Blood agar medium

It is a differenltial media that is prepared by adding blood in the nutrient agar.It does not involve pH related reactions but differentiates hemolytic and non haemolytic bacteria.The lysis of RBC produces a clear zone around the colony growing on the blood agar plate.Some of the bacteria can produce haemolysin that lyses red blood cells.Examples ,species of Streptococcus that reside harmlessly in the throat often cause a hemolysis called alpha hemolysis,which is characterized by a zone of greenish clearing around the colonies.

MacConkey agar
It is a differential media that differentiates between lactose fermenter and non-lactose fermenter.this agar contains lactose as a source of carbon.Those organisms that can utilize lactose for its carbon metabolism thereby producing lactic acid produce pink colonies and those that cannot utilize lactose produse yellow colonies.This is mainly due to the presence of pH indicator neutral red which is red or pink in acidic condition and yellow in alkaline pH.
The presence of bile salts and crystal violet in MacConkey agar inhibit the growth of Gram positive bacteria,allowing the selective growth of gram negative bacteria.Thus,MacConkey agar is also known as selective media. In addition to the selective growth of enterobacteria,MacConkey agar also differentiates lactose fermenting bacteria from non lactose fermenting bacteria.