Throat Culture Health Article

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.