What is the difference between the groups of Streptococcus (Strep)?

Streptococcus Group Classification

Streptococci are classified into groups based on their hemolytic patterns and Lancefield grouping, with Group A Streptococcus (GAS) being the most clinically significant cause of bacterial pharyngitis, while Groups C and G can also cause pharyngitis but less commonly. 1

Primary Classification Systems

Hemolytic Pattern Classification

Streptococci are fundamentally categorized by their hemolytic activity on blood agar:

• Beta-hemolytic streptococci: Complete hemolysis (clear zone around colonies) - includes Groups A, C, and G that cause pharyngitis 1
• Alpha-hemolytic streptococci: Partial hemolysis (greenish discoloration) - includes viridans streptococci and pneumococci 1
• Gamma-hemolytic streptococci: No hemolysis 1

Lancefield Grouping (Carbohydrate Antigen-Based)

Group A Streptococcus (Streptococcus pyogenes) is the most important for pharyngitis:

• Most common bacterial cause of acute pharyngitis (15-30% in children, 5-10% in adults) 1
• Only commonly occurring form of acute pharyngitis for which antibiotic therapy is definitely indicated 1
• Can be divided into more than 100 M-serotypes or emm types based on M proteins 2
• Virulence directly related to M protein on cell surface that inhibits phagocytosis 2

Groups C and G Beta-Hemolytic Streptococci:

• Also capable of producing acute pharyngitis but less common than Group A 1
• Clinical presentation overlaps with Group A 1

Group D Streptococci (Enterococcus):

• Resistant to penicillin 3
• Not a typical cause of pharyngitis 3

Clinical Significance by Group

Group A Streptococcus - Key Distinguishing Features

Complications unique to Group A:

• Acute rheumatic fever (ARF) - prevention is a primary goal of treatment 1, 4
• Post-streptococcal glomerulonephritis 2, 4
• Suppurative complications (peritonsillar abscess, cervical lymphadenitis, mastoiditis) 1
• Scarlet fever 1, 3
• Invasive disease including streptococcal toxic shock syndrome and necrotizing fasciitis 2, 5

Antibiotic sensitivity:

• Penicillin remains treatment of choice with no documented resistance 1, 3
• Groups A, C, G, H, L, and M are very sensitive to penicillin 3

Streptococcus Anginosus Group (SAG)

This is a distinct classification comprising three species:

• Streptococcus anginosus, Streptococcus intermedius, and Streptococcus constellatus 6
• Also known as Streptococcus milleri group 6
• Classified as viridans streptococci (alpha-hemolytic) 6
• Oral commensals that act as opportunistic pathogens 6
• Associated with abscess formation rather than typical pharyngitis 6

Pneumococcal Infections

Pneumococci (Streptococcus pneumoniae):

• Cause mild to moderately severe respiratory tract infections 3
• Alpha-hemolytic pattern 1
• Different clinical syndrome than typical streptococcal pharyngitis 3

Critical Clinical Distinctions

Why Group A matters most for pharyngitis management:

• Only group requiring antibiotic treatment to prevent rheumatic fever 1
• Accounts for majority of bacterial pharyngitis cases requiring treatment 1
• Laboratory confirmation specifically targets Group A streptococcal carbohydrate antigen 1

Common pitfall: Assuming all beta-hemolytic streptococci require the same management approach - while Groups C and G can cause pharyngitis, they do not cause rheumatic fever, making the urgency of treatment different 1

Carrier state consideration: Up to 20% of school-aged children may be asymptomatic Group A streptococcal carriers during winter and spring, complicating diagnosis when they develop viral pharyngitis 7