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Oh no, it's the purple people infector!

Gram positive stain of Streptococcus pyogenes, by Dr. William Schwan, University of Wisconsin-La Crosse.

The picture above is a wonderful example of the Streptococcus pyogenes being gram-positive.  A bacteria is gram-positive when it is a purple color after being stained.  As you can see, the Streptococcus pyogenes is a gram-positive bacteria.  It can also be seen that the bacteria has a coccoid shape and grows in chains.  This picture is a perfect example of what Streptococcus pyogenes looks like when it is properly gram stained.

In order to isolate Streptococcus pyogenes, a medium must contain blood but not reducing sugars.  In a benzidine reaction, a negative indicates that a gram-positive coccus is a member of the Streptococcaceae.  This species is the only member of serological group A.  A throat swab that is taken at the doctor is examined only for the presence of a beta-hemolytic streptococci (Starr 1572-1574).  After 18-24 hours of growth on blood agar the colonies are domed, grayish in color and approximately .5 mm in diameter.  The colonies are surrounded by a defined zone of beta hemolysis several times greater than the diameter of the colony.  Beta hemolysis is seen as a complete clearing of the blood agar medium around the colonies (Joklik and Willett 555-558).


Streptococcus pyogenes is a bacteria, so it reproduces by binary fission.  Binary fission is a form of asexual reproduction in a single-celled organisms.  This process occurs by one cell dividing into two cells of the same size.  Binary fission is used by most prokaryotes.


Streptococcus pyogenes species are spherical to ovoid microorganisms from .6 to 1.0 micrometers in diameter.  They have a rigid cell wall, inner plasma membrane with mesosomal vesicles, cytoplasmic ribosomes and a nucleoid (Joklik and Willett 555-558).


Streptococcus pyogenes are facultative anaerobes, which means that it can survive with and without oxygen (although most facultative anaerobes would greatly prefer aerobic conditions) and requires somewhat complex media for growth.  The metabolism is fermentative and the principal product of metabolism is lactic acid.  The minimal nutritional requirements of the streptococcus are very complex because of the organism's inability to synthesize many of its amino acids, purines, pyrimidines and vitamins.  At a temperature of 60C, Streptococcus pyogenes will be killed in 30 minutes (Joklik and Willett 555-558).  The cell wall is composed of repeating units of N-acetyglucosamine and N-acetylmuramic acid.    It is also non-motile and non-spore forming (Todar).

Culutural Characteristics

For optimal growth of Streptococcus pyogenes the temperature should be 37C and have a pH of 7.4 to 7.6.  Under these conditions colony growth will be the greatest.  The primary media that is used for isolation contains blood or blood products and the growth may be enhanced by reducing oxygen tension (Joklik and Willett 555-558).

Information on this page is from Joklik, Wolfgang K., and Hilda Willett (Zinsser Microbiology), Starr, Mortimer P (The Prokaryotes) and Todar, Kenneth.  "Streptococcus pyogenes." (Todar's Online Textbook of Bacteriology).