Different Strains of Strep Throat
Yes, there are multiple distinct strains of Group A Streptococcus (the bacteria causing strep throat), and these strains differ in their surface proteins, virulence factors, and tissue preferences—some strains preferentially cause throat infections while others cause skin infections. 1, 2
Strain Classification Systems
Group A β-hemolytic streptococci can be differentiated by several typing methods that identify distinct strains:
M Protein Typing
- M protein is the primary type-specific antigen used to classify Group A Streptococcus into over 200 different serotypes, with each M type representing a genetically and immunologically distinct strain 3
- The M protein determines virulence and resistance to phagocytosis, and antibodies to M protein provide type-specific immunity 3
- A newer classification scheme based on emm gene sequencing (which encodes M protein) has identified sequence clusters that correlate with clinical disease patterns—certain emm types are associated with pharyngitis while others predominantly cause skin infections 2
T Protein Typing
- T antigen provides an additional typing system that can distinguish strains, though it does not confer protective immunity like M protein 3
- T typing is particularly useful when M typing is unavailable, as strains can be classified by T-protein patterns 1
Clinical Significance of Different Strains
- Distinct throat strains and skin strains have been recognized for decades, with molecular differences in their virulence factors determining tissue tropism 2
- Certain M protein types are rheumatogenic (associated with acute rheumatic fever risk), while others are nephritogenic (associated with post-streptococcal glomerulonephritis) 4
- The structural features of M proteins, particularly class I M proteins, appear critical for rheumatogenic potential 4
Non-Group A Streptococcal Strains
Beyond Group A Streptococcus, other streptococcal groups can cause pharyngitis:
Groups C and G Streptococci
- Groups C and G β-hemolytic streptococci cause pharyngitis with clinical features similar to Group A strep 1, 5
- These organisms, particularly Streptococcus dysgalactiae subsp. equisimilis, possess M proteins analogous to Group A strains and can cause both throat and invasive infections 6, 4
- Acute rheumatic fever has not been described as a complication of group C or G pharyngitis, which is why routine antibiotic treatment is less critical 1
Emerging Pathogens
- Fusobacterium necrophorum is an emerging cause of pharyngitis in adolescents and young adults, representing a completely different bacterial species that can progress to life-threatening Lemierre syndrome 7, 8
Clinical Implications
The existence of multiple strains matters clinically because:
- Immunity to one M type does not protect against other M types, allowing repeated strep throat infections with different strains 3
- Strain typing should be saved when possible during clinical trials and outbreak investigations to track transmission patterns 1
- Treatment failure may represent reinfection with a new strain rather than persistence of the original strain 1