BY THROAT CULTURE
©David B. Fankhauser, Ph.D.,
Professor of Biology and Chemistry
University of Cincinnati Clermont College,
Batavia OH 45103
light in position,
alpha and beta
The Streptococci cause more disease than any other single group of bacteria. They are aerotolerant, but do not use O2 and lack catalase. Most streptococci are non-pathogenic, including lactic acid fermenters found in fermented milk products such as yogurt, buttermilk and sour cream.
(The image at the left is of Streptococcus pyogenes at 1000x, stained with methylene blue.)
Pathogenic strains of strep are characterized by two traits:
1. HEMOLYSIS: Pathogenic strains produce the exotoxin streptolysin which causes the complete lysis of red blood cells. When these strains are grown on blood agar, their colonies are surrounded by a yellowish halo of complete clearing on a background of the bright red agar, called beta hemolysis. Some strains produce partial hemolysis on blood agar, and produce turbid halos with a greenish cast around the colonies, termed alpha hemolysis. Those strains which produce no lysis are termed (for some reason) gamma hemolytic. Here is a labeled image displaying both alpha and beta hemolysis.
2. ANTIGENICITY: The M protein is part of the cell wall, and functions to mediate attachment and to resist phagocytosis. These M proteins have been serologically classified by Rebecca Lancefield into groups A through O. Pathogenic strains of strep are limited to those which carry the group A antigen.Thus, pathogenic strep is Group A, beta hemolytic. Streptococcus pyogenes is most common member of this group. It causes a wide variety of diseases including:
Strep throat: beefy red pharynx, fever, sore throat (80% Strep infections are asymptomatic)We will learn to perform a diagnostic test for beta hemolysis on blood agar in which a plate is inoculated with a throat swab (of the oropharynx and palatine tonsils) and incubated at 35C. Presence of nunerous colonies with the beta hemolytic reaction on the plate strongly suggests strep throat.
Puerperal fever: infection of the uterus following contamination during childbirth.
Rheumatic fever occurs in 3% of untreated strep-infected children with febrile exudative pharyngitis, and is thought to be an autoimmune manifestation. One to five weeks following a strep infection the sequelae may include rheumatoid arthritis, endocarditis, and/or pyelonephritis.
|Illustrate the structure of the oropharynx, including palatine tonsils, soft palate, oropharynx and the uvula. Review the subject in an anatomy text. Click on the image to see a labeled view of the oral cavity .|
|well-focused light to illuminate
Tongue depressors, sterile
Sterile swabs, 15 cm long
|Blood Agar Plates
fresh culture of Streptococcus pyogenes
|BLood agar plate with Streptococcus pyogenes streaked at the top and a throat swab on the bottom. Note the clarity of the clearing around the S. pyogenes colonies, and the characteristic greenish turbidity of alpha hemolysis in the throat swab.|
|A mixed streak of non-hemolytic (larger colonies) and smaller Streptococcus pyogenes colonies showing zones of clearing around each colony.|
|Mixed streak showing the clearing around a concentration of S. pyogenes colonies.|
|Close up of a student's throat swab with alpha hemolysis surrounding the streak.|
|Here is a student's culture showing considerable beta hemolysis. Streptococcus pyogenes is streaked at the top as a positive control.|