Group A Streptococcus in Blood Cultures: Almost Never a Contaminant
Group A streptococcus (GAS) in blood cultures should be considered a true pathogen, not a contaminant, and warrants immediate clinical attention and treatment. 1
Why GAS is Considered a True Pathogen
GAS (Streptococcus pyogenes) is consistently classified among organisms that are always considered pathogens when isolated from blood cultures, alongside other organisms like Streptococcus pneumoniae, Hemophilus influenzae, Neisseria meningitidis, and group B streptococcus. 1
This classification stands in stark contrast to organisms commonly regarded as contaminants, such as:
- Bacillus species 1
- Diphtheroids 1
- Propionibacterium 1
- Micrococcus 1
- Coagulase-negative staphylococci (unless specific risk factors present) 1
Clinical Significance and Disease Burden
GAS bacteremia represents serious invasive disease that demands urgent intervention:
GAS remains among the top ten causes of mortality from infectious disease globally, with more than 18 million people suffering from serious GAS disease worldwide 2
Invasive GAS infections carry high mortality rates despite effective antimicrobials, including life-threatening manifestations such as streptococcal toxic shock syndrome and necrotizing fasciitis 2, 3, 4
The organism causes more than 500,000 deaths annually worldwide 4
Critical Clinical Pitfall to Avoid
Never dismiss GAS in blood cultures as a contaminant or delay treatment while awaiting additional cultures. Unlike organisms such as coagulase-negative staphylococci or viridans streptococci (where multiple positive cultures strengthen clinical significance 5), a single blood culture positive for GAS should prompt immediate clinical action given the organism's pathogenic potential and association with rapidly progressive invasive disease. 2, 3, 1
The diverse clinical manifestations of GAS—ranging from invasive infections to toxin-mediated diseases and autoimmune sequelae—underscore why this organism must always be treated as clinically significant when isolated from normally sterile sites like blood. 2, 3