Streptococcus dysgalactiae is a bacterium gaining recognition in human medicine, though it remains less familiar than its relative, Streptococcus pyogenes. This organism belongs to the Streptococci family, which commonly causes various illnesses. S. dysgalactiae is responsible for a wide spectrum of human infections, ranging from mild throat and skin conditions to severe, life-threatening systemic disease.
Identity and Characteristics of the Group
The organism responsible for most human disease is Streptococcus dysgalactiae subspecies equisimilis. This subspecies is a Gram-positive, coccal bacterium, often classified serologically as a large-colony-forming Group C or Group G streptococcus. It is characterized by beta-hemolysis, meaning it completely lyses red blood cells when grown on blood agar plates.
S. dysgalactiae commonly colonizes the human body, residing in the throat, skin, and the female genital and alimentary tracts. Its presence often does not cause symptoms, establishing it as part of the normal human flora. It is genetically closely related to S. pyogenes (Group A Strep), sharing approximately 70% of its genome. This overlap allows it to express similar virulence factors, such as the M-protein, which facilitates host tissue attachment and immune evasion.
The organism’s pathogenicity is largely opportunistic, meaning it typically requires a breach in the body’s natural defenses to cause an infection. While S. dysgalactiae also infects animals, human-associated strains are genetically distinct from those causing conditions like bovine mastitis.
Range of Infections in Humans
S. dysgalactiae causes a broad range of clinical manifestations that closely mirror those caused by S. pyogenes. Infections are categorized as non-invasive or invasive. Non-invasive infections are the most common presentation and include localized conditions like pharyngitis (strep throat), which is difficult to distinguish from Group A Strep pharyngitis based on symptoms alone.
The bacterium frequently causes skin and soft tissue infections, particularly cellulitis and erysipelas, which involve inflammation of the deeper skin layers. These localized infections typically start when the bacterium gains entry through a break in the skin barrier. The incidence of invasive disease caused by S. dysgalactiae is rising in some regions, sometimes approaching the rate seen with Group A Strep.
Invasive infections occur when the bacterium breaches the initial site and enters normally sterile body sites, such as the bloodstream. Bacteremia (bacteria in the blood) is a serious manifestation of invasive disease. From the bloodstream, the infection can disseminate to cause focal infections in deep tissues.
Disseminated infections include:
- Septic arthritis (infection of the joints).
- Endocarditis (inflammation of the heart’s inner lining or valves).
- Necrotizing fasciitis (a rapidly progressing soft-tissue infection that destroys muscle fascia and fat).
- Streptococcal toxic shock syndrome (a severe illness characterized by a sudden drop in blood pressure and organ failure).
Sources of Transmission and Vulnerable Populations
Transmission of S. dysgalactiae typically occurs through person-to-person contact, often via respiratory droplets or direct contact with skin lesions. However, most infections are endogenous, meaning the bacteria originates from the patient’s own colonizing flora. The bacterium moves from its normal habitat into deeper tissue following an injury or underlying medical issue.
Individuals with specific pre-existing conditions are at a higher risk for developing severe, invasive disease. The elderly population is particularly vulnerable, often showing a higher incidence of bacteremia. People with chronic illnesses that compromise immune function are also highly susceptible.
Risk factors for invasive infections include:
- Diabetes mellitus.
- Cancer.
- Liver disease.
- Conditions causing a breakdown in the skin barrier (e.g., lymphedema, chronic wounds, or venous stasis ulcers).
While the bacterium infects animals, zoonotic transmission to humans is rare.
Diagnosis and Standard Management
The initial identification of an S. dysgalactiae infection begins with collecting a sample from the site of disease (e.g., a throat swab or blood culture). The sample is cultured in a laboratory, where the bacteria are observed for their characteristic beta-hemolytic pattern and large colony size. Serological testing (Lancefield grouping) typically classifies the isolate as Group C or Group G streptococcus.
Further laboratory analysis, such as mass spectrometry or molecular sequencing, is often necessary to confirm the species as S. dysgalactiae and distinguish it from other streptococcal species. Accurately identifying the species is important because the clinical severity of the illness can be high.
The standard management for all S. dysgalactiae infections is effective treatment with penicillin or ampicillin. Penicillin remains the drug of choice because the bacterium has not developed resistance to this class of antibiotics. Treatment duration varies, ranging from ten days for a throat infection to several weeks for deep-seated or systemic infections like endocarditis.
For patients with a documented penicillin allergy, alternative antibiotics are available. These include cephalosporins, clindamycin, or macrolides such as azithromycin. However, macrolide resistance is increasingly reported in some geographic areas, necessitating antibiotic susceptibility testing to ensure the chosen treatment is effective.