Group G strep is a type of bacteria that normally lives harmlessly on your skin, throat, and gut but can sometimes cause infections ranging from mild skin problems to serious bloodstream infections. The main species responsible for human infections is Streptococcus dysgalactiae subspecies equisimilis, a close relative of group A strep, the bacterium behind strep throat. While less well known than its group A cousin, group G strep is increasingly recognized as a significant cause of invasive disease, particularly in older adults and people with chronic health conditions.
How Group G Strep Is Classified
Streptococcal bacteria are sorted into groups based on a sugar molecule on their surface called the Lancefield antigen, a classification system first described in the 1930s. Group G strep carries the group G version of this antigen. The bacteria that cause human group G infections were previously split into several separate species, but modern taxonomy now places them together under one name: Streptococcus dysgalactiae subspecies equisimilis. This same subspecies can also carry a group C antigen, which is why researchers often study group C and group G strep together.
A smaller number of group G infections come from Streptococcus canis, which primarily infects animals but occasionally crosses over to humans. Members of the Streptococcus anginosus group can also carry the group G antigen, though these bacteria behave quite differently and tend to cause abscesses rather than the typical skin and bloodstream infections associated with the main group G species.
Where It Lives and How It Spreads
Group G strep is a commensal organism, meaning it colonizes the human body without causing problems in most people. It can be found in the throat, on the skin, and in the gastrointestinal and genital tracts. The bacteria spread in ways similar to group A strep: through direct contact with infected wounds, respiratory droplets, and contaminated surfaces. A 2024 outbreak at a long-term care facility in Pennsylvania, investigated by the CDC, found that the bacteria spread among residents through wound care, with screening revealing colonization in the throats and wounds of multiple residents who had no symptoms of infection.
Why It Resembles Group A Strep
Group G strep is genetically very similar to Streptococcus pyogenes (group A strep) and shares many of the same tools for causing disease. Both produce M-like proteins on their surface that help them evade the immune system, along with enzymes that break down tissue and toxins that damage cells. Group G strep also makes streptolysin, the same cell-destroying compound that gives group A strep its ability to burst open red blood cells. This genetic overlap explains why group G strep causes a strikingly similar range of infections.
One key difference: the skin barrier matters more. Disruption of the skin, whether from wounds, surgical sites, eczema, or leg ulcers, is one of the most common predisposing factors for group G strep entering the bloodstream. Group G strep is more frequently associated with skin lesions than group A strep is.
Types of Infections It Causes
The spectrum of group G strep infections mirrors group A strep closely. On the milder end, it causes sore throats (pharyngitis), superficial skin infections, and wound infections. More serious infections include cellulitis, erysipelas (a type of skin infection involving the upper layers), and abscesses. In rare but dangerous cases, it can cause necrotizing fasciitis, the rapidly spreading “flesh-eating” soft tissue infection.
When the bacteria enter the bloodstream, they can seed infections throughout the body. Septic arthritis (joint infection), endocarditis (heart valve infection), meningitis, and pneumonia have all been documented. Endocarditis caused by group G strep carries particularly high rates of embolic complications, where clumps of bacteria break off heart valves and travel to other organs. The severity profile is comparable to endocarditis caused by Staphylococcus aureus, one of the most dangerous forms.
Who Is Most at Risk
Group G strep infections disproportionately affect people whose immune defenses or skin integrity are compromised. The most common risk factors include diabetes, cancer, chronic skin conditions, peripheral vascular disease, and any condition that creates open wounds or chronic skin breakdown. People who are immunosuppressed, whether from medications or underlying disease, face higher risk. Residents of long-term care facilities are particularly vulnerable because of the combination of chronic wounds, close living quarters, and frequent hands-on care.
Age plays a major role. Invasive infections are far more common in adults over 65. A Finnish national surveillance study spanning 2006 to 2020 found that invasive group C and G strep infections had a mean annual incidence of 10.1 per 100,000, which was actually higher than both group A strep (4.1 per 100,000) and group B strep (5.2 per 100,000). The incidence rose by about 8% per year over that period, a sharp and sustained increase.
How It Is Diagnosed
Diagnosing group G strep starts with a culture, typically from a throat swab, wound swab, or blood sample. The bacteria grow on standard lab media and produce a characteristic clear zone of destruction around their colonies (called beta-hemolysis), which looks identical to group A strep under a microscope. To tell them apart, labs use antigen detection kits that identify the specific Lancefield group, or newer technology like mass spectrometry that can pinpoint the exact species within hours. Rapid strep tests designed for group A strep will not detect group G, so a standard throat culture is needed if group G is suspected.
Treatment and Antibiotic Options
Group G strep responds well to penicillin, which remains the first-line treatment. For mild skin infections, oral penicillin or a related antibiotic is typically sufficient. More serious infections like cellulitis require intravenous antibiotics, and invasive infections such as bloodstream infections or septic arthritis are usually treated with high-dose intravenous penicillin for 10 to 14 days. For people with penicillin allergies, alternatives include certain cephalosporins and clindamycin.
There are some important resistance patterns to be aware of. Group G strep does not respond to certain antibiotics in the penicillin family that are designed for staph infections (oxacillin and nafcillin are ineffective). Resistance to tetracycline, erythromycin, and fluoroquinolones has been documented, which limits backup options in some cases.
How Serious Are Invasive Infections
Most group G strep infections are manageable with prompt treatment. A prospective study of 159 bloodstream infections in Finland found a case-fatality rate of 6%, with 7% of patients requiring intensive care. That said, specific complications carry much higher risks. Necrotizing fasciitis caused by group G strep has mortality rates between 19% and 33%, even with aggressive surgical and antibiotic treatment. Endocarditis also carries significant mortality and a high rate of complications from bacterial fragments traveling to other organs.
The overall trend is concerning. The rising incidence of invasive group C and G strep infections across multiple countries, combined with an aging population with more chronic disease, means these infections are becoming a more common clinical problem rather than the rare curiosity they were once considered.