Lab Diagnosis of Staphylococcal Food Poisoning

(Apurba Sastry - Essentials of Medical Microbiology, Exam-Oriented)

Key Concept First

Specimens to Collect

Note: Since this is an intoxication by heat-stable toxin, the organisms may be absent from food after cooking - toxin detection from food is thus equally important.

Steps in Laboratory Diagnosis

1. Microscopy (Direct Smear)

• Gram stain of food/vomitus/stool
• Shows Gram-positive cocci in clusters ("bunches of grapes")
• Not diagnostic alone but supports further workup
• Limited value in toxin-mediated disease (organisms may already be dead)

2. Culture

Media Used:

Baird-Parker agar is the gold standard selective medium for isolation of S. aureus from food specimens.

Incubation:

• Aerobic, 37°C, 24-48 hours
• S. aureus grows rapidly on non-selective media

3. Identification of S. aureus

Coagulase test = hallmark of S. aureus. In food lab contexts, Thermonuclease (TNase) test is particularly valuable because the enzyme is heat-stable and persists even after organisms are killed by heat processing.

4. Enterotoxin Detection (Most Specific for Food Poisoning)

A. Serological Methods (Most Practical)

• Gel Diffusion / Ouchterlony Double Immunodiffusion - classic method; visible precipitin line between antigen (toxin in food) and anti-enterotoxin antibody
• Reverse Passive Latex Agglutination (RPLA) - latex particles coated with anti-enterotoxin antibodies; agglutination = toxin present; highly sensitive
• ELISA (Enzyme-Linked Immunosorbent Assay) - most widely used currently; detects nanogram quantities of enterotoxin

B. Bioassay (Old Method, Rarely Used)

• Intraperitoneal injection of food extract into kittens/monkeys → vomiting confirms toxin
• Emesis in monkeys is the classic test
• Only SEA requires ~2.3 µg to cause symptoms in animals; humans are more sensitive

C. Molecular Methods (Modern)

• PCR to detect enterotoxin genes (sea, seb, sec, sed, see)
• Nucleic acid hybridization / Colony blot hybridization using oligonucleotide probes targeting enterotoxin genes
• Faster and more sensitive than bioassay

D. Mass Spectrometry (MALDI-TOF)

• Used in reference labs for rapid identification of S. aureus isolates

5. Phage Typing (Epidemiological)

• Used in outbreak investigation to trace the source/food handler
• Most S. aureus strains causing food poisoning belong to phage group III (phage types 6, 7, 8, 9, 10, 53, 54)
• Phage type 80/81 = historically associated with epidemic strains

Summary Flowchart (Exam-style)

Suspected SFP
     ↓
Collect: Food remnant + patient vomit/stool + nasal swab of food handler
     ↓
Gram Stain → Gram +ve cocci in clusters
     ↓
Culture on Blood Agar + Baird-Parker Agar + Mannitol Salt Agar
     ↓
Identify colonies → Coagulase +ve, Catalase +ve, DNase +ve = S. aureus
     ↓
ENTEROTOXIN DETECTION from food:
  → ELISA / RPLA → Serotype A/B/C/D/E
  → PCR for toxin genes
     ↓
Phage typing for outbreak tracing

High-Yield Exam Points (Apurba Sastry Focus)

Lab Diagnosis of Staphylococcal Food Poisoning

Specimens

• Suspected food (leftover/vomitus) — most important
• Patient's vomit and stool
• Nasal swab of food handlers (for source tracing)

1. Microscopy

• Gram stain of food/vomitus
• Shows Gram-positive cocci in clusters
• Limited value as organisms may be dead (heat-stable toxin persists even after bacteria are killed)

2. Culture

• Aerobic, 37°C, 24-48 hrs

3. Identification of Isolate

4. Enterotoxin Detection from Food (Definitive Confirmation)

5. Phage Typing (Epidemiological - outbreak tracing)

• Traces the food handler/source
• Most strains belong to phage group III

Key Exam Points to Include

• Disease is an intoxication (preformed toxin), NOT infection
• Toxin is heat-stable - survives boiling 30 min; organisms may be absent from food but toxin remains
• Incubation: 1-8 hours (short because toxin is preformed)
• No fever (differentiates from Salmonella food poisoning)
• Enterotoxin A is most common in outbreaks
• Antibiotics NOT used (toxin already formed, no bacteria replicating in host)
• Minimum infective dose in food: >10⁵ CFU/g