Selective & Differential Media

Selective & Differential Media :

Two main types of Media for Differential and Selective Organisms the main types of Media for get desired and Screening of Microorganism the two main media are used in Microbiological Laboratory are :-

1) Selective Media

2) Differential Media

In this series we are going to take tour towards the Selective & Differential Media.

1) Selective Media :

Selective media can be define as the medium which is used to get the growth of only specific and desired microorganisms known as "Selective Media".

The Selective Media supresses the growth of unwanted bacteria and gives full opportunity to support growth of particular or specific type of organisms which are nothing but the Desired Microbes.

Examples : 

¹.MacConkey Agar - 

².Mannitol Salt Agar etc.

¹.MacConkey Agar -

MacConkey Agar is the very good example of Selective Medium which inhibits the growth of 'Gram Positive' bacteria while selective for "Gram Negative" bacteria the reason behind it because Bile Salts & Crystal Violet present in the compostion of MacConkey's agar which suppress the Gram Postive and Non-Lactose Fermenting Bacteria.

While Lactose Fermenters turns into red or pink color appearance on agar surface and non-lactose fermenters does not changes the color.

🤔 ¿ Then which ingredient Supports the Gram Positive  Bacterial Growth ?

².Mannitol Salt Agar -

Mannitol Salt Agar (MSA) have the concenttration of NaCl 7.5% which is acts as inhibitor to the most of human pathogens where Bile Salts inhibits the Gram Postive and Permitts the Gram Negative  rods to grow easily.

MSA is used to isolate the intestinal pathogens which contains the Bile Salts.

If we want to see the role of indicators dye in the medium which are nothing but the Methylene blue or Crystal Violet which plays the excellent role of inhibitory complexfor Gram Positive Bacteria (Staphylococci  which can ferment Mannitol so that they can produce acids which turns Phenol red into Bright Yellow Color)

While other side acetate plays the very important role of Carbon source in Selective Media which makes it Selective Medium for Staphylococci.

2) Differential Media :

The differential media which make evrything easy to get understand the growth identifiction of organisms and screening & distinguish purpose of colonies of desired microbes the different organisms grow on same media plate known as "Differential Media".

In other simple words the media which is used to cultivation of organism and help to differentiate the varios organism colonies on same media can be termed as "Differential Medium".

¹.MacConkey's Agar is Selective as well as Differential Media because its helps in 

Means Lactose fermenting & non-lactose

fermenting  organisms.

i.e.(E.coli & Shigella)

¿Thats why its Differential Which helped in Differentiate the above mentioned characters.

Where E.Coli is lactose fermenting bacteria which ferment lactose present in media & produces acids leads to turns the color into red where those dont ferment lactose or non-lactose fermenters produces white color colonies.

Lactose positive (pink colonies)

Lactose fermenting species will grow pink colonies. 

Lactose negative (white colonies)

Gram-negative bacterial species will still form colonies, but colonies will have a white appearance as there will be no change in pH in the absence of lactose fermentation.

².Mannitol Salt Agar is used to screen out the Staphylococcus aureus because its ability to ferment mannitol & turns color from pink to yellow.

As this is differnital media so the Dyes are aslo used agent of differetiastion.

                Mannitol turns pink to yellow

³.Blood Agar -(Enriched & Differential)

Blood Agar is used to differentiate  hemolytic & non-hemolytic bacteria are determined blood agar is enriched as well as differential medium.

Blood agar has RBC enriched while it helps to detect the hemolytic & non-hemolytic bacteria which shows the clear zone (Produces zone of clearance around their colonies because of hemolytic destruction of Red Blood Cells (RBCs).

Example :

Hemolytic Becteria which breaks down the RBC and Deatruct them are -

Streptococci & Staphylococci