3. COLLECTION AND TRANSPORT OF THROAT
AND MOUTH SWABS
In a hospital with a microbiology laboratory
1 In a good light and using the handle of a spoon to depress the
tongue, examine the inside of the mouth. Look for inflammation, and
the presence of any membrane, exudate, or pus.
– With diphtheria, a greyish-yellow membrane (later becoming greyish
green-black and smelly) can often be seen extending forwards over the
soft palate and backwards onto the pharyngeal wall.
– With a streptococcal sore throat, the tonsils are inflamed and often
covered in yellow spots.
– With Vincent’s angina, there is ulceration of the mouth, throat, or
lips.
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4. COLLECTION AND TRANSPORT OF THROAT
AND MOUTH SWABS
2 Swab the affected area using a sterile cotton wool swab. Taking care
not to contaminate the swab with saliva, return it to its sterile
container.
Important: For 8 hours before swabbing, the patient must not be
treated with antibiotics or antiseptic mouthwashes (gargles).
Caution: It can be dangerous to swab the throat of a child with acute
haemophilus epiglottitis because this may cause a spasm that can
obstruct the child’s airway. Blood for culture should be collected
instead.
3 Within two hours of collection, deliver the swab with a completed
request form to the laboratory.
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5. In a health center for dispatch to a
microbiology laboratory
1 Using a sterile swab (supplied in a tube of silica gel by the
microbiology laboratory), collect a specimen from the infected area as
described under the hospital collection of throat swabs.
2 Taking care not to contaminate the swab, return it to its tube. Seal
with adhesive tape and label the tube.
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6. Culture the specimen
Blood agar
– Inoculate the swab on a plate of blood agar Use the loop to make also a
few stabs in the agar (well area). Colonies of S. pyogenes growing below the
surface will show more distinct zones of haemolysis because of the
anaerobic conditions provided.
– When a swab is received in silica gel (e.g. from a health centre), moisten it
first with sterile nutrient broth and then inoculate the plate.
– Add a 0.05 unit bacitracin disc to the plate. This will help in the
identification of S. pyogenes. Some workers also add a co-trimoxazole disc
(as used for susceptibility testing) which prevents the growth of other
bacteria, making it easier to see betahaemolytic S. pyogenes colonies.
– Incubate the plate preferably anaerobically or, when this is not possible, in
a carbon dioxide enriched atmosphere overnight at 35–37 C. Candle jar
incubation will detect
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7. Culture the specimen
Culture : Blood agar:
S. pyogenes produces betahaemolytic colonies, i.e. the colonies are
surrounded by a zone of complete haemolysis with decolorization of
the haemoglobin. Colonies are usually small (0.5–1 mm), colourless,
dry, shiny or mucoid. Haemolysis is more marked under anaerobic
conditions as seen in colonies growing below the agar surface
(following stabs made in the culture medium.
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8. Laboratory Examination
First Day
Culture on Blood agar and incubate anaerobic or in C02 condition (add
Bacitracin Disc)
When diphtheria suspected culture on Tellurite Blood Agar
Second Day
Gram stain: G+ve pleomorphic Rod when diphtheria suspect
Examine and report culture: look for Beta Hemolysis and sensivity to
Bacitracin.
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