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Throat Culture

Throat culture

Definition

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.

Purpose

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 present in the patient's throat and to identify people who are carriers of organisms that cause meningitis and whooping cough , among other diseases.

Besides the use of throat cultures in diagnosis, the bacteria identified are used to determine antibiotic sensitivity, allowing physicians to select the most appropriate and effective antibiotic to treat a specific infection. It is common for physicians to order culture and sensitivity tests at the same time.

Description

A throat culture will often be performed on an individual who has a severe sore throat or known symptoms of strep throat. These symptoms include a sore throat that may be accompanied by fever , body aches, and loss of appetite. The tonsils and the back of the throat may appear red, swollen, and streaked with pus. Symptoms usually appear one to three days after being exposed to the group A streptococcus S. pyogenes. Strep throat occurs more often among children than adults, with incidence at peak in fall and winter when school is in session and contact with other children is highest. Because strep is highly contagious, family members and close contacts of individuals diagnosed with strep throat may also be advised to have throat cultures if they show signs of sore throat or other symptoms.

The specimen for throat culture is obtained by wiping the child's throat with a sterile cotton swab. The child is asked to tilt the head back and open the mouth wide. With the tongue depressed and the child saying "ah," the care provider wipes the back of the throat and the tonsils with the sterile swab, applying it to any area that appears either very red or is discharging pus. 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. The swabbing procedure may cause gagging but is not painful. Obtaining the specimen takes less than 30 seconds. Laboratory results will be available as soon as bacteria grow in a special plate that has been streaked with the contaminated swab, usually within two to three days. Sometimes the organism cultured is not strep as suspected. The microbiology laboratory may use samples of the bacteria grown to perform other tests that will help identify the disease causing organism.

S. pyogenes is known to grow well in growth media such as rich broths or gels (agars) that are supplemented with blood. When strep is suspected, the throat material is cultured on blood agar that has been prepared as a broth and poured into petri dishes (plates) where it solidifies into a gel. Blood agar is usually made from the cell walls of red algae (also trypticase soy, heart infusion, or Todd-Hewitt agar) and sheep's blood. When the throat swab reaches the laboratory, the microbiologist uses it to make streaks directly across a blood agar plate. The covered plate is allowed to incubate at a specific temperature (35°37°C) for 24 to 48 hours to foster the growth of bacteria. The bacteria will grow in clusters called colonies. If the organism is a group A hemolytic streptococcus, an area immediately around the bacterial colony will show hemolysis (the breaking up or lysing of red blood cells), leaving a clear zone surrounding the colony. This helps a technician identify a hemolytic strep organism visually. Other types of bacteria may grow in differently sized or shaped colonies, allowing the microbiologist to differentiate the bacteria. A sample of the bacterial colony may also be examined microscopically to evaluate bacterial type or morphology. Samples of the bacteria may be restreaked on another agar plate with small disks of specific antibiotics to see which antibiotics destroy the bacteria (sensitivity testing). The physician may then prescribe the most effective antibiotic.

When strep throat is suspected, it may be screened in a quick test in the doctor's office. These tests allow direct detection of streptococcal antigens in body fluids such as urine or blood serum or from a throat swab. The test uses a strip or disc that is chemically coated with an antibody specific for the strep antigen. If strep is present, a visible reaction occurs with the antibody on the strip when combined with material from the throat. Depending upon the manufacturer's method, results may be available in about ten to 30 minutes. These "instant" tests are not as definitive as cultures but their reliability has improved since they were first introduced. If an instant throat test is negative, however, a throat culture will still be performed to verify the negative results or to identify non-strep organisms.

Precautions

Gargling to clear the throat or treatment with antibiotics will affect culture results and may make identification of the bacteria impossible. The child should not gargle immediately before the culture.

The child's throat should be swabbed and the culture performed before any antibiotics are taken. The laboratory should be informed if the patient has recently taken antibiotics for the current infection or any other infection. After the culture, however, the physician may initiate early treatment by prescribing a broad spectrum antibiotic to be started before results of the culture are available. After the organism has been identified and sensitivity testing has indicated the most effective antibiotic, a different, more specific antibiotic can be prescribed.

The child's immunization history should be checked to evaluate the possibility that diseases other than strep are causing the sore throat. The care provider should wash his or her hands carefully after swabbing the throat and handling the specimen to prevent the spread of any infectious organisms. Hand washing should be done at home also to reduce contact with infective material. Spreading is usually from contact with droplets of material from the nose and throat of affected individuals.

Preparation

There is no special preparation involved before performing a throat culture. The individual does not need to avoid food or fluids before the test.

Aftercare

There are no special care recommendations after throat swab and culture have been performed. There are no unusual effects expected from having the throat swabbed, though the child may have a mild sensation of something present in the throat for several hours after it has been swabbed.

Risks

Healthcare professionals, parents, or other contacts are at risk of exposure to the child's illness. Strep throat is highly contagious and easily spread through contact with droplets from the nose or throat.

