Throat Culture Health Article

Definition

A throat culture is a technique for identifying disease-causing microorganisms in material taken from the throat. Most throat cultures are performed to identify infections caused by Group A beta-hemolytic streptococci, which cause strep throat.

Purpose

The primary purpose of a throat culture is to isolate and identify organisms from the throat that cause infection of the posterior pharynx and tonsillar areas. Since most sore throats are caused by viral infections rather than by bacteria, a correct diagnosis is important to prevent unnecessary use of antibiotics. The bacterium that most often causes a sore throat is Streptococcus pyrogenes or Group A beta-hemolytic streptococcus. In many circumstances, the throat culture is performed for the purpose of identifying this organism only. Throat cultures are also performed to identify people who are carriers of the organisms that may cause meningitis (Neisseria meningitidis, Streptococcus pneumoniae) and whooping cough (Bordetella pertussis).

Precautions

Throat cultures should be taken before the patient is given any antibiotic medications. In addition, the patient's immunization history should be checked to evaluate the possibility that diseases other than strep are causing the sore throat. The health care provider should use a mask and gloves for infection control, as the patient may cough or gag when the throat is swabbed. Swabs for rapid strep tests should be made of dacron or rayon.

Description

Throat cultures are performed for isolation of bacteria that cause throat infections. Throat washings or swabs are also required for culture of viruses that cause throat infections, but these viral cultures are not commonly performed. Most bacterial throat infections are caused by Group A streptococci. Strep throat is more common in children (ages five to 15) than in adults, and is spread by droplets of mucus and other respiratory secretions. The tonsils and the back of the throat often appear red, swollen, and streaked with pus. The symptoms usually appear within three days after being exposed to group A strep and include an abrupt sore throat, headache, fever, loss of appetite and malaise. Group A strep infections may be associated with complications called sequelae, if not treated promptly with antibiotic therapy. In addition to causing sore throat (pharyngitis), this group of strep can also cause scarlet fever, rheumatic fever, glomerulonephritis, or abscesses around the tonsils

Other bacteria may cause pharyngitis, but do so less frequently. These include Groups B, C and G streptococci, Neisseria gonorrhoeae, Corynebacterium diptheriae, Haemophilis influenzae, Mycoplasma pneumonia, and Clamydia trachomatis. In addition, anaerobic bacteria are often implicated as the cause of Vincent's angina, a form of tonsillitis. Many other pathogenic bacteria can be isolated from sites in the upper respiratory tract other than the pharynx such as the sinuses, nasopharynx, and epiglottis.

The specimen for culture is obtained by swabbing the throat with a sterile swab. The patient is asked to tilt the head back and open the mouth wide. A tongue depressor is used to hold down the tongue and the swab tip is rubbed against the area behind the uvula (posterior pharynx) and tonsillar areas on both sides of the throat. Any red or whitish patches on the throat should also be swabbed. The swab is removed gently without touching the teeth, gums, or tongue. It is then placed in a sterile tube for immediate delivery to a laboratory. For optimal recovery, especially if the laboratory is located off-site, the tube should contain Stuart's or Cary-Blair transport medium in order to maintain the viability of the organisms. The swab tip is used to break the ampoule and is immersed in the fluid. If a rapid strep test (streptococcal antigen test) is being performed, two swabs should be taken of the throat. One is used for the rapid test, and the other is used for culture should the rapid test result be negative. Obtaining the specimen takes less than 30 seconds. The swabbing procedure may cause gagging but is not painful. The physician or nurse should indicate if any disease organisms other than strep are suspected, because some bacteria require special culture media and growth conditions.

S. pyogenes, group A beta hemolytic streptococcus, is cultured on a growth medium called blood agar. Agar is a gel that is made from the cell walls of red algae. Blood plates are made from agar that contains a low carbohydrate nutrient such as trypticase soy and 5% sheep red blood cells. When the throat swab reaches the laboratory, it is wiped across a blood agar plate. An inoculating loop is used to streak the plate and stab the agar. This process separates the bacteria so that individual colonies can be isolated. An antibiotic disk containing bacitracin (A disk) is placed on the agar in an area containing the initial inoculum. Blood agar allows differentiation of streptococci based upon the characteristic hemolysis that they produce. Beta hemolytic strep releases products into the medium called beta hemolysins, which lyse the red blood cells and cause a clear zone to form around the colonies. Alpha strep releases alpha hemolysins, which causes a green discoloration to the blood around the colonies. Gamma hemolysis (no hemolysins produced) refers to no zone of discoloration around the colonies. Blood agar is nonselective and permits the growth of normal throat flora as well as other potential pathogens. For identification of Group A strep, a selective medium such as strep selective agar (SSA) is used. This medium contains colistin, crystal violet, and trimethoprin-sulfamethoxazole (SXT). These antibiotics inhibit the growth of most normal flora and all streptococci except groups A and B. Plates are allowed to incubate for 18 hours at 35°C in 10% carbon dioxide or under anaerobic conditions.

Plates should be examined after 18 hours of incubation, and if negative, again after an additional 24 hours incubation. Group A streptococci produce small oval-shaped transparent colonies that produce beta hemolysis and will not grow around the bacitricin disk. The colonies are catalase and coagulase negative and pyroglutamyl aminopeptidase (PYR) positive which differentiates them from the genera Staphylococcus and Micrococcus, which may appear similar on blood agar. Colonies of beta hemolytic strep isolated from the medium should be tested with group specific antibodies to confirm that they are group A. Antibiotic susceptibility testing is not usually necessary because Group A strep are susceptible to penicillin and related antibiotics such as ampicillin. Persons who are allergic to penicillin may be given erythromycin.

Rapid strep tests are enzyme immunoassays that detect Group A streptococcal antigens. The specificity of these tests if very high (approximately 98%), but the sensitivities have been reported to be from 60-96%. Consequently, negative tests can occur in the presence of Group A streptococcal infections, and culture should be performed on samples that test negative. These tests can be performed in a medical office or clinic and results can be available within 10 minutes, allowing for quicker diagnosis and treatment. Usually, the physician will order a throat culture if the rapid strep test is negative, but the patient has clinical symptoms that are suggestive of strep

infection. If the rapid strep test is positive, then treatment is ordered immediately.

Rapid strep tests are based upon the principle of double antibody sandwich immunoassay. The first step of a rapid strep test is the extraction of specific Group A streptococcal antigen from the swab. The swab is placed in a test tube containing the extracting reagents (usually dilute acid). The swab is rotated vigorously in the solution while pressing the tip against the sides of the test tube. After all fluid is pressed from the swab, it is discarded and the extract is applied to a nitrocellulose membrane containing both immobilized antibodies and nonimmobilized antibodies to different regions of the Group A strep antigen. The non-immobilized antibodies are conjugated to colored particles or colloidal gold. If Group A streptococcal carbohydrate antigen is present in the extract, the conjugated antibodies bind to it, forming antigen-antibody complexes. These migrate along the pad until they reach the reaction zone containing immobilized antibodies to the same Group A strep antigen. These antibodies capture the antigen-antibody complexes, forming a colored bar or line in the reaction zone area.