Comparison between Chocolate agar and Blood agar.

Hello everyone.

Today we will be learning something about blood and chocolate agar and try to compare them.

So, What are Chocolate and blood agar.

Basically both are enriched non-selective media  which are used to isolate fastidious and pathogenic organisms, that do not grow on usual nutrient medias. In case you want to identify whether a particular isolate is pathogenic, you can simply isolate it on Blood or chocolate agar.

How are they made?

Well, to make the two medias you will need to prepare a basal medium. And this basal media is nothing but our Nutrient agar.

For blood agar. We need to add 5% of sterile and defibrinated ( Clotting factors like fibrin are removed) sheep blood is added into the molten agar and then dispended into petri plates.

As for Chocolate agar, we add sterile and molten 100% chocolate into the agar. You can use your favorite one and melt it...nah, I am just kidding.

No, considering your behaviour, I am sure you would have done this.

Hehehe...shut up.

Sadly chocolate agar does not contains Chocolate :(

Chocolate agar, you can say is more of a lysed blood agar. The blood that is added into the medium is heated to a point till the RBCs are lysed. This changes the color of blood from bright red to brown, which is why the media is named as Chocolate agar. This the basic difference between the two.

What temperature should the blood be heated?

For blood agar, the blood is heated at 50oC -55oC. 

For chocolate agar, the blood should be heated at 80oC to lyse all the RBCs till the color of blood changes.

PS. Not necessary to add sheep blood only. You can add bovine blood too or even human blood if you can source it from a hospital. They generally provide you with expired blood, which comes useful for you :p

How are they nutritionally different?

Chocolate agar is more nutritionally enriched than blood agar. Blood agar contains whole blood which consists various complex nutrients that helps in growth of fastidious organisms.

But the reason that makes chocolate agar more nutritious is lysed RBCs. Lysed RBCs releases intracellular nutrients like hemoglobin, hemin or other porphrins (factor X) and Nicotinamide dinucleotide i.e.NAD (Factor V).

These factors facilitates growth of pathogenic organsims like Hemophilus and Neissria species which are unable to grow on blood agar. Addition of certain antibiotics will make it  selective for a particular strain like N.gonohorrea.

Another variation of These agars is Thayer Martin Agar, which contains similar components but it is made selective by addition of antibiotics like vancomycin, bacitracin, etc.

I hope this somewhat helps you all.

High six.

-Staph :D