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Streptococcus pyogenes and Streptococcal Disease (page 1)
(This chapter has 4 pages)
© Kenneth Todar, PhD
Introduction
Streptococcus pyogenes (Group A streptococcus) is
a Gram-positive, nonmotile, nonsporeforming coccus that occurs in
chains
or in pairs of cells. Individual cells are round-to-ovoid cocci,
0.6-1.0
micrometer in diameter (Figure 1). Streptococci divide in one plane and
thus occur in pairs or (especially in liquid media or clinical
material)
in chains of varying lengths. The metabolism of S. pyogenes is
fermentative;
the organism is a catalase-negative aerotolerant anaerobe (facultative
anaerobe), and requires enriched medium containing blood in order to
grow.
Group A streptococci typically have a capsule composed of hyaluronic
acid
and exhibit beta (clear) hemolysis on blood agar.
Figure 1. Streptococcus
pyogenes.
Left. Gram stain of Streptococcus pyogenes in a clinical
specimen.
Right. Colonies of Streptococcus pyogenes on blood agar
exhibiting
beta (clear) hemolysis.
Streptococcus pyogenes is one of the most frequent pathogens
of humans. It is estimated that between 5-15% of normal individuals
harbor
the bacterium, usually in the respiratory tract, without signs of
disease.
As normal flora, S. pyogenes can infect when defenses are
compromised
or when the organisms are able to penetrate the constitutive defenses.
When the bacteria are introduced or transmitted to vulnerable tissues,
a variety of types of suppurative infections can occur.
In the last century, infections by S. pyogenes claimed many
lives
especially since the organism was the most important cause of puerperal
fever (sepsis after childbirth). Scarlet fever was formerly
a severe complication of streptococcal infection, but now, because of
antibiotic
therapy, it is little more than streptococcal pharyngitis
accompanied
by rash. Similarly, erysipelas (a form of cellulitis
accompanied
by fever and systemic toxicity) is less common today. However, there
has
been a recent increase in variety, severity and sequelae of Streptococcus
pyogenes infections, and a resurgence of severe invasive
infections,
prompting descriptions of "flesh eating bacteria" in the news media. A
complete explanation for the decline and resurgence is not known.
Today,
the pathogen is of major concern because of the occasional cases of
rapidly
progressive disease and because of the small risk of serious sequelae
in
untreated infections. These diseases remain a major worldwide health
concern,
and effort is being directed toward clarifying the risk and mechanisms
of these sequelae and identifying rheumatogenic and nephritogenic
strains
of streptococci.
Acute Streptococcus pyogenes infections may present as pharyngitis
(strep throat), scarlet fever (rash), impetigo
(infection
of the superficial layers of the skin) or cellulitis (infection
of the deep layers of the skin). Invasive, toxigenic infections can
result
in necrotizing fasciitis, myositis and streptococcal
toxic
shock syndrome. Patients may also develop immune-mediated
post-streptococcal
sequelae, such as acute rheumatic fever and acute glomerulonephritis,
following acute infections caused by Streptococcus pyogenes.
Streptococcus pyogenes produces a wide array of virulence
factors and a very large number of diseases. Virulence factors of
Group
A streptococci include: (1) M protein, fibronectin-binding
protein
(Protein F) and lipoteichoic acid for adherence; (2) hyaluronic
acid capsule as an immunological disguise and to inhibit
phagocytosis;
M-protein
to inhibit phagocytosis (3) invasins such as streptokinase,
streptodornase
(DNase B), hyaluronidase, and streptolysins; (4)
exotoxins,
such as pyrogenic (erythrogenic) toxin which causes the rash of
scarlet
fever and systemic toxic shock syndrome.
Classification of Streptococci
Hemolysis on blood agar
The type of hemolytic reaction displayed on blood agar has long been
used to classify the streptococci. Beta -hemolysis is
associated
with complete lysis of red cells surrounding the colony, whereas alpha-hemolysis
is a partial or "green" hemolysis associated with reduction of red cell
hemoglobin. Nonhemolytic colonies have been termed gamma-hemolytic.
Hemolysis
is affected by the species and age of red cells, as well as by other
properties
of the base medium. Group A streptococci are nearly always
beta-hemolytic;
related Group B can manifest alpha, beta or gamma hemolysis. Most
strains
of S. pneumoniae are alpha-hemolytic but can cause
ß-hemolysis
during anaerobic incubation. Most of the oral streptococci and
enterococci
are non hemolytic. The property of hemolysis is not very reliable for
the
absolute identification of streptococci, but it is widely used in rapid
screens for identification of S. pyogenes and S. pneumoniae.
Antigenic types
The cell surface structure of Group A streptococci is among the most
studied of any bacteria (Figure 2). The cell wall is composed of
repeating
units of N-acetylglucosamine and N-acetylmuramic acid, the standard
peptidoglycan.
Historically, the definitive identification of streptococci has rested
on the serologic reactivity of "cell wall" polysaccharide antigens as
originally
described by Rebecca Lancefield. Eighteen group-specific antigens
(Lancefield
groups) were established. The Group A polysaccharide is a polymer
of
N-acetylglucosamine and rhamnose. Some group antigens are shared by
more
than one species. This polysaccharide is also called the C substance
or group carbohydrate antigen.
