Effective Date: 
Thu, 08/01/2013
Fri, 01/05/2018
Tue, 01/10/2017

Purpose and Scope:

The primary purpose of a throat culture is identification of the specific organisms that cause strep throat. These organisms are Group A streptococci, specifically Streptococcus pyogenes. Since most sore throats are caused by viral infections rather than by S. pyogenes, a correct diagnosis is important to prevent unnecessary use of antibiotics and to begin treatment of strep infections as soon as possible. Group A streptococcal infections are potentially life-threatening, often involving other parts of the body in addition to the throat. Besides causing sore throat (pharyngitis), streptococci can also cause scarlet fever, rheumatic fever, kidney disease, or abscesses around the tonsils. Other beta hemolytic streptococci have also been implicated in pharyngitis.
This test may also be useful in the detection of agents of thrush (yeast) and pharyngitis due to Arcanobacterium. Haemolyticum.


Specimen Requirement:

Throat exudate collected on a sterile cotton or polyester swab from the posterior pharynx, tonsils, tonsillar fossae, or areas of inflammation and exudate, avoiding the lips and tongue.

Quality Control:

Perform Quality Control as required for Bacitracin susceptibility testing.

Reagents and Supplies:

5% Sheep Blood plate
Inoculating stick or loop (disposable)
37 C Incubator
Bacitracin disks


Setup Procedure:

1. Be sure BAP plates have come to room temperature.
2. Use the swab to inoculate the first quadrant. Inoculate the entire quadrant rolling the swab while swabbing.
3. Streak for isolation using a sterile disposable stick or loop. Use a light touch. Don't penetrate or scrape the agar surface.
4. Streak four quadrants as demonstrated in the diagram below.


5. Place the Bacitracin disk at the junction of the 1st and 2nd quadrants.
6. Stab the first quadrant with the loop or stick to create an environment for microaerophilic beta strep to grow.
7. Incubate aerobically at 35°C


Culture Examination

1. Observe plates at 24-48 hours for presence of beta hemolytic Streptococcus that is susceptible to the Bacitracin disk.
2. Group A Strep colonies are small, transparent or translucent, smooth, and dome shaped; have an entire edge; and are surrounded by a relatively wide zone of complete β-hemolysis.
3. Perform a catalase test on beta hemolytic colonies. If negative, check a wet mount or gram stain to ensure GPC morphology.
4. Beta hemolytic colonies that do not grow in the vicinity of the Bacitracin disk can be identified with the Rapid Strep test. These colonies must also have catalase and wet mount/gram stain done before reporting them as Group A strep.
5. Rare or mixed colonies suspected of beta hemolysis must be sub-cultured to BAP plate and evaluated on the subsequent day for identification.
6. Beta hemolytic colonies that are catalase negative, GPC morphology and either Bacitracin or Rapid Strep negative can be reported as “Beta-hemolytic strep not Group A.”
7. If the β-hemolytic colonies are catalase positive GPR in significant amount send to Quest on BAP to identify Arcanobacterium haemolyticum.
8. Suspected yeast colonies can be identified by wet mount. Report as few, moderate or yeast. Hold plate to see if clinician wants further identification.

Reporting results

If negative or no beta hemolytic colonies are present, report as “No beta-hemolytic streptococci found”.

If beta hemolytic catalase negative GPC colonies are positive for Group A strep, report as “(Rare, Few, Moderate, or Many) Group A Streptococcus”

If beta hemolytic catalase negative GPC colonies are negative for Group A, report as “(Rare, Few, Moderate, or Many) Beta Strep not Group A”


Key Points: 

Color Atlas and Textbook of Diagnostic Microbiology, Koneman, 5th edition, Chapter 3, pages 122-131.

Bailey & Scott’s Diagnostic Microbiology, Forbes, 11th edition, Chapters 56 and 57, pages 884-906.

Textbook of Diagnostic Microbiology, Mahon, C.R., Lehman, D.C. & Manuselis, G., 3rd
Ed., Saunders Elsevier, 2007.

Duncan J, (ed.), Laboratory Diagnosis of Upper Respiratory Tract Infections, Cumitech 10, pp. 1-
10, American Society for Microbiology, 1979

Isenberg HD (ed.), Clinical Microbiology Procedures Handbook, 2:1.14.1-1.14.4, American Society for Microbiology, 1992.

Murray PR (ed.), Manual of Clinical Microbiology, 7th Ed, p., 74, pp. 283-296, American Society
for Microbiology, 1999.

Williams GS. Group C and G Streptococci Infections: Emerging Challenges, Clinical Laboratory
Science, Vol. 16, NO 4 Fall 2003.