Differential and Selective Media

Prepared BYMs.c: Abed Al Rahman I.

Hamad

Type of media Purpose

Define : The media used in the laboratory have to be chosen to suit the nutritional requirements of the species of organism to be grown. Isolation from a mixture can sometimes be facilitated by the use of media designed for a special purpose.

Types of media:a) Transport media: its media for preserve the microbe in same number and

condition as in patient (no nutrient, inhibition of lethal effect of destructive enzymes, and preserve the microbe from effect of oxidation).

b) Isolated media: 1) Simple media (basic media):Peptone (peptone + NaCl)Nutrient broth (extract meet + peptone + NaCL)Nutrient agar (nutrient broth + solidyfing agent)2) Enrichment mediaBlood agar: nutrient agar +5% blood (red color) Chocolate agar : heated blood agar (brawn color) specific media for

Hemophilus influenzae and Neisseria gonorrhoeae

Differential vs Selective Media• Selective media: Media – is a type of media that contains

certain chemicals that will permit only certain groups of genus of bacteria to grow on it. Lowenstein Jensen media for T.B. which contain malachite green as inhibitor.

• Differential: Media – is a media type that will distinguish biochemically between related groups of organisms. Some organisms will incorporate some of the chemicals in their make up creating specific colored colonies.

• Ex: MSA, Mac Conkey,EMB

• Trypticase Soya agar Nutritious medium for a growth of a huge variety of bacteria.

• No differential • No selective

http://www.austincc.edu/microbugz/assets/images/MSA.jpg

TSA

Mannitol Salt Agar

• MSA: Mannitol Salt Agar• Selective medium for Gram positives. Inhibit

Gram negatives.• Differential: mannitol fermenters• Medium specific for staphylococcus contains

7.5% salt which is inhibitory for most other bacteria.

• Contains the sugar mannitol and the pH indicator phenol red fermentation of mannitol acid produced media turn yellow

• Positive: Growth, yellow

• Negative: No growth, no change of color

http://www.austincc.edu/microbugz/assets/images/MSA.jpg

MSA

Eosin Methylene Blue

• EMB: Eosin Methylene Blue

• Selective medium for Gram negatives. Inhibit Gram positives.

• Differential medium for lactose fermenters– Lactose fermenter will produce dark red

colonies

• E. coli produce a characteristic green sheen

EMB

• 1: E. coli green• 2: no lactose

fermentation• 3: lactose

fermentation• 4: no growth

http://www.microchemlab.net/bact5.jpg

http://iws2.ccccd.edu/dcain/CCCCD%20Micro/EMBplate.jpg

McConkey agar

• Selective medium for Gram negatives

• Differential medium for lactose fermenter bacteria

• Positive for lactose fermentation =Colonies bright pink

• Negative for lactose fermentation= colorless colonies

http://www.emlab.com/m/media/Ecoli_Feb07.jpg

• Mueller-Hinton agar plate (MHA)

• Use : For antimicrobial susceptibility testing of a variety of nonfastidious, rapidly-growing microorganisms.

• Control organisms :   Escherichia coli   ATCC25922 , Staphylococcus aureus ATCC 25923, and Pseudomonas aeruginosa ATCC 27853 : Growth ,Inhibition Zone followed Standard interpretative  

• Salmonella-Shigella agar plate (SS)

• Use : For the selective isolation and differentiation of pathogenic enteric bacilli, especially those belonging to the genus Salmonella. This media is not recommended for the primary isolation of Shigella species.  Lactose-fermenting   bacteria such as  Escherichia coli or Klebsiella pneumoniae appear as small pink or red colonies. Lactose-nonfermenting bacteria such as Salmonella species, Proteus species and Shigella species appear as colorless colonies. Production of H2S  by Salmonella species turns the center the colonies black.

• Control organisms :   Salmonella typhi : Colorless colonies, black centers.      Escherichia  coli : Pink colonies 

• Sabouraud Dextrose Agar plate (SDA)

• Use : For the cultivation of pathogenic and nonpathogenic fungi, especially dermatophytes. The medium may be made more selective for fungi by the addition of specific antibiotics such as chloramphenicol. For the cultivation of yeast and filamentous fungi.