Change of Colour of Agar Plate?

Question

After a 1-2 days, the agar jelly changed colour from a bright red to a darker brownish red, while the bacteria had grown extremely quickly. Any explanations of why it changed colour?

Answer 1

the by-products of the bacterial metabolic processes hemolyzed the blood in the agar.

Answer 2

AS  you have mentioned, the bacteria had grown rapidly... this is due to proteins and sugar present in the agar. As they grow , they will divide , reproduce, excrete, age and dye. The young bacteria was the initial colour... the later colour was probably due to enzymic reactions of all the end products of digestion ,reproduction, and death on the sugar and protein of the agar. Hope I've helped you.

Answer 3

Blood agar plates have blood in them.  When the red blood cells are broken, when they lyse, they change color to brown.  Chocolate agar plates are plates where the blood has been lysed by heat.  There's no chocolate, just the lysed blood.  

Of course, the different ways that different bacteria grow on the various plates is what allows for their identification.

Here are some references.

Answer 4

What kind of agar plate are you using??