Normal results

Normal results would include finding organisms that grow in healthy throat tissues (normal flora). These organisms include non-hemolytic and alpha-hemolytic streptococci, some Neisseria species, staphylococci, diphtheria and hemophilus organisms, pneumococci, yeasts, and Gram-negative rods.

Abnormal results

In addition to S. pyogenes, other disease agents may be identified in the throat culture. Besides other varieties of strep organisms, these organisms may include Candida albicans, which can cause thrush; Corynebacterium diphtheriae, which can cause diphtheria; and Bordetella pertussis, which can cause whooping cough . Inaddition, the appearance of a specific normal organism in very high numbers may also be regarded as an abnormal result.

Parental concerns

Parents may be concerned that effective treatment will be delayed because of waiting for the throat culture results, which can take up to 48 hours. Physicians may prescribe a broad spectrum antibiotic as initial treatment rather than waiting for culture results. When the culture results are available and sensitivity tests indicate a more effective antibiotic, the physician will likely prescribe a new antibiotic specific for the strep or other organism identified.

KEY TERMS

Agar A gel made from red algae that is used to culture certain disease agents in the laboratory.

Antibiotics Drugs that are designed to kill or inhibit the growth of the bacteria that cause infections.

Antigen A substance (usually a protein) identified as foreign by the body's immune system, triggering the release of antibodies as part of the body's immune response.

Carrier A person who possesses a gene for an abnormal trait without showing signs of the disorder. The person may pass the abnormal gene on to offspring. Also refers to a person who has a particular disease agent present within his/her body, and can pass this agent on to others, but who displays no symptoms of infection.

Diphtheria A serious, frequently fatal, bacterial infection that affects the respiratory tract. Vaccinations given in childhood have made diphtheria very rare in the United States.

Hemolytic Able to break down or dissolve red blood cells.

Morphology Literally, the study of form. In medicine, morphology refers to size, shape, and structure rather than function.

Streptococcus Plural, streptococci. Any of several species of spherical bacteria that form pairs or chains. They cause a wide variety of infections including scarlet fever, tonsillitis, and pneumonia.

Thrush An infection of the mouth, caused by the yeast Candida albicans and characterized by a whitish growth and ulcers.

Whooping cough An infectious disease of the respiratory tract caused by a bacterium, Bordetella pertussis. Also known as pertussis.

See also Strep throat.

Resources

ORGANIZATIONS

American Academy of Pediatrics. 141 Northwest Point Boulevard, Elk Grove Village, IL 600071098. Web site: <www.aap.org>.

Centers for Disease Control. 200 Independence Avenue, SW, Washington, DC, 20201. Web site: <www.cdc.gov>.

Centers for Disease Control and Prevention. 1600 Clifton Rd., NE, Atlanta, GA 30333. Web site: <www.cdc.gov>.

WEB SITES

Rutherford, Kim. "Strep Throat." KidsHealth, May 2001. Available online at <http://kidshealth.org/parent/infections/lung/strep_throat.html> (accessed December 1, 2004).

Wener, Kenneth. "Throat Swab Culture." MedlinePlus August 11, 2003. Available online at <www.nlm.nih.gov/medlineplus/ency/article/003746.htm> (accessed December 1, 2004).

L. Lee Culvert Cindy L. A. Jones, PhD

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Throat Culture

Throat Culture

Definition

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.

Purpose

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, throat cultures are sometimes used to test antibiotics for their effectiveness in treating different infections.

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 care provider should wash the hands carefully after taking the specimen to prevent the spread of any infectious organisms.

Description

A throat culture test should be done on anyone who has symptoms of a strep throat. These symptoms include a sore throat that may be accompanied by a fever, body aches, and loss of appetite. Age is a consideration, in that strep throat is more common in children than in adults. The tonsils and the back of the throat often appear red, swollen, and streaked with pus. These symptoms usually appear one to three days after being exposed to group A strep. Because strep is highly contagious, family members and close contacts of patients diagnosed with strep throat should also have throat cultures performed if they show signs of the disease.

The specimen for throat culture is obtained by wiping the patient's throat with a cotton swab. The patient is asked to tilt the head back and open the mouth wide. With the tongue depressed and the patient saying "ah," the care provider wipes the back of the throat and the tonsils with a sterile swab. The swab is applied to any area that appears either very red or discharging pus. 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. Obtaining the specimen takes less than 30 seconds. Laboratory results are usually available in two to three days. The swabbing procedure may cause gagging but is not painful. The doctor makes a note for the laboratory to indicate if any disease organisms other than strep are suspected, because some require special growth conditions in the laboratory.

S. pyogenes is cultured on a growth medium called blood agar. Agar is a gel that is made from the cell walls of red algae. Blood agar is made from agar gel and sheep's blood. When the throat swab reaches the laboratory, it is wiped across a blood agar plate. The plate is allowed to incubate for 24-48 hours to allow the growth of bacteria. If the organism is a Group A hemolytic streptococcus, the area immediately around the bacterial colony will be cleared of red blood cells. Hemolytic streptococci dissolve (lyse) red blood cells, leaving a clear zone surrounding the colony.

