What are group A streptococci and how are they classified?
Group A streptococcal (GAS) infections are caused by Streptococcus pyogenes, a bacterium that is commonly found on the skin or in the upper respiratory tract (nose and throat). The term group A refers to the presence of a surface antigen on the bacterium, which helps in the microbiological identification of GAS. Streptococci are grouped according to these surface antigens, each type of antigen being allocated a letter of the alphabet; hence we have Lancefield groups A to W (excluding I and J), named after Dr Rebecca Lancefield who devised this classification scheme. Streptococci are also classified into pyogenic (‘pus-forming’) streptococci, which include groups A, B, C, and G, and non-pyogenic (non ‘pus-forming’) streptococci which encompasses the mitis, anginosus, bovis, salivarius and mutans sub-groups. Streptococcus pneumoniae, otherwise known as pneumococcus, forms its own distinct group.
