Throat Culture (Encyclopedia of Medicine)
A throat culture is a technique for identifying disease bacteria in material taken from the throat. Most throat cultures are done to rule out infections caused by beta-hemolytic streptococci, which cause strep throat. Hemolytic means that these streptococci destroy red blood cells.
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.
Throat cultures can also be used to identify other disease organisms that are present in the patient's throat; and to identify people who are carriers of the organisms that cause meningitis and whooping cough.
Besides their use in diagnosis,...
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Throat Culture (Encyclopedia of Children's Health)
A throat culture is a microbiological procedure for identifying disease-causing bacterial organisms in material taken from the throat. A throat swab will capture the causative organism in most cases and the culture will allow the specific organism to be grown in the microbiology laboratory under certain conditions. The bacteria can then be identified, and results from antibiotic sensitivity tests on the bacteria will determine the appropriate treatment to be prescribed.
The primary purpose of a throat culture is to identify the specific bacterial organisms that are causing a sore throat or throat infection, particularly to identify or to rule out the presence of group A, beta-hemolytic streptococci, the bacterial organisms that cause strep throat. Hemolytic means that these streptococci are capable of destroying red blood cells.
Since most sore throats are caused by viral infections rather than by strep organisms, a correct diagnosis is important to prevent unnecessary use of antibiotics for viruses that do not respond to them, and to begin effective treatment of strep or other throat infections as soon as possible. Throat cultures can also be used to identify other disease organisms that are...
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Throat Culture (Encyclopedia of Nursing & Allied Health)
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.
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).
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.
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.
Recent gargling or treatment with antibiotics will adversely affect the culture results. The laboratory should be notified if the patient has recently taken antibiotic medications.
Antibiotic drug given to stop the growth of bacteria. Antibiotics are ineffective against viruses.
Antigen substance that interacts with an antibody and causes an immune reaction.
Carrier person harboring an infectious disease who may be immune to it but who can transmit the disease to others.
Diphtheria serious disease of the throat, nose, and lungs caused by a bacterium, Corynebacterium diphtheriae.
Streptococcus category (genus) of sphere-shaped bacteria that occur in pairs or chains.
Thrush disease occurring in the mouth or throat that is caused by a yeast, Candida albicans, and is characterized by a whitish growth and ulcers.
Whooping coughn infectious disease of the respiratory tract caused by a bacterium, Bordetella pertussis.
No specific aftercare is needed.
There is a minor risk to the health care provider collecting the specimen of contracting a viral or bacterial infection from the patient.
The results from throat cultures identify the presence of any pathogenic bacteria growing on the media. Nondisease-producing organisms that grow in healthy throat tissues include non-hemolytic and alpha-hemolytic streptococci, some Neisseria species, Moraxella catarrhalis, coagulase negative staphylococci, and diphtheroids. These organisms are described on the culture report as normal flora. Group A streptococci are identified as described previously. Unless the culture is done solely for the identification of Group A strep, any other potential pathogen (e.g., Haemophilus influenzae, Neisseria gonorrhoeae) is isolated on appropriate growth media, and the colonies that grow are identified by a selection of biochemical tests. Antimicrobial susceptibility testing is performed on a standardized growth of the isolated organism to determine which antibiotics will be effective in treating the infection.
Health care team roles
A physician, nurse, or physician assistant collects the throat swab. A health care provider such as a nurse will usually perform the rapid strep test in the outpatient setting. Cultures are performed by a clinical laboratory scientist, CLS(NCA)/medical technologist, MT(ASCP). Culture results are reported directly to the ordering physician who will select the appropriate antibiotic therapy if required.
Because strep is highly contagious, the health care provider should stress that other family members and close contacts of patients diagnosed with strep throat also seek medical attention if they have similar symptoms. The health care provider should stress that the patient should wash hands frequently (especially after coughing or sneezing), stay home, and follow the treatment regimen prescribed by the physician.
Fischbach, Frances. "Throat Cultures (Swab or Washings)." In A Manual of Laboratory & Diagnostic Tests. 6th ed. Philadelphia: Lippincott Williams & Wilkins, 2000, pp. 551-553.
Forbes, Betty A., Daniel F. Sahm, and Alice S. Weissfeld. "Streptococcus, Enterococcus, and Similar Organisms." In Bailey & Scott's Diagnostic Microbiology. 10th ed. St. Louis: Mosby, 1998, pp. 620-635.
Hayes, Cynthia S., and Harold Williamson, Jr. "Management of Group A Beta-Hemolytic Streptococcal Pharyngitis." American Family Physician 63 (April 15, 2001): pp.1557-1565.
The American Society for Clinical Laboratory Science. 7910 Woodmont Ave., Suite 523, Bethesda, MD 20814. (301) 657-2768. <<a href="http://www.ascls.org">http://www.ascls.org>.
Division of Bacterial and Mycotic Diseases, National Center for Infectious Disease, Centers for Disease Control and Prevention. 1600 Clifton Road NE, Atlanta, GA 30333. (800) 311-3435. <<a href="http://www.cdc.gov">http://www.cdc.gov>.
National Institute of Allergy and Infectious Diseases, National Institutes of Health. Building 31, Room 7A-50, 31 Center Drive MSC 2520, Bethesda, MD, 20892. <<a href="http://www.niaid.nih.gov">http://www.niaid.nih.gov>.
Linda D. Jones, B.A., PBT (ASCP)