Streptococcus

Streptococcus is a genus of Gram-positive spherical bacteria belonging to the family Streptococcaceae within the order Lactobacillales, a group commonly known as the lactic acid bacteria. These organisms divide along a single axis and typically form chains or pairs of cells, producing arrangements that differ from the irregular clusters seen in Staphylococcus. Most species are catalase-negative, non-motile and facultatively anaerobic, enabling growth in a wide range of environments. The genus was first described in 1877 by the Viennese surgeon Theodor Billroth, who combined the Greek term streptos (“twisted”) with coccus (“berry”).
A major taxonomic revision in the 1980s reassigned several species to the genera Enterococcus and Lactococcus, yet the genus today still includes more than fifty recognised species. Many members form part of the normal microbial populations of the mouth, upper respiratory tract, gastrointestinal tract and skin, and some play useful roles in the production of fermented foods such as Emmental cheese. Others, however, are significant human and veterinary pathogens.

Pathogenesis and clinical importance

Species within this genus are responsible for a wide range of important infections. Among the most notable diseases are streptococcal pharyngitis, conjunctivitis, meningitis, bacterial pneumonia, otitis media, sinusitis, peritonitis, endocarditis, erysipelas and necrotising fasciitis. Pathogenicity varies widely between species, while many other streptococci exist harmlessly as part of the normal human microbiota.

Classification by haemolysis

A primary means of classifying streptococci in clinical microbiology is through their haemolytic characteristics on blood agar:

• Alpha-haemolytic streptococci produce partial or “green” haemolysis, causing a greenish discolouration of the agar. This group includes Streptococcus pneumoniae (pneumococcus) and the viridans streptococci, many of which are commensal inhabitants of the oral cavity.
• Beta-haemolytic streptococci cause complete lysis of red blood cells around colonies, creating clear zones. This effect is mediated by exotoxins known as streptolysin O and streptolysin S.
• Gamma-haemolytic streptococci show no haemolytic activity.

Lancefield grouping

Beta-haemolytic streptococci are further categorised by the Lancefield system, which classifies species according to distinct carbohydrate antigens found in their cell walls. More than twenty groups exist, labelled A to W (with some letters omitted). Two groups are particularly important in medical practice:

• Group A Streptococcus (GAS), usually Streptococcus pyogenes, associated with pharyngitis, scarlet fever, rheumatic fever and severe invasive infections.
• Group B Streptococcus (GBS), typically Streptococcus agalactiae, a major cause of neonatal sepsis and meningitis.

Major alpha-haemolytic groups

Among alpha-haemolytic streptococci, two groups are especially notable:

• Pneumococci (Streptococcus pneumoniae) — a leading cause of community-acquired bacterial pneumonia and an important pathogen in otitis media, sinusitis and meningitis.
• Viridans streptococci — a diverse group of largely commensal oral bacteria, frequently implicated in dental plaque formation and subacute bacterial endocarditis.

Beta-haemolysis and virulence factors

Beta-haemolysis is linked to the action of two main exotoxins:

• Streptolysin O (SLO): an oxygen-labile cytotoxin that binds to cholesterol in eukaryotic cell membranes and typically operates beneath the agar surface.
• Streptolysin S (SLS): an oxygen-stable toxin responsible for surface clearing on blood agar and capable of damaging a variety of immune cells.

Together with other virulence determinants—including capsules, adhesins and enzymatic factors—these toxins contribute to the pathogenic potential of several streptococcal species.

Species diversity

The genus includes a wide array of species found in humans, animals and diverse ecological niches. Examples range from well-known pathogens such as S. pyogenes, S. pneumoniae, S. mutans and S. agalactiae to numerous commensal or host-specific species including S. salivarius, S. mitis, S. canis, S. suis, S. equi, S. dysgalactiae, S. oralis, S. sanguinis and many others associated with specialised animal hosts or environments.
These organisms collectively illustrate the biological breadth of the genus, spanning harmless commensals, opportunistic pathogens and major causes of acute human disease.