Streptococcus

Streptococcus is one of the bacteria commonly present as normal flora of the human body. They are gram positive, nonmotile, catalyst negative cocci that occur in pairs of chains, bacterial cells join together to form a long chain of cocci. Reason behind it is that cell division in streptococcus takes place along a single axis.

Older cultures of streptococcus may lose their gram-positive character. Most of the streptococci are facultative anaerobes, and some are obligate anaerobes. Most require enriched media (blood agar). Group A streptococci have a hyaluronic acid capsule.

Classification

Species of streptococcus are classified on the basis of their property of hemolytics. 

Alpha hemolytic species causes oxidation of iron in hemoglobin of red blood cells so gives a greenish color on blood agar. Beta hemolytic species causes complete rupture of red blood cells. On blood agar beta hemolysis appears as wide areas clear of blood cells surrounding bacterial colonies. Gamma hemolytic species cause no hemolysis.

Beta hemolytic streptococcus are further classified by Lancefield grouping, a serotype classification. The 21 serotypes of streptococcus are named Lancified groups A to W (excluding I and J).

In the medical setting, the most important groups are of alpha hemolytic. And the beta hemolytic streptococcus of Lancefield group A and B (also known as “group A strep” and “group B strep”.

Pathogenesis 

Although 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, which causes the rash of scarlet fever; and (4) streptokinase, streptodornase (DNase B), and streptolysins. Some strains are nephritogenic (causing nephritis). Immune-mediated sequelae do not reflect dissemination of bacteria. Nongroup A strains have no defined virulence factors.

Epidemiology

Group A beta-hemolytic streptococci are found in the nasal way of humans so they can easily transmit by respiratory secretions and focuses. At childhood respiratory and skin infections are on peak. Group B streptococci are common in normal flora of vagina and occasionally causes invasive neonatal infections.

Diagnosis

Diagnosis of streptococci is based on clinical specimens and culture. Serological method can detect group A and B antigen. Acute glomerulonephritis and acute rheumatic fever are identified by anti streptococcus antibody titers.

Control 

Penicillin treatment of streptococcal pharyngitis reduces the antigenic stimulus and therefore prevents glomerulonephritis and acute rheumatic fever. Vaccines are under development.

Gaurav Singh

Editor in Chief Medical Microbiology & RDT Labs - RDT Labs Magazine

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