A wide range of bacteria belong to the genus Streptococcus, many of which are commonly found in the human body without causing illness. Scientists categorize these bacteria using the Lancefield grouping system, based on specific carbohydrates on the bacterial cell wall, designating them with letters like A, B, C, and G. While Group A and Group B Strep are widely known, Group C Streptococcus (GCS) represents a distinct, though less frequent, cause of human infection.
What Defines Group C Strep
Group C Streptococcus bacteria are Gram-positive cocci, meaning they appear purple under a microscope and have a spherical shape. They are classified within the Lancefield system because they possess a unique C carbohydrate antigen on their cell surface. This group includes several species, with the two most relevant to human infection being Streptococcus dysgalactiae subsp. equisimilis (SDSE) and Streptococcus zooepidemicus.
These species are typically beta-hemolytic, meaning they cause complete lysis of red blood cells when grown on a blood agar plate. GCS species are naturally found as part of the normal bacterial flora in many animals, particularly livestock. They can also colonize the human throat, skin, or intestines without causing symptoms.
How Common Is Group C Strep
Group C Strep is generally considered less common in causing disease than Group A Streptococcus (GAS), the primary cause of strep throat, and Group B Streptococcus (GBS), a major concern for neonatal infections. However, GCS is not rare, and it accounts for a measurable portion of overall streptococcal infections, with some reports suggesting GCS and GGS combined represent approximately six percent of cases. The prevalence of invasive GCS infection has been estimated at around 10.1 cases per 100,000 people annually in some regions.
Human infection with GCS often occurs through zoonotic transmission from animals to humans. Exposure to livestock or contaminated food products, such as unpasteurized milk, is a common route, which limits widespread person-to-person transmission compared to GAS. The primary GCS species causing human disease shares evolutionary traits with GAS, explaining why it can cause similar types of infections. While GCS is less frequently isolated from throat cultures than GAS, it is a recognized cause of pharyngitis.
Symptoms and Who Is Most at Risk
The clinical presentation of a Group C Strep infection can range from mild to life-threatening, mirroring the spectrum of diseases caused by other streptococci. The most common manifestations include pharyngitis, which presents as a sore throat, and skin and soft tissue infections, such as cellulitis and abscesses. GCS is also known to cause more severe, deep-seated skin infections.
A significant concern is invasive GCS disease, where the bacteria penetrate normally sterile body sites. This can lead to serious conditions like bacteremia, sepsis, and endocarditis. Invasive GCS infections can progress rapidly and have a high risk of severe outcomes.
Certain individuals face a higher risk of developing severe or invasive GCS disease. The elderly population and people with underlying chronic medical conditions are disproportionately affected. This includes individuals with diabetes, cancer, chronic liver disease, and compromised immune systems.
Treating a Group C Strep Infection
The standard approach for managing a Group C Strep infection involves antibiotic treatment. Penicillin is the first-line medication of choice for GCS infections, a treatment regimen similar to that used for Group A Strep. GCS bacteria have maintained high susceptibility to penicillin.
In cases where a patient has a documented penicillin allergy, alternative antibiotics are available. Macrolides, such as azithromycin or clindamycin, are commonly used as effective substitutes. The duration of treatment, typically a 10-day course, is important to ensure the complete eradication of the bacteria and prevent potential complications.
Any suspected bacterial infection requires a prompt medical evaluation. Timely diagnosis and initiation of appropriate antibiotic therapy are paramount, especially for invasive infections like bacteremia, where delays can lead to significantly worse outcomes.