Selective and Differential Media


Media can be used not only to grow bacteria but also as a way to select and differentiate between different species.  A selective media contains something in the formulation that will inhibit growth of different bacteria....ie Gram (-) can grow but Gram (+) bacteria are inhibited.  A differential media will contain something in the media that will cause one species of bacteria to look different form the rest....ie one species of bacteria will be able to break a product down causing a precipitant, color change in the media, colonies to grow a differnt color form the rest, etc.  A media can be selective, diifferential or both.

MacConkey Agar (MAC):
•-Important Ingredients: crystal violet, bile salts, neutral red (color indicator), lactose
•-Differential Media: used to identify and isolate lactose fermenting G- enteric bacteria mainly Enterobacteriaceae.  Strong lactose fermentation produces pink colonies with bile salt precipitation (hot pink).  Weak fermentation produces pink colonies.
•-Selective Media: crystal violet and bile salts inhibit G+ growth and some G- bacteria.


•-Results:
       •Top Left: E. coli: strong lactose fermentation resulting in bright pink halo (bile ppt) and pink growth. 
       •Top Right:  Salmonella arizonae: organism can grow but does not ferment lactose, white transparent colonies, turns agar yellow.
       • Lower Right:  Enterobacter aerogenes: ferments lactose, colonies are pink.
       • Lower Left:  Proteus vulgaris: organism can grow but does not ferment lactose, white transparent colonies, turns agar yellow.


E. coli colonies with characteristic precipitant, pink halo.

Proteus vulgaris grows with no color change.
(A)                                                                                     (B)

(A) Enterbacter aerogenes week fermentor that grows pink with no precipitant.  (B) Klebsiella pneumoniae, week fermentor that grows pink.  Klebsiella pneumoniae is highly mucoid and easier recognized on MAC.

Mannitol Salt Agar (MSA)

•Importatn Ingredients:  Phenol red (color indicator), mannitol (sugar), 7.5% NaCl (inhibits most bacterail growth.
•IDifferential:  Between pathogenic S. aureus and S. epidermidis.
•ISelective: NaCl inhibits non-halophylic organisms.  Organism that can grow are considered halophylic and are usually Gram (+) organisms.
•IMannitol:  fermentation of acid causes a drop in pH of the media and the indicator (phenol red) turns yellow.



Top Center:  Staphylococcus aureus:  large yellow halo around growth indicating mannitol fermentation.
Right center:  Staphylococcus epidermidis:  Growth but no color change to the media.  
Bottom Center:  Staphylococcus saprophyticus:  samll yellow halo around growth (10% of S. saprophyticus can ferment mannitol)
Left Center:  E. coli: no growth, inhibited by the 7.5% NaCl
Note:  Staphylococcus spp are considered halophies because they are able to grow on high salt concentrations.


Blood agar

• Differential Media:Blood agar contains 5% sheep blood, is used to grow fastidious organisms, and can differentiate between different species based on their hemolytic properties.  Many of these fastidious organisms require particular nutrients found in blood and considered more difficult to grow.  Blood agar is considered a differential agar and can be used to differentiate between different Strep spp.


Streak plate of Streptococcus pyogenes (incubated @ 37 degrees C for 48 hrs) showing beta hemolysis due to hemolysin O and S, complete lysis of red blood cells, around the isolated colonies. Image taken using transmitted light.  S. pyogenes causes Strep throat.

Streak plate isolation of Streptococcus mitis on blood agar demonstrating alpha hemolysis seen as a greenish color around the growing colonies due to a reduction of the hemoglobin to methemoglobin in the surrounding agar. Image taken using transmitted light.


Enterococcus faecalis grown on blood agar for 48 hrs at 37 degree's C. Small, pin point, circular colonies can be seen with gamma hemolysis (no hemolysis of the blood agar). Colonies were white with a glossy translucent appearance.

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All images copyright T. Sturm

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