Streptococcus Spp.
Streptococci are Gram-positive, nonmotile, nonsporeforming, catalase-negative cocci that
occur in pairs or chains. Older cultures may lose their Gram-positive character. Most
streptococci are facultative anaerobes, and some are obligate (strict) anaerobes. Most require
enriched media (blood agar).
Streptococci are subdivided into groups by antibodies that recognize surface antigens. These groups may include one or more species. Serologic grouping is based on antigenic
differences in cell wall carbohydrates (groups A to V), in cell wall pili-associated protein, and
in the polysaccharide capsule in group B streptococci. Rebecca Lancefield developed the
serologic classification scheme in 1933. β-hemolytic strains possess group-specific cell wall antigens,
most of which are carbohydrates. These antigens can be detected by immunologic assays and have
been useful for the rapid identification of some important streptococcal pathogens. The most
important groupable streptococci are A, B and D. Among the groupable streptococci,
infectious disease (particularly pharyngitis) is caused by group A. Group A streptococci have
a hyaluronic acid capsule. Streptococcus pneumoniae (a major cause of human pneumonia)
and Streptococcus mutans and other so-called viridans streptococci (among the causes of
dental caries) do not possess group antigen. Streptococcus pneumoniae has a polysaccharide
capsule that acts as a virulence factor for the organism; more than 90 different serotypes are
known, and these types differ in virulence.
Group A Streptococci causes:
- Strep throat - a sore, red throat, sometimes with white spots on the tonsils
- Scarlet fever - an illness that follows strep throat. It causes a red rash on the body.
- Impetigo - a skin infection
- Toxic shock syndrome
- Cellulitis and necrotizing fasciitis (flesh-eating disease)
Rheumatic fever is a nonsuppurative complication of S. pyogenes pharyngitis. Rheumatic
fever is an inflammatory disease affecting primarily the heart and joints. Although severe, it
can take an extended period of time to develop. The mechanism of chronic immunopathology
of rheumatic fever is not resolved. M protein cross-reacts with heart myosin leading to
autoimmunity. Also the group A streptococcal cell wall is highly resistant to degradation in
the host. These antigens persist for months in vivo and experimentally elicit diseases that
resemble rheumatic arthritis and carditis. Rheumatic arthritis should not be confused with the
most common rheumatic disease - rheumatoid arthritis. Early termination of throat infections
with penicillin therapy decreases the incidence of the subsequent development of rheumatic
carditis. Acute glomerulonephritis is an immune complex disease of the kidney.
Group B Streptococci can cause blood infections, pneumonia and meningitis in newborns.
Adults can also get group B strep infections, especially if they are elderly or already have
health problems. Strep B can cause urinary tract infections, blood infections, skin infections
and pneumonia in adults.
Group D Streptococci is now classified as an Enterococcus. Enterococci are distantly related
to other streptococci and have been moved into the genus Enterococcus; the most commonly
isolated are E. (S1
.) faecalis and E. (S.) faecium. E. (S.) faecalis can cause nosocomial
infections, urinary tract infections, bacteremia, endocarditis, meningitis, and can be found in
wound infections along with many other bacteria.
Streptococcus pneumoniae causes pneumonia, acute sinusitis, otitis media, meningitis,
bacteremia, sepsis, osteomyelitis, septic arthritis, endocarditis, peritonitis, pericarditis,
cellulitis, and brain abscess. S. pneumoniae is the most common cause of bacterial meningitis
in adults and children, and is one of the top two isolates found in ear infection, otitis media.
STREPTOCOCCI - GRAM STAIN (Click to read)
The genus Streptococcus is a diverse collection of Gram-positive cocci typically arranged in
pairs or chains (in contrast to the clusters formed by Staphylococcus).
Streptococcus pneumoniae are lancet shaped (ovoid) cocci in short chains, diplococci and
single cocci.
STREPTOCOCCI - BLOOD AGAR CULTURE
Most species of streptococci are facultative anaerobes, and some grow only in an atmosphere
enhanced with carbon dioxide (capnophilic growth). The classification of species within the
genus is complicated because three different schemes are used:
- hemolytic patterns: complete (β) hemolysis, incomplete (α) hemolysis, and no (γ)
hemolysis;
- serologic properties: Lancefield groupings (originally A to W);
- biochemical (physiologic) properties.
HEMOLYTIC PATTERNS
Streptococci are divided into three groups:
β-hemolytic streptococci, which are classified by Lancefield grouping,
α-hemolytic and γ-hemolytic streptococci, which are classified by biochemical testing.
STREPTOCOCCI - SEROLOGIC PROPERTIES TESTING
STREPTOCOCCI - PHYSIOLOGIC (BIOCHEMICAL)
PROPERTIES TESTING
Streptococcus pneumoniae, Streptococcus salivarius, Streptococcus viridans are referred to
collectively as viridans streptococci, a name derived from viridis (Latin for "green"),
referring to the green pigment formed by the partial hemolysis of blood agar. The α-hemolytic
(viridans) streptococci are classified by biochemical testing.
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