Alternative procedures

So-called instant strep tests are now available to help diagnose strep throat. They can be used in the doctor's office and take about 10-30 minutes to perform. Instant tests detect an antigen associated with the streptococcus. These tests are relatively new and not available at all clinics. Their reliability has improved since they were first introduced. If an instant throat test is negative, however, a standard throat culture can be performed to verify the results.

Preparation

The patient does not need to avoid food or fluids before the test. Recent gargling or treatment with antibiotics, however, will affect the culture results. The laboratory should be notified if the patient has been recently taking antibiotic medications.

Aftercare

No specific aftercare is needed.

Risks

There is a minor risk for the health professional of exposure to the patient's illness.

Normal results

Normal results would include finding organisms that grow in healthy throat tissues. These organisms include non-hemolytic and alpha-hemolytic streptococci, some Neisseria species, staphylococci, diphtheria and hemophilus organisms, pneumococci, yeasts, and Gram-negative rods.

Abnormal results

In addition to S. pyogenes, other disease agents may be identified in the throat culture. Infectious agents that can be identified include Candida albicans, which can cause thrush; Corynebacterium diphtheriae, which can cause diphtheria; and Bordetella pertussis, which can cause whooping cough. In addition, the appearance of a normal organism in very high numbers may also be regarded as an abnormal result.

Resources

ORGANIZATIONS

American Medical Association. 515 N. State St., Chicago, IL 60612. (312) 464-5000. http://www.ama-assn.org.

Centers for Disease Control and Prevention. 1600 Clifton Rd., NE, Atlanta, GA 30333. (800) 311-3435, (404) 639-3311. http://www.cdc.gov.

KEY TERMS

Agar A gel made from red algae that is used to culture certain disease agents in the laboratory.

Antibiotic A drug given to stop the growth of bacteria. Antibiotics are ineffective against viruses.

Antigen A substance that interacts with an antibody and causes an immune reaction.

Carrier A person harboring an infectious disease who may be immune to it but who can give it to others.

Diphtheria A serious disease caused by a bacterium, Corynebacterium diphtheriae.

Hemolytic Able to dissolve red blood cells. The bacteria that cause strep throat are hemolytic organisms.

Streptococcus A category (genus) of sphere-shaped bacteria that occur in pairs or chains.

Thrush A disease occurring in the mouth or throat that is caused by a yeast, Candida albicans.

Whooping cough An infectious disease of the respiratory tract caused by a bacterium, Bordetella pertussis.

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Throat Culture

Throat Culture

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 95 °F (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 is 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 non-immobilized 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 antigenantibody complexes, forming a colored bar or line in the reaction zone area.

Preparation

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.

Aftercare

No specific aftercare is needed.

Complications

There is a minor risk to the health care provider collecting the specimen of contracting a viral or bacterial infection from the patient.

Results

The results from throat cultures identify the presence of any pathogenic bacteria growing on the media. Non-disease-producing organisms that grow in healthy throat tissues include nonhemolytic 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.

Patient education

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.

KEY TERMS

Antibiotic— A drug given to stop the growth of bacteria. Antibiotics are ineffective against viruses.

Antigen— A substance that interacts with an antibody and causes an immune reaction.

Carrier— A person harboring an infectious disease who may be immune to it but who can transmit the disease to others.

Diphtheria— A serious disease of the throat, nose, and lungs caused by a bacterium, Corynebacterium diphtheriae.

Streptococcus A category (genus) of sphereshaped bacteria that occur in pairs or chains.

Thrush— A 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 cough— An infectious disease of the respiratory tract caused by a bacterium, Bordetella pertussis.

Resources

BOOKS

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.

PERIODICALS

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.

ORGANIZATIONS

The American Society for Clinical Laboratory Science. 7910 Woodmont Ave., Suite 523, Bethesda, MD 20814. (301) 657-2768. 〈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. 〈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. 〈ttp://www.niaid.nih.gov〉.

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Throat Culture

Throat culture

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.

Preparation

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.


KEY TERMS


Antibiotic —A drug given to stop the growth of bacteria. Antibiotics are ineffective against viruses.

Antigen —A substance that interacts with an antibody and causes an immune reaction.

Carrier —A person harboring an infectious disease who may be immune to it but who can transmit the disease to others.

Diphtheria —A serious disease of the throat, nose, and lungs caused by a bacterium, Corynebacterium diphtheriae.

Streptococcus —A category (genus) of sphere-shaped bacteria that occur in pairs or chains.

Thrush —A 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 cough —An infectious disease of the respiratory tract caused by a bacterium, Bordetella pertussis.


Aftercare

No specific aftercare is needed.

Complications

There is a minor risk to the health care provider collecting the specimen of contracting a viral or bacterial infection from the patient.

Results

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.

Patient education

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.

Resources

BOOKS

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.

PERIODICALS

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.

ORGANIZATIONS

The American Society for Clinical Laboratory Science. 7910 Woodmont Ave., Suite 523, Bethesda, MD 20814. (301) 657-2768. <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. <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. <http://www.niaid.nih.gov>.

Linda D. Jones, B.A., PBT (ASCP)

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