Throat Culture
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Throat culture refers to the growth and isolation of a micro-organism from a specimen taken from the throat. The results guide appropriate therapy in infectious and inflammatory diseases of the throat.
Cultures from the respiratory tract can be very difficult to interpret. These specimens are not easily collected without introducing contamination from adjacent areas. Furthermore, the contaminating organisms from these types of specimens are often considered pathogenic. At times, evaluation of the specimen by examining the direct Gram stain can help determine the quality of the specimen and therefore the potential significance of the isolates.
An organism is best isolated if the sample is collected as soon as possible, preferably within 5 days from disease onset.
The procedure of obtaining a throat culture (see the video below) is simple and straightforward.
The patient is explained the procedure and reassured that there will be no pain or discomfort, only a mild tickling sensation as the swab is taken. As the mouth is kept wide open and the tongue is at rest, a sterile swab is passed around the tonsils and posterior pharyngeal wall. A separate swab may be passed a little superiorly into the nasopharynx. For patients with a bulky tongue and in infants and small or uncooperative children, a tongue depressor is used to provide better exposure of the pharynx.
The swab is carefully replaced back into the sterile test-tube and capped.
Aside from sterile containers with swabs, petri dish, spirit lamp, streaking wire, and incubator, the other equipment necessary for throat culture are different types of culture media. The constituents of bacteriologic culture media are as follows: [1]
Amino-nitrogen base (digested protein) (eg, peptone)
Growth factors (eg, blood, serum, yeast extract)
Energy source (eg, sugars, carbohydrates)
Buffer salts (eg, phosphate, citrate)
Mineral salts (eg, calcium, magnesium, iron)
Selective agents (eg, chemicals, dyes, antimicrobials)
Indicators (eg, phenol red)
Gelling agent (for solid media) (eg, agar)
Transport media are usually not used for throat culture, which is principally performed for bacterial isolation, but are essential for the handling of specimens to identify viral antigens and immunoglobulins. An important transport medium is Dulbecco modified Eagle medium viral transport medium (DMEM VTM). [2]
A bacteriological culture may be performed using a throat swab, expectorated throat sputum, or throat washings the respiratory tract of patients with pharyngitis, croup, bronchitis, or pneumonia.
For throat washing, the patient gargles 10 mL of sterile saline, which is then expectorated into a sterile sputum container. It may be refrigerated and processed within 48 hours.
To obtain pure colonies, aseptic transfer is done and a streak plate produced first.
Depending on the source of specimen, treatment with antibiotics, as indicated for the purpose of determining sensitivity, may be required before inoculation of tissue cultures. After removing the swab, 0.9 mL of specimen is added to 0.1 mL antibiotic. It is vortexed and allowed to stand for 30 minutes at room temperature. If a large amount of cellular material is seen in the transport medium (cloudy appearance), the specimen is centrifuged for 5 minutes at full speed and the supernatant used for tissue culture inoculation.
Culture media include blood agar (BA), chocolate agar (CA), and Mueller-Hinton tellurite blood agar or Tinsdale agar (if Corynebacterium diphtheriae infection is suspected). Incubation on BA is better for streptococcal hemolysis. If CA is used, it may be kept for 18-48 hours at 35-37°C.
Microscopy is of no value for throat swabs unless Ludwig angina (a mixed infection caused by a spirochaeta and a gram-negative anaerobic fusiform) is suspected. [1]
The throat represents the pharynx in medical and scientific terms and may be further divided into the nasopharynx, oropharynx and hypopharynx. It is the oropharynx, and to some extent the nasopharynx, that houses the lymphoid tissues in the upper aero-digestive tract, the so-called Waldeyer ring, consisting of the various groups of tonsils. These tonsillar tissues are among the first lines of defense against pathogens entering the body.
The pharynx communicates anteriorly with the oral cavity, superiorly with the nose, laterally with the Eustachian tubes (and hence the ears), and inferiorly with the larynx and esophagus. It therefore forms the crossroads where infective agents, or pathogens, may gain access to any of the above relatively sterile areas. This access is prevented by, in addition to the tonsils and scattered lymphoid tissues, resident or nonpathogenic micro-organisms, specifically bacteria, that constitute what is known as the normal flora of the throat. This resident flora produces antigens that stimulate the formation of antibodies in the tonsils, producing humoral immunity, and provokes the accumulation and constant turnover of T-lymphocytes, macrophages, and mast cells, producing cellular immunity.
The normal flora of the respiratory tract is made up of the following: [1]
Common residents (>50% of the healthy general population): Oral streptococci, Neisseria species, Branhamella, Corynebacteria, Bacteroides, anaerobic cocci (Veillonella), fusiform bacteria, Candida albicans, Streptococcus mutans, Haemophilus influenza
Occasional residents (< 10% of the healthy general population): Streptococcus pyogenes, Streptococcus pneumonia, Neisseria meningitides
Uncommon residents (< 1% of the healthy general population) C diphtheriae, Klebsiella pneumonia (as well as Pseudomonas, Escherichia coli, and C albicans [especially after antibiotic treatment])
Residents in latent state in tissues, lungs and lymph nodes: Pneumocystis carinii, Mycobacterium tuberculosis, cytomegalovirus (CMV)
In sensory neurones and glands connected to mucosae: Herpes simplex virus, Epstein-Barr virus
The resident flora of the throat may be altered and attenuated by any insult to the body, leading to incompetence of the above mechanisms of immunity—a state of immunosuppression. Acute causes of this include viral infection, overuse or misuse of antibiotics and/or steroids, radiation, thermal or chemical insults, and the acute leukemias. Chronic immunosuppression is caused by anemia, diabetes, renal or hepatic disease, long-term steroid use, and malignancy. While this by itself has little clinical significance, it is essential to determine if a pathogenic strain of bacteria is present in the throat.
