Selective media:

They are Solid mediu to which inhibitory substance are added.Thus, the media will allow only the growth of wanted bacteria and the unwanted bacteria are inhibited.

Example MacConkey medium for gram negative bacteria.

TCBS thiosulphate citrate bile salts sucrose agar for vibrio cholerae.

Differential medium This is a special medium that helps us to distinguish between bacteria.The medium is incorporated with substance enabling the differentiation of bacteria by producing different colony appearance.Hence,this medium is called indicator medium.

Example MacConkey medium distinguishes between lactose fermenters pink colour colonies and non lactose fermenters colourless colonies.

Some important bacteriological culture media

MacConkey Agar It is both a selective and differential medium.It is selective for gram negative bacteria.It further differentiates bacteria as lactose fermenters those bacteria that have the ability to ferment lactose and non lactose fermenters.

Component 

Bile salts are added to inhibit most gram positive, except enterococcus and some species of staphylococcus aureus.

Crystal violet dye also inhibit certain gram positive bacteria.

Lactose is a sugar used to differentiate lactose fermenters and non lactose fermenters.some bacteria can ferment lactose resulting in acid end product, others cannot.

Neutral pH red indicator produces pink colour when the lactose is fermented producing acid end product.

Peptones is a source of amino acid and organic nitrogen for microbial growth.

The bacteria such as Escherichia coli,enterobacter and klebsiella ferment lactose in the medium and will produce acid in the medium,which lowers the pH of the agar and results in the appearance of pink colonies.The bile salts in the medium precipitate causing the medium surrounding the colony to become hazy appearance.Non lactose fermenting bacteria,such as proteus spp, salmonella,pseudomonads aeruginosa and shigella, cannot utilize lactose in the medium leads to the formation of white or colourless colonies in the plate.

Blood agar 

Blood agar is both differential and enriched medium.here sterile 5 percent sheeps blood is incorporated into the basal medium to supply special nutrients required for the growth of fastidious Organisms Such as the streptococcus species.

Many streptococcus species produce haemolysin that will lyse the red blood cells causing the certain characteristics haemolysis pattern in the medium that helps to differentiate them.

Some streptococci produce only partial haemolysis around the colonies showing a distinct green haemolytic zone around the colonies.This type of haemolysis is called alpha haemolysis.

Some species of streptococci cause complete haemolysis around the colonies showing clear zone around the colonies.They are said to be a beta haemolysis.remaining streptococci which do not produce any haemolysis around the colonies are called non haemolytic or gamma haemolytic streptococci.