Beta-haemolytic bacitracin sensitive strains of Streptococcus salivarius: a source of confusion with Streptococcus pyogenes in throat swab cultures.

Thirty strains of Streptococcus salivarius were beta-haemolytic when initially isolated from throat swabs and, because they were also bacitracin sensitive, they could be confused with the pathogenic Streptococcus pyogenes on primary culture plates. These strains were isolated form approximately 3000 throat swab cultures made in 6 mth, during which time 220 Lancefield group A organisms were also detected. Other features of these organisms, including loss of beta-haemolysis on subculture, penicillin resistance in half of the strains and ability to grow on MacConkey bile salt agar, are not shared by Strep. pyogenes, and their demonstration enables strains of the 2 species to be simply distinguished.