Monday, December 3, 2012

Difference between Blood agar and Chocolate agar

Hi everyone!

To understand what is the difference between Blood agar and Chocolate agar, we'll need to know about nutrient agar first.

Nutrient agar: It is a simple basal medium used for growth of common pathogens. It constitutes peptone water, meat extract and agar.

So now, let's see how blood agar and chocolate agar is made, and the difference between the two -

How is Blood agar made?
Nutrient agar is sterilized by autoclave, cooled to 50°C and sterile sheep blood (5-10%) is added gradually and poured into plates.

How is Chocolate agar made?
Nutrient agar is sterilized by autoclave, cooled to 75-80°C and sterile sheep blood (5-10%) is added gradually and poured into plates.

Difference between Blood Agar and Chocolate Agar
What is the similarity between Blood agar and Chocolate agar?
- Blood agar and Chocolate agar are enriched media.
- They are used for the growth of Gram positive cocci and Gram negative fastidious organisms (see below) like Neisseria and Haemophilus species.
- They can be used to indicate hemolysis.

(If you can't remember the organisms that grow on chocolate agar, here's a mnemonic:
“Nice Homes have Chocolate” - Neisseria and Haemophilus grow on Chocolate agar)

What are fastidious organisms?
Fastidious organisms require specialized environments due to their complex nutritional requirement.

How is the difference in temperature used in the manufacturing of blood agar and chocolate agar significant?

Certain organisms such as Haemophilus species require V factor for growth (complex nutritional requirement). Factor V is present in blood but it is present inside the red blood cells (RBC).

These organisms can not utilize V factor which is trapped inside the RBC's in Blood agar.
When Blood agar is heated to 80-90°C for a few minutes (boiled blood agar), the V factor is released from within the erythrocytes and made available to the organism for utilization.

Some strains of Neisseriae and Diphtheroids require V factor too. That is why, these media are superior to plain Blood agar for growing organisms requiring V factor.

What is V fatcor?
V fatcor is a coenzyme, Nicotinamide Adenine Dinucleotide (NAD) or Nicotinamide Adenine Dinucleotide Phosphate (NADP) which acts as a hydrogen acceptor in the metabolism of cell.

Chocolate agar is a type of Blood agar in which the blood cells have been lysed by heating for growing fastidious organisms.

Did you know?
Modified Thayer Martin is a type of chocolate agar which contains antibiotics (Vancomycin, Nystatin and Colistin) to suppress growth of other bacteria and promote growth of  N gonorrhoeae.

*phew* That's all for today!

The image is my first Photoshop work on the site, hope you like it =D
Just to remind you guys, Chocolate agar contains no chocolate, they simply named it after the yummy color :P


Questions asked by readers:
Which blood is used for the production of blood agar?
The blood of any animal can be used.
For example - Rabbit blood, sheep blood, ox blood or human blood.

Wait, you can use even human blood? (No, nothing creepy! :P)
Human blood is usually the one which is unused in blood banks for a long period of time and can no longer be used for transfusions.


  1. Thanks! Very helpful information!

  2. Neisseria & Haemophilus are gram-negative bacteria

    1. Yes. I meant it is used for the growth of Gram positive cocci AND fastidious Gram negative organisms like Neisseria & Haemophilus species.

    2. dats exactly wats written

  3. Nice and easy explanation,was very helpful.

  4. Why do we need to cool the nutrient agar to 50°C before adding blood to it? What will happen if we pour the blood directly without cooling?

  5. Thank you for clarifying on this. I'd like to ask why does Thayer Martin agar suppress other bacteria but enhances the growth of N.gonorrhea

  6. What is the difference between chocolate agar and blood agar in respect to nutrient agar


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