Final answer:
Strep throat is caused by Streptococcus pyogenes, which forms chains of cocci. These bacteria belong to the group A streptococci and are the most common cause of bacterial pharyngitis.
Step-by-step explanation:
Strep throat, a severe pharyngitis, is caused by the bacteria Streptococcus pyogenes. These bacteria appear as chains of cocci and belong to the group A streptococci (GAS) or Lancefield group A. GAS is the most common cause of bacterial pharyngitis and is responsible for various skin infections as well. Streptococci are distinguished from other Gram-positive cocci by their negative response to the catalase test.