Throat Swab Culture

 



1. Objective

To isolate and identify bacterial pathogens from the oropharynx, especially Group A Streptococcus (GAS), and other causes of bacterial pharyngitis.

2. Principle

A sterile swab is used to collect a sample from the posterior pharynx and tonsillar areas. The specimen is cultured on selective and differential media to isolate pathogenic organisms such as:

  • Streptococcus pyogenes (GAS)
  • Streptococcus pneumoniae

  • Haemophilus influenzae
  • Corynebacterium diphtheriae

  • Neisseria gonorrhoeae (rare cases)

Colonies are then identified by morphology, hemolysis, Gram staining, and biochemical tests.

3. Materials

  • Sterile throat swab

  • Culture media:
    • Blood agar (for hemolysis pattern)
    • Chocolate agar (for H. influenzae)

    • Tellurite agar (if C. diphtheriae suspected)
  • Incubator (35–37°C, 5% CO₂ recommended)

  • Inoculating loop (for subculture)

  • Gram staining reagents

  • Bacitracin disc (for presumptive S. pyogenes ID)

  • Biochemical kits or rapid identification system

  • PPE

4. Procedure

A. Specimen Collection

  1. Ask patient to tilt head back and open mouth wide.
  2. Use a sterile swab to gently rub posterior pharynx, tonsils, and inflamed areas, avoiding the tongue and cheeks.

  1. Place swab in transport medium if delayed.

B. Culture and Incubation

  1. Roll swab across Blood agar plate and optionally on Chocolate agar.
  2. Place a bacitracin disc on the first quadrant of the Blood agar (for GAS).

  1. Incubate at 35–37°C with 5% CO₂ for 18–24 hours.

C. Identification

  1. Observe colony morphology and hemolysis (GAS = beta-hemolysis).
  2. Perform Gram stain (GAS = Gram-positive cocci in chains).

  1. Confirm with catalase test, PYR test, or group-specific latex agglutination.

5. Result Interpretation

Organism

Key Features on Blood Agar

S. pyogenes (GAS)

Beta-hemolytic, bacitracin-sensitive

S. pneumoniae

Alpha-hemolytic, optochin-sensitive

H. influenzae

Grows on Chocolate agar, not on Blood agar

C. diphtheriae

Gray colonies on Tellurite agar

6. Uses

  • Diagnose streptococcal pharyngitis
  • Prevent rheumatic fever or post-streptococcal complications

  • Detect diphtheria, gonorrhea, or other pathogens (if suspected)
  • Rule out carrier states

7. Conclusion

Throat swab culture is a standard test to diagnose bacterial sore throat, especially Streptococcus pyogenes. Rapid antigen tests can supplement culture but are less sensitive. Proper swab technique and timely processing are critical for accuracy.

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