Selective media are used for the growth of only select microorganisms. For example, if a microorganism is resistant to a certain antibiotic, such as ampcillin or tetracycline , then that antibiotic can be added to the medium in order to prevent other cells, which do not possess the resistance, from growing.

Selective media

•Media contains some dyes such as crystal violet , methylene blue and eosin Y. Other media contains bile salts which inhibits the growth of Gram positive bacteria

Thiosulfate citrate bile salt Agar (TCBS)

Selective media for Vibrio Cholerae because the high pH which inhibit Gram +ve bacteria &most of Enter bacteria

Aspergillus Niger sabouraud dextrose agar

Selective media for fungi because the pH5.6

Differential media

• Differential media or indicator media distinguish one microorganism type from another growing on the same media.

•This type of media uses the biochemical characteristics of a microorganism growing in the presence of specific nutrients or indicators (such asneutral red ,phenol red, methylene blue or eosin Y) added to the medium to visibly indicate the defining characteristics of a microorganism.

Blood agar plates are often used to diagnose infection. On the right is a positive Streptococcus culture; on the left a positive Staphylococcus culture

Examples for selective and differential media

• Eosin methylene blue agar (EMB)

• Mannitol Salt Agar (MSA)

• MacConkey Agar (Mac)

• Salmonella Shigella Agar(SSAgar

Eosin methylene blue agar (EMB)

• Eosin methylene blue agar (EMB) is a selective and differential medium used to isolate fecal coliforms.

• Selective because the contains of Eosin Y and methylene blue are inhibit growth of most gram positive bacteria

Differential because the contains of :

1-pH indicator are E & MB dyes which combine to form a dark purple precipitate at low pH

2-lactose carbohydrate sources which encourage the growth of fecal coli forms and provide a means of differentiating them as:

• Lactose fermenter produced acid and gives pink colors.

• Non-lactose fermenter normally-colored or colorless colonies

Eosin methylene blue agar (EMB)

• E. coli results in a metallic green sheen on EMB .

Enterobacter aerogenes and Enterobacter cloacae both form pink

colonies on EMB.

Salmonella typhimurium and Pseudomonas aeruginosa both form colorless colonies on EMB.

MacConkey Agar

• MacConkey Agar (MAC) is a selective and differential medium designed to isolate and differentiate enterics based on their ability to ferment lactose.

• Selective because the contains of Bile salts and crystal violet inhibit the growth of Gram positive organisms.

MacConkey Agar

Differential because the contains of :

•Lactose as a source of fermentable carbohydrate, allowing for differentiation.Lactose fermenter will gives red colors lactose non- fermenter will give normally- colored or colorless colonies.

•Neutral red is a pH indicator that turns red at a pH below 6.8 and is colorless at any pH greater than 6.8.

• Lactose fermenters will produce pink colors colonies

•Non-fermenters will produce normally-colored or colorless colonies.

Klebsiella pneumoniae Micrococcus gluteus does not grow

on Mac agar

Escherichia coli on Mac

E.Coli & Serratia marcesens on Mac agar

Mannitol Salt Agar (MSA)

• Mannitol Salt Agar (MSA) is a selective and differential medium.

• Selective because contain the high concentration of salt (7.5%) which selects for members of the genus Staphylococcus, since they can tolerate high saline levels.

• Organisms from other genera may grow, but they typically grow very weakly.

MSA differential because the contains of :

1. pH indicator phenol red which gives yellow color at low pH.

2. Mannitol sugar as a carbon sources which encourage the growth Staphylococcus aureus and provide a means of differentiating them from the other Staphylococci which will not ferment mannitol.

Staphylococcus epidermidisStaphylococcus aureus yellow color

The Staphylococcus aureus will give yellow.  The Serratia marcesens does not

grow on MSA

Streptococcus agalactiae does not grow on MSA. Staphylococcus epidermidis grows but does not ferment mannitol

Salmonella Shigella Agar (S.S Agar) Salmonella Shigella Agar is a selective and

differential medium.

•Selective for Salmonella & Shigella because its contain Na-citrate & bile salts which inhibit Gram +Ve & most of Gram - Ve bacteria .

•Differential between Salmonella & Shigella because Salmonella can utilize the citrate a carbon source & give a black color as a result H2S produces .But Shigella can not utilize the citrate a carbon source & give colorless

Salmonella on SS agar give a black color as a result H2S produces

Never Ever Trust The Selectivity Of The Selective Media

References:-http://en.wikipedia.org/wiki/Mannitol_Salt_Agar“http://gateway.nlm.nih.gov/MeetingAbstracts/102265145.html2003http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Media.html 2003http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Media.html#Bristol's_Medium http://books.google.com/books?id=AtjDUn5KfG0C&pg=PA172&ots=H0ClQyGxd4&dq=selective+and+differential+media&sig=6f8CshJIEPuoTNVVerDwYNttk_o#PPP15,M1http://www.arabslab.com/vb/showthread.php?t=2415http://en.wikipedia.org/wiki/Image:Proteus_McConkey.jpg#filehttp://www.biomedcentral.com/content/figures/1471-2180-4-6-4.jpg

الحمد لله الذي هدانا الحمد لله الذي هدانا لهذا وما كنا لنهتدي لهذا وما كنا لنهتدي

لوال ان هدانا اللهلوال ان هدانا الله