STREPTOCOCCUS

 

INTRODUCTION:  Streptococci are gram-positive cocci which grow in chains in liquid medium. Man is the most susceptible of all the animals to streptococcal infections.  No organ or tissue of the body is completely immune to infection by streptococci. In addition, infections caused by it may lead to post infection syndromes of acute rheumatic fever, rheumatic heart disease and acute glomerulonephritis.

CLASSIFICATION:  The genus Streptococcus is the only genus of medical importance in the family Streptococcaceae. These organisms are gram-positive, spherical to ovoid in shape and less than 2 μm in diameter. Their inability to produce catalase is an important feature that distinguishes these from staphylococci.  Staphylococcus

These organisms divide in one plane to form chains or pairs. The genus Streptococcus is the only one of the five genera of family Streptococcaceae that contains organisms pathogenic to man.  These organisms can as well be classified basing on haemolysis.  On the basis of their action on blood agar medium and lysis of red blood cells, streptococci have been classified into three groups:

1)  Alpha Haemolytic Streptococci:  The colonies produced by this group on blood agar medium are surrounded by a narrow zone of haemolysis, with unhaemolysed RBCs persisting in the inner zone and complete haemolysis in the outer zone.  A greenish discolouration takes place because of the formation of reductase of haemoglobin. The greenish discoloration gives the name viridans to these streptococci (viridan: green).  The most common species belonging to this group is Strept. salivarius.

2) Beta Haemolytic Streptococci:  These organisms produce complete haemolysis of RBCs. Streptococci and colonies are surrounded by a clear zone of haemolysis (beta haemolysis).  This lysis of RBCs is due to the production of two types of streptolysins by the organisms: streptolysin O and streptolysin S.  Streptolysins 

To this group belongs the most important species for man: Streptococcus pyogenes. The beta haemolytic streptococci have further been subdivided into a number of immunologic groups designated by latters A to V. These groups are known as Lancefield groups after the name of Rebecca Lancefield who introduced this type of serological method of classification.

 

RELATED;

1.  BACTERIOLOGY  2.  STAPHYLOCOCCUS

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