Blood agar is an enriched, bacterial growth medium. Fastidious organisms, such as streptococci, do not grow well on ordinary growth media. Blood agar is a type of growth medium (trypticase soy agar enriched with 5% sheep blood) that encourages the growth of bacteria, such as streptococci, that otherwise wouldn’t grow.
Blood 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). Heating of blood agar converts it into chocolate agar (heated blood turns a chocolate color) and supports the growth of these bacteria.
Blood agar consists of a base containing a protein source (e.g. Tryptones), soybean protein digest, sodium chloride (NaCl), agar, and 5% sheep blood.
Composition of Blood Agar
- Pancreatic digest of casein
- Papaic digest of soy meal
- Distilled water
Combine the ingredients and adjust the pH to 7.3. Boil to dissolve the agar, and sterilize by autoclaving.
Procedure for the preparation of Blood Agar
- Prepare the blood agar base as instructed by the manufacturer. Sterilize by autoclaving at 121°C for 15 minutes.
- Transfer thus prepared blood agar base to a 50°C water bath.
- When the agar base is cooled to 50°C, add sterile blood aseptically and mix well gently. Avoid the formation of air bubbles. You must have warmed the blood to room temperature at the time of dispensing to the molten agar base.
(Note: If you are planning to prepare a batch of blood agar plates, prepare few blood agar plates first to ensure that blood is sterile).
- 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).
- Store the plates 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.
Quality control of Blood Agar
- The pH of the blood agar range from 7.2 to 7.6 at room temperature.
- Inoculate the plates with 5-hour broth cultures of Streptococcus pyogenes and S. pneumoniae. Inoculate also a plate with H. influenzae and streak with S. aureus (i.e. Satellitism Test).
- Incubate the plates in a carbon dioxide enriched atmosphere at 35-37°C overnight.
- Check for the growth characteristics of each species
- S. pyogenes: Beta-hemolysis
- S. pneumoniae: Alpha-hemolysis
- Satellitism of H. influenzae
Uses of Blood Agar
Blood agar has two major uses:
- Isolation, identification (with the use of either Optochin disc or Bacitracin disc and testing the sensitivity of the isolate), and antimicrobial susceptibility of Streptococci.
- Determine the type of hemolysis, if any.
Blood Agar and Hemolysis
Certain bacterial species produce extracellular enzymes that lyse red blood cells in the blood agar (hemolysis). These hemolysins (exotoxin) radially diffuses outwards from the colony (or colonies) causing complete or partial destruction of the red cells (RBC) in the medium and complete denaturation of hemoglobin within the cells to colorless products.
Four types of hemolysis are produced in sheep blood agar by Streptococci namely; alpha (α) hemolysis, beta (β) hemolysis, gamma (γ) hemolysis, and alpha prime or wide zone alpha hemolysis.
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?
Note: To know the type of hemolysis, the blood agar plate must be held up to a light source and observed with the light coming from behind (transmitted light).
If either type of hemolysis is present, then one will observe a zone of hemolysis surrounding a growing colony.
- Alpha (α) hemolysis: Partial lysis of the RBC to produce a greenish-grey or brownish discoloration around the colony. Alpha hemolysis is due to the reduction of RBC hemoglobin to methemoglobin in the medium surrounding the colony. Many of the alpha-hemolytic streptococci are part of the normal body flora. But Streptococcus pneumoniae which is also alpha-hemolytic causes serious pneumonia and other deadly infectious diseases.
- Beta (β) Hemolysis: Complete lysis of red blood cells causing a clearing of blood from the medium under and surrounding the colonies e.g. Group A beta-hemolytic streptococci-Streptococcus pyogenes and Group B, beta-hemolytic streptococci-Streptococcus agalactiace.
For group A streptococci maximal activity of both the hemolysins; Oxygen labile (SLO) and oxygen stable (SLS) hemolysins are observed only in anaerobic conditions.
- Gamma (γ) or non-hemolysis: No hemolysis of RBC. No change of the medium under and surrounding the colonies.
- 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 β-hemolysis.
- Target Hemolysis: Clostridium perfringens are readily identified in the laboratory by its characteristic “double zone” hemolysis also known as target hemolysis.