(Solved): Laboratory Exercise 13: Blood Agar Blood agar is used for isolation and cultivation of many types of ...

Laboratory Exercise 13: Blood Agar Blood agar is used for isolation and cultivation of many types of fastidious bacteria. It is also used to differentiate bacteria based on their hemolytic characteristics. Blood agar is best known for its role in the differentiation of Streptococci. In fact, it is so important in this capacity that Streptococci are grouped according to their hemolytic properties on blood agar- often determined at the time of isolation. Several species of Gram-positive cocci produce exotoxins called hemolysins, which are able to destroy red blood cells (RBCs) and hemoglobin. Three major types of hemolysis are ß hemolysis, a hemolysis, and y hemolysis. ß hemolysis is the complete destruction of RBCs and hemoglobin, and results in a clearing of the medium around the colonies. Alpha (a) hemolysis is the partial destruction of RBCs and produces a greenish discoloration of the agar around the colonies. Gamma (y) hemolysis is actually non-hemolysis and appears as simple growth with no change to the medium. Hemolysins produced by Streptococci are called streptolysins. Streptolysins come in two forms-type O and type S. Streptolysin O is oxygen labile and expresses maximal activity under anaerobic conditions. Streptolysin S is oxygen stable, but expresses itself optimally under anaerobic conditions as well. Materials: 1 blood agar plate Sterile cotton swabs Procedure: Use a marking pen to divide the plate into four sectors. Label throat, nose, nails, and behind ear, and your name. Marks should be made on the plate's base. Swab your throat and spot inoculate on the sector labeled throat on the blood agar plate. Repeat the procedure for the nails, nose, and behind ear. Tape the lid down to prevent if from opening accidentally. Invert the plate and incubate at 37°C for 48 hours. 1. 2. 3. 4. Second lab day: 1. Examine the plate using the information in the table of results. Blood agar Result Interpreta Clearing around growth Greening around growth No change in the medium tion Organism hemolyzes RBCs completely Organism hemolyzes RBCs partially Organism does not hemolyze RBCs Photographic atlas reference page 61. Symbol B hemolysis a hemolysis y hemolysis (no hemolysis