BACTERIAL IDENTIFICATION

BACTERIAL IDENTIFICATION

1. Gram positive bacteria

   • Catalase test
     • Use to identify organisms that produce enzyme catalase.
     • This enzyme detoxifies hydrogen peroxide to produce water and oxygen gas.
   • Mannitol Salt Agar (MSA)
     • MSA will select orgnisms which can live in high salt concentration
     • Organisms capable of using mannitol as a food source will produce acidic byproducts of fermentation
     • Acid will cause the pH indicator, phenol red, to turn yellow
   • Coagulase test
     • Coagulase is an enzyme that clots blood plasma
     • Coagulase is a virulence factor of some Gram positive bacteria
     • The formation of clot around an infection caused by this bacteria likely protects it from phagocytosis

2. Gram negative bacteria

   • Oxidase test
     • This test is used to identify microorganisms containing the enzyme cytochrome oxidase
     • Cytochrome oxidase transfers electrons from the electron transport chain to oxygen and reduces it to water
     • In the oxidase test, artificial electron donors and acceptors are provided. When the electron donor is oxidized by cytochrome oxidase it turns a dark purple
   • Motility agar
     • Used to determine whether an organism is equipped with flagella and thus capable of swimming away from a stab mark
     • If the entire tube is turbid, this indicates that the bacteria have moved away from the stab mark
   • Urease test
     • This test is used to identify bacteria capable of hydrolyzing urea using the enzyme urease
     • The hydrolysis of urea forms the weak base, ammonia, as one of its products
     • This weak base raises the pH of the media above 8.4 and the pH indicator, phenol red, turns from yellow to pink