Blood Agar: Composition, Preparation, Uses and Types of Hemolysis

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.

Beta Hemolysis in Sheep Blood Agar.
Beta hemolysis in sheep blood agar.

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
  • NaCl
  • Agar
  • 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

  1. Prepare the blood agar base as instructed by the manufacturer. Sterilize by autoclaving at 121°C for 15 minutes.
  2. Transfer thus prepared blood agar base to a 50°C water bath.
  3. 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).
  4. Dispense 15 ml amounts to sterile Petri plates aseptically
  5. Label the medium with the date of preparation and give it a batch number (if necessary).
  6. 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

  1. The pH of the blood agar range from 7.2 to 7.6 at room temperature.
  2. 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).
  3. Incubate the plates in a carbon dioxide enriched atmosphere at 35-37°C overnight.
  4. Check for the growth characteristics of  each species
    1. S. pyogenes: Beta-hemolysis
    2. S. pneumoniae: Alpha-hemolysis
    3. Satellitism of H. influenzae
Optochin and Bacitracin Sensitivity of the isolates in Blood Agar
Optochin and bacitracin sensitivity of the isolates in Blood agar

Uses of Blood Agar

Blood agar has two major uses:

  1. Isolation, identification (with the use of either Optochin disc or Bacitracin disc and testing the sensitivity of the isolate), and antimicrobial susceptibility of Streptococci.
  2. 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.

Types of hemolysis (α, β and γ)
Types of hemolysis (α, β and γ)

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.

Various types of Hemolysis
Various types of Hemolysis
  • 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.
Double Zone hemolysis produced by Clostridium perfringens
Double zone hemolysis produced by Clostridium perfringens
  • 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 HemolysisClostridium perfringens are readily identified in the laboratory by its characteristic “double zone” hemolysis also known as target hemolysis.
About Acharya Tankeshwar 458 Articles
Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please mention in the comments below.


  1. when an incubate blood agar .it turns to black colour… every thing is like preparation. plz take out from this problem.

  2. I have a friend who claims that adding Blood at 50 degrees centigrade causes heamolysis in the final product i.e. Blood Agar. The microbiologist using the product complain that they cannot differentiate the type of bacterial growth if its beta or alpha. Any suggestions on this issue? thank you in advance.

    • Thank you for your comment. There are two possibilities..
      1. The temperature of Blood agar base is more than 50°C; You can test adding the blood after it’s temperature is 45°C
      2. The problem might be in the Blood you are using. Do you use sheep blood or human blood?

    • Your teacher might be saying media containing eggs such as Lowenstein Jensen medium uses inspissator. For Blood Agar, we sterilized blood agar base, and later you mix the blood. As blood is sterile body fluid, you do not need to sterilize it.

    • Jiskirat, you might have mistaken, your teacher might not have said so. He/She might have says, you have to sterilize egg containing medium such as Lowenstein Jensen medium using inspissator. In the case of Blood Agar, you sterilize and prepare blood agar base, in which you add required amount of Blood (Blood is sterile body fluid, so you need not to sterilize it). For procedure, please go through the post again.

  3. Can you please help me with easy steps on analysis of plant extracts on anti-bacteria activity against Heliobacter pylori, thank you

    • Dear Sam
      Culturing Helicobacter pylori in the laboratory is not a easy process and we use other tests, mainly Urea breathe test to diagnose if the patients admitted in our hospital have H.pylori infection. As i have not grown H.pylori and also not involved in extraction of plant products to check their antibacterial effects, I am sorry, i am not able to guide you with easy steps you have to follow. I am hopeful you will find it in pubmed or other similar portals of scientific interests .

  4. thank you this very good work. please could include the preparation of layered blood agar and the advantages it has over blood agar?

  5. 1. why do one need to pour water unto the nutrient agar imside the erlenmeyer flask and not the other way/
    2. what is the medium colour brfore and after adding the blood/

  6. Hello Mr. Acharya,
    Your blog is useful. I saw it for preparation of Manuals for NABL accreditation of laboratory.

    Could you please help me:
    1. You mentioned about shelf life of blood agar as 4 weeks. Can you give a standard reference for this
    2. Do we have to pack this blood agar in single packing…..
    3. Sterility Check of blood agar should be done for how many percent of plates prepared.

    If the same principles can be applied for other media like MHA

    • Dear Abdul Wasiu
      Thank you for your query, but my areas of expertise is only on pathogenic bacteria, So i may not be the right person to guide on the project you are planning to begin.

  7. Sir why they named delta hemolysis as one of the type of hemolysis. They are not mentioned properly in any books . Could you help me??

  8. Thank you sir..can you help me out comparion between sheep blood agar, horse blood agar , human blood agar

  9. What anticoagulant should be used while collecting sheep blood please ?
    Does any can be harmful to the media ?

  10. If blood agar was to be used to culture S. aureus and P. aeruginosa will there be a possibility that another type of bacteria will grow???

    • Yes, if the sample contains other bacteria, they will also grow on it. Blood agar is an enriched culture medium and supports growth of most of the pathogenic bacteria except few fastidious bacteria like Haemophilus influenzae and obligate intracellular bacteria.

2 Trackbacks / Pingbacks

  1. Practical week 5 (02/07/18-05/07/18) – TYBSC18/19
  2. Differential Media for Different Microorganisms - Labmonk

Do you have any queries? Please leave me in the comments section below. I will be happy to read your comments and reply.

This site uses Akismet to reduce spam. Learn how your comment data is processed.