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Streptococci

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Overview

Streptococci are a genus of Gram-positive coccoid bacteria found in pairs or chains. They can be divided into alpha and beta haemolytic and non-haemolytic types depending on their appearance on blood agar.

Alpha-haemolytic streptococci

Alpha-haemolytic streptococci cause incomplete haemolysis on blood agar and include Streptococcus pneumoniae and Streptococcus viridans.

Beta-haemolytic streptococci

Beta-haemolytic streptococci cause complete haemolysis on blood agar and can be subdivided based on their antigens into groups A-H. Groups A, B, and D are clinically important in humans.

Streptococcus pneumoniae

Also known as the pneumococcus, Streptococcus pneumoniae is a Gram-positive coccus that grows in pairs (diplococci) found in the upper respiratory tract (the nasopharynx), with increases in colonisation in the winter.

It is the most common cause of pneumonia and is associated with:

Streptococcus pyogenes (Group A Streptococcus)

Also known as Group A Streptococcus, Streptococcus pyogenes is a Gram-positive coccus that is often found in the upper respiratory tract and is associated with:

Streptococcus bovis

Streptococcus bovis is a Gram-positive coccus found in the gut of cows and sheep and sometimes in infective endocarditis associated with colorectal cancer. It is unclear as to why people with malignancy can have increased Streptococcus bovis carriage and it is thought to be due to the proteins helping it adhere to valves and abnormal colorectal tissue.

Any person with Streptococcus bovis bacteraemia should be screened for colorectal cancer and infective endocarditis.

Viridans Streptococci

Also known as the oral streptococci, Viridans Streptococci are a large group of Streptococci that colonise the respiratory tract, with large numbers in the mouth. This includes Streptococcus mitis and Streptococcus sanguinis. They are associated with poor dental hygiene, infective endocarditis, and abscesses.

Streptococci agalactiae (Group B Streptococcus)

Also known as Group B Streptococcus (GBS), Streptococcus agalactiae is the most common cause of neonatal sepsis in children <3 months. 20-40% of women are colonised with GBS in the genital or lower gastrointestinal tracts.

Risk factors for GBS infection include:

  • Prematurity
  • Diabetes mellitus
  • Premature rupture of the membranes
  • Intrapartum fever (e.g. due to chorioamnionitis)

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