Almost 70% of upper respiratory infections are caused by viruses such as herpes simplex, adenoviruses, and respiratory syncytial virus (RSV). Viral infections caused by these common viruses may be precipitated by a temporary breakdown of immunity as happens in times of stress, sudden changes in weather, or exposure to chill. More severe viral infections may occur in epidemic or pandemic forms and are caused by extremely virulent virus strains.
A viral infection is usually self-limiting but may be complicated by a bacterial superinfection due to poor immunity or a severe viral load. S pyogenes, the most important bacterial pathogen of the throat, is a group A beta-hemolytic Streptococcus (GABHS), and the infection is commonly known as strep throat or streptococcal sore throat. It appears on a blood agar plate as a small, convex, translucent, whitish colony surrounded by a clear zone of beta-hemolysis (see image below). All beta-hemolytic colonies on throat culture must be checked to rule out S pyogenes. Prompt diagnosis and treatment of streptococcal pharyngitis is imperative in the prevention of complications, namely quinsy, otitis media, mastoiditis, sinusitis, scarlet fever, rheumatic fever, rheumatic heart disease, and acute glomerulonephritis.
Rarer forms of beta-hemolytic streptococci have been discovered, but GABHS have the potential to cause the dreaded complications of streptococcal sore throat. While a viral infection is short-lived and may be relieved by symptomatic treatment, bacterial superinfection requires antibiotic therapy instituted as soon as possible after identification of the organism and its antibiotic sensitivity.
Another significant pathogen in bacterial pharyngitis is Neisseria gonorrhoeae. To isolate this organism, special media such as Thayer-Martin or Martin-Lewis are required. [3] The colonies of this organism are small to medium in size, flat, gray to tan, and buttery or slightly rubbery. Colonies of this type growing on the selective media should be further identified to rule out Neisseria meningitidis, which may also be a resident throat pathogen. Carriers of this organism may be identified with throat culture during outbreaks of meningitis, although it does not cause pharyngitis. Colonies of the two Neisseria species may be differentiated with biochemical tests.
Other possible pathogens suspected in a case of acute pharyngitis (eg, C diphtheriae) may also be identified with throat culture. The colonies produced by this pathogen are small, opaque, and white or gray. They are catalase-positive, and Gram staining shows gram-negative rods with a club shape. Individual cells are aligned parallel to each other in palisades. Further tests are required for confirmation, such as Albert stain on smear, growth in Loeffler serum, culture in Mueller-Hinton tellurite agar, Schick test for susceptibility, and Elek test for toxigenicity.
The principal uses of a throat culture are in the following:
Confirmation of a viral infection (if no pathogenic bacteria are isolated)
Diagnosis of a strep throat or diphtheria
Exclusion of a carrier state
Determining the antibiotic sensitivity of an organism
Detection of meningococci
Testing kits for throat culture should undergo stringent evaluation by the concerned laboratories before widespread use in order to ensure optimum sensitivity and specificity. [4] An important contraindication to throat culture is suspected acute epiglottitis, in which attempts to obtain a throat swab may be catastrophic and kill the patient, especially if it is a child, as it may precipitate a total respiratory obstruction.
It is meaningless to obtain a sample for culture in cases where empirical antibiotic therapy has already been started.
Mims, Playfair, Roitt et al. Medical Microbiology. 2nd Edition. Mosby;
Lisa Anne Shimeld. Essentials of Diagnostic Microbiology. Delmar Publishers; 1999.
Microbiology-A Laboratory Manual. Cappuccino & Sherman. Pearson Education. 6th Edition.
Forward KR, Haldane D, Webster D, Mills C, Brine C, Aylward D. A comparison between the Strep A Rapid Test Device and conventional culture for the diagnosis of streptococcal pharyngitis. Can J Infect Dis Med Microbiol. 2006 Jul. 17(4):221-3. [Medline].
Jayita Poduval, MS, MBBS, DNB(ENT), DORL Assistant Professor, Department of ENT, Pondicherry Institute of Medical Sciences, India
Disclosure: Nothing to disclose.
Eric B Staros, MD Associate Professor of Pathology, St Louis University School of Medicine; Director of Clinical Laboratories, Director of Cytopathology, Department of Pathology, St Louis University Hospital
Eric B Staros, MD is a member of the following medical societies: American Medical Association, American Society for Clinical Pathology, College of American Pathologists, Association for Molecular Pathology
Disclosure: Nothing to disclose.
Throat Culture
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