Blood Agar: Introduction, Composition, Preparation, Principle and Interpretation, Uses , Types of Hemolysis and Keynotes
Introduction of Blood Agar
Blood agar is an enriched medium for bacteria. Fastidiousorganisms, such as streptococci, do not grow well on ordinary growth media. It is a type of growth medium i.e. trypticase soy agar enriched with 5% sheepblood or blood agar base with 5-10 % sterile sheep blood that encourages the growth of bacteria and their hemolysis, such as streptococci, that otherwise wouldn’t grow.
Composition of Sheep Blood Agar Base
Ingredients Gms / Litre
Casein enzymic hydrolysate 14.000
Peptic digest of animal tissue 4.500
Yeast extract 4.500
Sodium chloride 5.000
Final pH (at 25°C) 7.3±0.2
Procedure for the Preparation of Blood Agar
Suspend 40.5 grams in 1000 ml distilled water or deionized water.
Heat to boiling to dissolve the medium completely.
Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes.
Cool to 45-50°C and aseptically add 7% sterile sheep blood.
Mix well and pour into sterile Petri plates. Avoid the formation of air bubbles. You must have warmed the blood to room temperature at the time of dispensing to a molten agar base.
Dispense 15 ml amounts to sterile Petri plates aseptically
Label the medium with the date of preparation and give it a batch number (if necessary).
Storage and Shelf Life
Store below 30°C in a tightly closed container and the prepared medium at 2-8°C, preferably in sealed plastic bags to prevent loss of moisture. The shelf life of thus prepared blood agar is up to four weeks. Use before the expiry date on the label.
Principle and Interpretation
Hemolysins are exotoxins produced by bacteria that lyse red blood cells. The hemolytic reaction can be visualized on blood agar plates observing through the bright transmitted light. On blood agar plates colonies of hemolytic bacteria may be surrounded by a clear, colorless zone where the red blood cells have been lysed and the hemoglobin destroyed to a colorless compound and which is beta hemolysis. Other types of bacteria can reduce hemoglobin to methemoglobin which produces a greenish zone around the colonies and is called alpha hemolysis. Gamma hemolysis is lacking hemolysis where no change in the medium is observed. Sheep blood agar base with added sheep blood was developed to allow maximum recovery of organisms without interfering with their hemolytic reactions. Sheep blood agar base was formulated to be compatible with sheep blood and give improved hemolytic reactions of organisms. Casein enzymic hydrolysate and yeast extract provide nitrogen, carbon, amino acids, and vitamins. Peptic digest of animal tissue (PDAT) is the nitrogen source. Sodium chloride (NaCl) maintains the osmotic balance. Sheep blood agar base showed considerable improvement and the expected beta-hemolytic reactions with S. pyogenes in comparison to other blood agar bases supplemented with blood
It is also used for the isolation and cultivation of other than streptococci like Neisseria and other fastidious microorganisms.
It is also useful for the preparation of Salmonella Typhi antigens.
Blood is a good constituent of enriched medium for fastidious organisms even though contains inhibitors for certain bacteria such as Neisseria and Haemophilus genera and the blood agar must be heated to inactivate these inhibitors and to release essential growth factors (e.g., V factor). The heating of blood agar converts it into chocolate agar (75°C for 15 minutes turns a chocolate color) and supports the growth of these bacteria.
Hemolysis on blood agar: Mainly three types of hemolysis are produced in Sheep blood agar by Streptococci namely; Alpha hemolysis, Beta hemolysis, gamma hemolysis but sometimes alpha prime or wide zone alpha hemolysis may be encountered. Hemolysis is best observed by examining colonies grown under anaerobic conditions or inspecting sub-surface colonies. How does one know if the colonies they are observing on a plate have caused alpha hemolysis or beta hemolysis or gamma ?-Follow up principle and interpretation.
To check the type of blood agar hemolysis, the blood agar plate must be held up to a bright transmitted light source and observed with the light coming from behind.
Alpha hemolysis: Streptococcus pneumoniae
Beta Hemolysis: Group A beta-hemolytic streptococci-Streptococcus pyogenesand Group B, beta-hemolytic streptococci–Streptococcus agalactiace.For the group, A streptococci maximal activity of both the hemolysins; Oxygen labile SLO, and oxygen stable SLS hemolysins is observed only in anaerobic conditions.
Gamma or non-hemolysis: Enterococcus species
Alpha prime or wide zone alpha hemolysis: A small zone of intact erythrocytes immediately adjacent to bacterial colony, with a zone of complete red-cell hemolysis surrounding the zone of intact erythrocytes. This type of hemolysis may be confused with Beta hemolysis.
The double zone of hemolysis on it is also seen by some organisms like Clostridium perfringens and Aeromonas hydrophilia called target hemolysis.
If you are planning to prepare a batch of blood agar plates, prepare few blood agar plates first to ensure that blood is sterile.
CAMP test for Streptococcus agalactiae. -positive on blood agar as shown below-
S. penumoniae antimicrobial susceptibility testing (AST) on blood agar as shown below-
S. penumoniae optochin test on blood agar as shown below-
Growth of Staphylococcus (pinhead colony) and Streptococcus (pinpoint colony) both are beta-hemolytic on blood agar as shown below-
Growth of Streptococcus pyogenes on blood agar, beta-hemolytic colonies, and 0.04 Unit bacitracin sensitive as shown below-
Staphylococcus and Enterococcus on blood agar and use of catalase test as shown below-
Viridans Streptococci growth on blood agar and optochin resistant as shown below-
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