Answer [1+4+5 = 10 marks]
Scenario: A 28-year-old woman with severe abdominal pain, vomiting, and fever. Stool reveals Gram-negative bacteria. History of eating contaminated food.
1. Likely Causative Agent (1 mark)
The most likely causative agent is Salmonella enterica (non-typhoidal Salmonella, e.g., serovar Typhimurium or Enteritidis).
- Gram-negative, facultative anaerobic rod
- Transmitted via contaminated food (poultry, eggs, dairy products are the most common sources)
- Causes gastroenteritis/food poisoning with the classic triad of abdominal pain, vomiting, and fever
- Onset typically 24-48 hours after ingestion of contaminated food
Other possibilities to mention: Shigella spp. or Campylobacter spp. can present similarly, but Salmonella is most classically associated with contaminated food ingestion with Gram-negative rods on stool culture.
2. Laboratory Diagnosis (4 marks)
A. Specimen Collection
- Stool sample is the primary specimen (fresh stool or rectal swab in Cary-Blair transport medium)
- Blood culture may be added if bacteremia is suspected (fever, systemic illness)
B. Direct Examination
- Gram stain of stool: Gram-negative short rods (though not typically diagnostic due to normal flora)
C. Culture on Selective & Differential Media
Because stool contains enormous numbers of normal microbiota, selective media are required:
| Medium | Feature |
|---|---|
| MacConkey agar | Salmonella grows as colorless (non-lactose fermenting) colonies |
| Salmonella-Shigella (SS) agar | Selective; Salmonella produces colorless colonies with black centers (H₂S production) |
| Hektoen Enteric (HE) agar | Salmonella produces blue-green colonies with black centers |
| XLD agar (Xylose Lysine Deoxycholate) | Pink-red colonies with black centers due to H₂S |
| Selenite-F broth | Enrichment broth used before plating to enhance Salmonella recovery |
Key identifying features on culture:
- Non-lactose fermenter (colorless colonies on MacConkey)
- H₂S production (black precipitate on SS/HE/XLD agar)
- Oxidase negative
D. Biochemical Identification
- Motility: positive (most strains)
- TSI (Triple Sugar Iron): Alkaline slant / Acid butt / H₂S positive / gas positive
- Urease: negative (distinguishes from Proteus)
- Indole: negative
- Citrate: positive
- Lysine decarboxylase: positive
E. Serological Typing (Kauffmann-White scheme)
- Antisera for O (somatic), H (flagellar), and Vi (capsular) antigens
- Confirms species and serovar identification
- All isolates should be referred to public health labs for confirmation and epidemiologic tracing
F. Molecular Methods
- Multiplex nucleic acid amplification tests (NAATs) / enteric PCR panels - considered the gold standard in developed settings
- Whole-genome sequencing for epidemiologic outbreak investigation
(Medical Microbiology 9e, p. 294; Sherris & Ryan's Medical Microbiology 8e, p. 1326)
3. Treatment and Preventive Measures (5 marks)
Treatment
For Salmonella Gastroenteritis (non-typhoidal):
- Antibiotics are NOT routinely recommended for uncomplicated gastroenteritis - antibiotic therapy may prolong the duration of illness and promote the carrier state
- Primary therapy = supportive care:
- Oral/IV fluid and electrolyte replacement (most important)
- Control of nausea and vomiting
- Rest
When antibiotics ARE indicated (severe or high-risk cases):
- Severe/systemic disease, bacteremia, immunocompromised patients, infants < 3 months, elderly, those with hemoglobinopathies or implanted vascular grafts
- Preferred agents: Fluoroquinolones (e.g., ciprofloxacin), extended-spectrum cephalosporins (ceftriaxone, cefixime)
- Trimethoprim-sulfamethoxazole or ampicillin (if susceptible - resistance is increasingly common)
- Drug selection should be guided by in vitro susceptibility testing (multidrug resistance is rising globally)
(Sherris & Ryan's Medical Microbiology 8e, p. 1327)
Preventive Measures
Personal / Food Hygiene:
- Thorough cooking of poultry, eggs, and meat (heat kills Salmonella)
- Avoid cross-contamination between raw and cooked foods
- Refrigerate food promptly; do not leave food at room temperature (warm months are peak season)
- Wash hands thoroughly before and after handling food
Public Health / Environmental:
- Safe water supply and proper sewage disposal
- Identification and treatment of Salmonella carriers (especially food handlers)
- Surveillance and outbreak investigation in communities
- Proper food processing standards in slaughterhouses and food industry
Vaccination:
- Typhoid vaccine (oral Ty21a or injectable Vi polysaccharide) - recommended for travelers to endemic areas; reduces risk of Salmonella Typhi infection specifically
- No licensed vaccine currently exists for non-typhoidal Salmonella gastroenteritis
(Medical Microbiology 9e, p. 294; Sherris & Ryan's Medical Microbiology 8e, p. 1328)
Summary Table
| Aspect | Key Point |
|---|---|
| Causative agent | Salmonella enterica (Gram-negative rod) |
| Transmission | Contaminated food (poultry, eggs, dairy) |
| Lab diagnosis | Stool culture on selective media (SS, HE, XLD); H₂S producing, non-lactose fermentor; serology; NAAT |
| Treatment | Supportive (fluids); antibiotics only in severe/high-risk cases (ciprofloxacin, ceftriaxone) |
| Prevention | Proper food cooking, hand hygiene, carrier identification, typhoid vaccine for travelers |
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