Posterior pharynx cultures are most commonly submitted to detect which organism?

• A. Streptococcus pneumoniae
• B. Streptococcus pyogenes
• C. Corynebacterium diphtheriae
• D. Staphylococcus aureus

Answer: (B)

Detecting Group A Streptococcus in the posterior oropharynx is the typical purpose of throat cultures done for suspected strep throat. This organism commonly colonizes the back of the throat during acute pharyngitis and is the primary bacterial cause whose early identification guides antibiotic treatment and helps prevent complications like rheumatic fever. In the lab, the sample from the posterior pharynx is plated on standard media; Streptococcus pyogenes often shows beta-hemolysis and, in many settings, bacitracin sensitivity helps confirm it as the causative agent. Other organisms listed—Streptococcus pneumoniae, Corynebacterium diphtheriae, and Staphylococcus aureus—are associated with different infections or colonization patterns and are not the usual targets of routine posterior pharyngeal cultures for pharyngitis.