The journey of a throat swab

A sore throat is a common affliction many of us will suffer from time to time. It’s a well-known fact that the majority of sore throats are viral, where rest and symptomatic relief are prescribed, rather than antibiotics.

However in around 20 per cent of cases a bacterial infection will be the culprit, and a throat swab needed to confirm the diagnosis. 

Mackenzie Nicol, team leader of Microbiology, tells us about throat swabs and what happens once they arrive at the laborator.

A clinical indication

A throat swab is taken when your doctor suspects a bacterial infection is the cause of your discomfort. Together with your clinical details, the throat swab is sent to the laboratory for processing.

The main pathogen of interest is Group A Streptococcus. Its occurrence in the throat is commonly referred to as ‘strep throat’.

Breeding bacteria on a plate

At the laboratory an attempt is made to grow the bacteria. The specimen is placed on a blood agar plate and ‘streaked’ out, so single colonies of the bacteria can be recognised with the naked eye. The plate also contains a disc of bacitracin, an antibiotic.

The blood agar plate then spends the night in a carbon dioxide incubator, set to the normal body temperature of 37°

A positive result

The next morning we analyse the blood agar plate, looking for beta-haemolytic streptococci. Beta-haemolysis is when a pathogen has the ability to destroy red blood cells.

Remember the disc of bacitracin added to the plate? Group A Streptococcus is sensitive to the antibiotic bacitracin. If this bacterium is present there will be a clear zone of no growth surrounding the bacitracin disc.

If a zone of inhibition is observed, testing will be done to confirm the bacterium.

The results, along with a comment on antibiotic sensitivity, will be with your doctor around lunchtime the day after receiving your sample

“The thing with microbiology,” says Mackenzie, “is that you have to wait for bugs to grow!”