What bacterium causes strep throat?

The bacterium that causes strep throat is Streptococcus pyogenes. It is also called group A Streptococcus, or GAS, and is a Gram-positive bacterium that commonly grows in chains.

Strep throat is an infection of the pharynx and tonsils. Typical symptoms include a sudden sore throat, fever, red or swollen tonsils, and tender lymph nodes in the front of the neck. Some cases produce a sandpaper-like rash called scarlet fever. The infection spreads mainly through respiratory droplets and close contact.

Not every sore throat is strep throat: viruses cause most sore throats, and antibiotics do not treat viral infections. A rapid antigen test or throat culture can identify GAS. Some people carry S. pyogenes in the throat without symptoms, which can complicate interpretation of test results.

The same bacterium can cause skin infections such as impetigo, cellulitis, and erysipelas. Rarely, it causes severe illnesses including necrotizing fasciitis or streptococcal toxic shock syndrome. Untreated strep throat can also lead to complications such as rheumatic fever.