Streptococcus anginosus: An Overview and Its Complex Identity
*Streptococcus anginosus* is a Gram-positive, facultative anaerobic bacterium that is a ubiquitous and prevalent member of the human microbial flora. It belongs to the Anginosus group of viridans streptococci, which historically caused significant taxonomic confusion and was commonly referred to as the “Streptococcus milleri group.” This latter name is now considered pseudotaxonomic, and the Anginosus Group (SAG) officially comprises three distinct species: *S. anginosus*, *S. constellatus*, and *S. intermedius*. While primarily found as a commensal in healthy individuals, *S. anginosus* is widely recognized as an opportunistic pathogen with a unique propensity to cause serious, deep-seated purulent infections, a trait that fundamentally distinguishes the SAG from other streptococci. This characteristic ability to form abscesses in multiple body sites has led to *S. anginosus* being referred to as the “unrecognized pathogen.”
Habitat and Commensal Role
As a part of the normal flora, *Streptococcus anginosus* colonizes various mucosal surfaces throughout the human body. The primary sites of colonization include the oral cavity, the upper respiratory tract, the gastrointestinal tract (gut and fecal samples), and the female urogenital/vaginal tract. In these environments, the organism often exists in a harmless, symbiotic relationship with the host, potentially offering protection against colonization by more virulent pathogens. However, the organism’s presence is not always benign; it has been found to be more abundant in the vaginal microbiota of postpartum females and has been associated with placental inflammation and chorioamnionitis. Crucially, the transition from commensal to pathogen occurs when the bacteria breach the mucosal barrier and gain access to normally sterile sites within the body.
Recent genomic analysis of *S. anginosus* isolates has further highlighted the diversity within the species, revealing two phylogenomically distinct groups. One group is comprised of strains isolated from throughout the human body and contains the type strain and previously identified subspecies. The second group, in contrast, consists predominantly of strains isolated from the urinary tract, suggesting that different populations of *S. anginosus* may be adapted to specific anatomical niches within the human host, a finding that provides greater insight into the species’ complex relationship with its human host.
Morphology and Biochemical Characteristics
*Streptococcus anginosus* cells are small (approximately 0.5–1.0 µm in diameter), Gram-positive cocci. They exhibit the characteristic streptococcal arrangement of successive division planes that are parallel, resulting in the cells being arranged in short chains, with longer chains being observed when grown in liquid culture media. Culturally, *S. anginosus* forms small, minute colonies, typically white to grey and round with an average diameter of 0.5–2.0 mm on blood agar. The hemolytic pattern is highly inconsistent and variable within the species, which has historically complicated its laboratory identification. Most strains display alpha-hemolysis (partial or ‘greening’ hemolysis) or are non-hemolytic (gamma-hemolysis), but some strains are known to produce beta-hemolysis (complete lysis of red cells).
Key biochemical and physiological characteristics are essential for its identification. *S. anginosus* is catalase-negative and oxidase-negative, typical of *Streptococcus* species. It is a facultative anaerobe, meaning it can grow in both aerobic and anaerobic conditions, and it is known to be Voges Proskauer (VP) positive. Furthermore, the cell wall contains peptidoglycan, teichoic acid, and various carbohydrates, and some strains may produce a polysaccharide capsule. The lack of reliable, consistently differentiating phenotypic markers across the Anginosus group is one reason why clinical laboratories often find it challenging to identify these streptococci to the species level.
Clinical Significance and Abscess Formation
The principal clinical hallmark of an infection caused by the *Streptococcus anginosus* group is its strong and unique association with the formation of abscesses—localized collections of pus surrounded by inflamed tissue—in deep-seated anatomical sites. These purulent infections can occur throughout the body, making the organism an important consideration in various clinical presentations. Common sites of infection include the liver and other abdominal viscera, the lungs (leading to lung abscess and empyema), the central nervous system (causing brain abscesses, particularly with *S. intermedius*), the head and neck (dental abscesses, sinusitis, and fasciitis), and the genitourinary tract. The group is also a cause of bacteremia, which is frequently associated with an underlying, often occult, deep-seated abscess, necessitating investigation and drainage.
The seriousness of these infections is often under-recognized, and without proper management, infections like dental abscesses and head/neck fasciitis can become life-threatening. Treatment for these serious purulent infections typically involves aggressive surgical drainage of the abscess in combination with appropriate antibiotic therapy. While little antibiotic resistance has been noted in the Anginosus group, some strains are less susceptible to penicillin than other streptococci, with preferred treatments remaining Penicillin G or Ceftriaxone. Alternatives include Clindamycin and Vancomycin.
Pathogenic Mechanisms and Virulence Factors
The molecular basis for the remarkable ability of *S. anginosus* to cause abscesses is an area of ongoing research. Several virulence factors and mechanisms have been identified that contribute to its pathogenicity. One key factor involves the generation of hydrogen sulfide (Htextsubscript{2}S) from L-cysteine by *S. anginosus*, a mechanism thought to increase abscess formation. Other critical virulence factors include the production of hydrolytic enzymes that destroy components of the extracellular matrix, such as nucleic acids, hyaluronic acid, and chondroitin sulfate. These enzymes likely facilitate the tissue destruction necessary for abscess formation and the spread of the infection. Furthermore, comparative genomic analysis has suggested the presence of genes homologous to virulence factors known from other, more recognized streptococcal species, like *Streptococcus pyogenes*. The entire pathogenic pathway is likely regulated by environmental signals, potentially including a quorum sensing system, which allows the bacteria to sense their population density and coordinate a collective pathogenic response.
Taxonomic Clarity and Diagnostic Approach
Given the clinical importance of the *S. anginosus* group, accurate identification is paramount, yet its taxonomic heterogeneity and inconsistent phenotypic presentation often lead to misidentification in clinical laboratories. The group encompasses multiple subspecies and genomosubspecies, all of which are phylogenetically distinct. Therefore, classical identification based solely on morphological characteristics, culture appearance, and limited biochemical tests is often insufficient for reliable species-level differentiation. The observation of the organism under the microscope, followed by its culture and biochemical tests (like the VP test), remains the most common method in a clinical setting, but it lacks the resolution to distinguish between the three species of the SAG.
For more definitive identification, modern microbiological approaches increasingly rely on molecular methods such as Multilocus Sequence Analysis (MLSA) and Average Nucleotide Identity (ANI) analysis. These genomic techniques provide the high phylogenetic resolution necessary to distinguish the three species and their underlying genomic groups, ensuring a more accurate correlation between the bacterial species and the patient’s clinical outcome. In conclusion, *Streptococcus anginosus* is a clinically important, complex, and highly adaptable organism. While usually a quiet member of the body’s flora, its latent pathogenic potential, particularly its unique and strong association with destructive, deep-seated abscess formation, establishes it as a major opportunistic pathogen that demands prompt recognition and aggressive medical-surgical management when it causes infection.