Streptococci Eva L. Dizon, M.D.,D.P.P.S Department 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

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