The genus Streptococcus represents a diverse group of Gram-positive, spherical bacteria belonging to the family Streptococcaceae. These organisms typically divide along a single axis, causing them to arrange themselves in pairs or chains that resemble a string of beads when viewed under a microscope. They are ubiquitous in nature, existing as harmless commensals in the human microbiome, particularly in the throat, nose, skin, and gastrointestinal tracts. While many species are benign, others are significant human pathogens capable of causing a wide range of diseases. Classification allows clinicians to rapidly identify specific strains, which is necessary for predicting disease severity and administering appropriate treatment.
The Basis of Serological Classification: Lancefield Grouping
The primary method for classifying many Streptococcus species is the Lancefield grouping system, a serological method developed by Rebecca Lancefield in the 1930s. This system is founded upon the unique carbohydrate composition of bacterial antigens embedded within the organism’s cell wall. The specific molecular target is the C carbohydrate antigen, a polysaccharide structure chemically distinct in different streptococcal species.
Lancefield discovered this component, which she termed “C-substance,” while classifying beta-hemolytic streptococci. She found that antibodies produced against one strain reacted only with the C carbohydrate of that specific strain. This specificity allowed her to differentiate between various streptococcal strains.
The methodology involves a precipitin test where extracts of the C carbohydrate are mixed with specific, pre-made antibodies. If the antigen matches the antibody’s target, an immune reaction occurs, resulting in visible clumping or agglutination. This reaction confirms the presence of a specific serogroup, designated by a letter, such as Group A or Group B.
The system initially assigned a letter code to these serotypes, identifying over 20 groups from A to V (excluding E, I, and J). Although developed primarily for beta-hemolytic streptococci, the Lancefield system remains a standard clinical method. Its efficiency in quickly determining the pathogenic potential of an isolate based on its characteristic cell wall antigen ensures its continued use.
Major Pathogenic Groups and Clinical Importance
The Lancefield system retains prominence because it correlates specific serogroups with significant human pathogens.
Group A Streptococcus (GAS)
Group A Streptococcus (GAS), or Streptococcus pyogenes, is defined by the Group A carbohydrate. S. pyogenes causes common infections like strep throat (pharyngitis) and impetigo. Its clinical importance extends to severe, life-threatening invasive infections (iGAS).
Invasive diseases occur when the bacteria penetrate deep tissues or enter the bloodstream, causing conditions like necrotizing fasciitis and streptococcal toxic shock syndrome. Furthermore, an untreated S. pyogenes infection can sometimes trigger post-infectious complications, known as sequelae, such as acute rheumatic fever which can damage heart valves, and post-streptococcal glomerulonephritis.
Group B Streptococcus (GBS)
Group B Streptococcus (GBS), or Streptococcus agalactiae, is defined by the Lancefield Group B antigen and is a major concern in perinatal medicine. While often a harmless commensal in healthy adults, GBS is the leading cause of culture-confirmed bacterial infection in newborns. In neonates, GBS causes severe infections, including sepsis, pneumonia, and meningitis.
In adults, GBS is an increasing cause of invasive infection, particularly in the elderly or those with underlying conditions like diabetes. Adult infections frequently manifest as urinary tract infections, skin and soft-tissue infections, or bacteremia.
Group D Streptococcus
Group D Streptococcus presents a complex classification history. Organisms like Enterococcus faecalis and Enterococcus faecium, formerly Group D streptococci, were taxonomically separated into the genus Enterococcus in the mid-1980s. The remaining non-enterococcal Group D organisms, such as the S. bovis group, are now primarily known as Streptococcus gallolyticus. Isolation of S. gallolyticus from blood cultures, often associated with endocarditis, has a strong association with underlying conditions like colorectal cancer. The classification system thus provides immediate clinical information, guiding doctors in both treatment and the search for related underlying diseases.
Complementary Traits for Species Identification
While Lancefield grouping is highly informative for common pathogens, it is often insufficient for definitive species identification and must be combined with other laboratory methods.
Hemolysis Patterns
One traditional trait is the pattern of hemolysis, which describes the organism’s ability to lyse red blood cells on a blood agar plate. Hemolysis helps narrow down identification, as Group A and Group B streptococci are typically beta-hemolytic.
Beta-hemolysis causes a complete clearing of red blood cells around the colony due to the full breakdown of hemoglobin.
Alpha-hemolysis is a partial breakdown that oxidizes the iron in hemoglobin, resulting in a characteristic greenish discoloration surrounding the colony. Species causing no visible change are termed gamma-hemolytic, or non-hemolytic.
Biochemical Tests
For species lacking a Lancefield antigen, such as Streptococcus pneumoniae or the Viridans group streptococci, identification relies almost entirely on other methods. Biochemical tests observe the organism’s metabolic capabilities when exposed to specific chemical compounds.
For instance, S. pneumoniae, an alpha-hemolytic organism that causes pneumonia and meningitis, is differentiated from other alpha-hemolytic streptococci by its unique sensitivity to the chemical agent Optochin.
Another important test is the bile esculin hydrolysis test, which detects the ability of certain bacteria, including Group D organisms, to grow in the presence of bile and hydrolyze esculin. Complementary tests, like the Bacitracin sensitivity test used to presumptively identify Group A Streptococcus, are used in sequence with serological grouping. The combination of serological, hemolytic, and biochemical data provides a comprehensive picture of the organism’s identity and its potential threat to human health.