Streptococci

Eva L. Dizon, M.D.,D.P.P.SDepartment of Microbiology

Structure

Gram-positive, nonmotile, nonsporeforming, catalase-negative cocci that occur in pairs or chains.

Most streptococci are facultative anaerobes, and some are obligate (strict) anaerobes.

Most require enriched media (blood agar).

Morphology

Cell surface structure of S. pyogenes

Medically Important Streptococci

Infections

Infections

Pathogenesis: Streptococci are members of the normal flora.

Virulence factors of group A streptococci include

(1) M protein and lipoteichoic acid for attachment(2) a hyaluronic acid capsule that inhibits phagocytosis

(3) other extracellular products, such as pyrogenic (erythrogenic) toxin

(4) streptokinase, streptodornase (DNase B), and streptolysins.

Host defenses: Antibody to M protein gives type-specific

immunity to group A streptococci. Antibody to erythrogenic toxin prevents

the rash of scarlet fever. Immune mechanisms are important in

the pathogenesis of acute rheumatic fever.

Maternal IgG protects the neonate against group B streptococci.

Classification and Antigenic Type

Colony morphology Hemolysis, Biochemical reactions Serologic specificity.

Classification and Antigenic type

Three groups by the type of hemolysis on blood agar:

1.b-hemolytic (clear, complete lysis of red cells) 2.a hemolytic (incomplete, green hemolysis)

3. g hemolytic (no hemolysis).

Classification and Antigenic Structure

Serologic grouping is based on: 1.antigenic differences in cell wall carbohydrates (groups A to V) 2.in cell wall pili-associated protein 3.in the polysaccharide capsule (in group B streptococci)

Host defenses: Antibody to M protein gives type-specific

immunity to group A streptococci. Antibody to erythrogenic toxin prevents

the rash of scarlet fever. Immune mechanisms are important in

the pathogenesis of acute rheumatic fever.

Maternal IgG protects the neonate against group B streptococci.

Epidemiology: Group A b-hemolytic streptococci are spread by

respiratory secretions and fomites. The incidence of both respiratory and skin

infections peaks in childhood. Infection can be transmitted by asymptomatic

carriers. Acute rheumatic fever was previously common

among the poor; susceptibility may be partly genetic.

Group B streptococci are common in the normal vaginal flora and occasionally cause invasive neonatal infection

Diagnosis: Diagnosis is based on cultures from clinical

specimens. Serologic methods can detect group A or B

antigen; definitive antigen identification is by the precipitin test.

Bacitracin sensitivity presumptively differentiates group A from other b-hemolytic streptococci (B, C, G);

Group B streptococci typically show hippurate hydrolysis;

Group D is differentiated from other viridans streptococci by bile solubility and optochin sensitivity.

Control Prompt penicillin treatment of

streptococcal pharyngitis reduces the antigenic stimulus and therefore prevents glomerulonephritis and acute rheumatic fever.

Vancomycin resistance among the enterococci is an emerging microbial threat.

Vaccines

Streptococcal pneumoniae

Pneumococci are lancet-shaped Catalase-negative Capsule-forming a-hemolytic cocci or

diplococci. Autolysis is enhanced by adding bile

salts.

Classification and Antigenic Types

There are more than 85 antigenic types of S pneumoniae , which are determined by capsule antigens.

There is no Lancefield group antigen.

Pathogenesis

S pneumoniae is a normal member of the respiratory tract flora; invasion results in pneumonia.

The best defined virulence factor is the polysaccharide capsule, which protects the bacterium against phagocytosis

Host Defenses

Protection against infection depends on a normal mucociliary barrier and intact phagocytic and T-independent immune responses.

Type-specific anti-capsule antibody is protective

Epidemiology

Pneumococcal pneumonia is most common in elderly, debilitated, or immunosuppressed individuals.

The disease often sets in after a preceding viral infection damages the respiratory ciliated epithelium

Incidence therefore peaks in the winter

Diagnosis:

Based on a sputum Gram stain and culture ( blood or cerebrospinal fluid).

Capsular antigen can be detected serologically.

Pneumococci are distinguished from viridans streptococci by the quellung (capsular swelling) reaction, bile solubility, and optochin inhibition.

Streptococci pneumoniae

Identify

Bacitracin test