Blood Agar

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What is Blood agar

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Blood agar is an general purpose enriched, bacterial growth medium

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History

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In 1919, Brown experimented with blood agar formulations for the effects of colony formation and hemolysis.

method of adding blood to agar media is described in Bulloch’s 1938

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Purpose or use of Blood agar

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Blood agar has two major uses:

1. To grow Fastidious or "picky". organisms, such as s Streptococci.

2. To determine the type of hemolysis, if any.

3. used to grow a wide range of pathogens particularly those that are more difficult to grow such as Haemophilus influenzae, Streptococcus, pneumoniae andNeisseria species.

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Composition of Blood Agar:

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0.5% Peptone, 0.3% beef extract/yeast extract, 1.5% agar, 0.5% NaCl, Distilled water, 5% Sheep Blood,

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Function of blood in the medium

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The variety of complex nutrients found in blood supports the growth ofmost bacteria, f

Blood contains inhibitors for certain bacteria such as Neisseria and Haemophilus genera and the blood agar must be heated to inactivate these inhibitors. Heating of blood agar converts it into chocolate agar (heated blood turns a chocolate agar) and supports the growth of these bacteria.

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Theory

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"Blood Agar" is not a consistently defined medium. The term "blood agar" generally refers to an enriched base medium to which defibrinated mammalian blood has been added.

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pH of the medium

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pH should be from 7.2 to 7.6 (7.4)

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Incubation Period

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18- 24 and 48 hours

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What is Hemolysis

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Hemolysis is the breakdown of red blood cells (RBC). A substance that causes hemolysis is a hemolysin. Brown (1919) introduced three terms alpha, beta and gamma to indicate three types of streptococci based on haemolytic reactions observed on blood agar plates.

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Beta-hemolysis (β-hemolysis)

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complete lysis of red cells surrounding the colony.

Beta hemolysis is caused by two hemolysins O and S

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hemolysins O

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inactive in the presence of oxygen.

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hemolysins S

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Oxygen-stable cytotoxin.

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Zone seen in β-hemolysis

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It exhibit a wide zone (2-4 mm wide).

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Example of Organisms showing β-hemolysis

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Streptococcus pyogenes, or Group A beta-hemolytic Strep (GAS ), Staphylococcus aureus.

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Weakly beta-hemolytic species:

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Streptococcus agalactiae, Clostridium perfringens, Listeria monocytogenes

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Alpha hemolysis

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is a partial or “green” hemolysis associated with reduction of red cell hemoglobin. Alpha hemolysis is caused by hydrogen peroxide produced by the bacterium, oxidizing hemoglobin to green methemoglobin.

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Zone seen in Alpha-hemolysis

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It exhibit incomplete haemolysis with 1-2 mm wide. Persistence of some unhaemolysed RBC’s can be seen microscopically.

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Examples Alpha-hemolysis

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Streptococcus pneumoniae and a group of oral streptococci (Streptococcus viridans or viridans streptococci)

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Gamma-hemolysis

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Colonies show neither typical alpha nor beta haemolysis. There may be, however, slight discoloration in the medium. The streptococci included in this group are usually not pathogenic

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Examples of Gamma- hemolysis

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Enterococcus faecalis (formerly called “Group D Strep”)

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Limitations of the Procedure

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1. Hemolytic reactions of some strains of group D streptococci have been shown to be affected by differences in animal blood. Such strains are beta-hemolytic on horse, human, and rabbit blood agar and alpha-hemolytic on sheep blood agar.

2. Incubation atmosphere can influence hemolytic reactions of beta-hemolytic streptococci.

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Alpha-prime hemolysis (ά)

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is a small zone of complete hemolysis surrounded by an area of partial lysis.

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