• What does selective media and differential tests do?

Helps us id. features and characteristics of bacteria. 

• artificial environments are created to exploit bacterial features.
• Not intended  to mimic natural environments. 

What are the 6 fundamental features of media?

•  Selectivity
• Nutrient components
• inhibitors
• subrates
• indicators
• positive or negative reactions

Selectivity

(selective media decriptors)

• selective
• differential
• defined/chemically defined
• undefined

Encourages some growth and discourages others.

 example: mannitol salt 

Allows growth for all but distinguishes between types of bacteria

examples: mannitol (substrate of ferminatation) color change to yellow

Ingredients known but NOT ALL components

Example: beef and yeast extract. 

Paticular cocktail of nutrients can help select for a particular bacteria. 

- Yeast extract: added to media to supply specific vitamins. Microbes that need specific vit.=fastidious.

-NaCL: Added to media for organisims that require osmotic equilibrium. 

Will make media particularlly selective (exploits certain bacterial weak points.) 

• Will commonly effect: DNA synthesis/ Gene expression. 
• Metabolic enzymes
• Membrane permability.

Therefore bacteria vary in these traits!!!!!! 

• Makes a media particularlly differential
• 
  Bacterial (cells) are bag of biochemical reactions!!!
• Some bacterial harbor particular enzymes needed for metabolic reactions (reactions needed for survial)
• Those enzymes are directed to work on particular substrates 
• controlling the presence or absence of substrates allow you to control which types of bacteria to grow.

Bacterial growth is not the best indicator of a successful selection event. 

• Some unknown contaminents can grow under selective conditions = false positive
• Dyes sensitive to PH change( phenol red) 
• color reaction in the media upon production of a metabolite example: ferric sulfate 

Media can contain particular senarios that only certain bacteria can survive manipulate and exploit.

Example: Some bacteria produce gas under certain conditions

-If bubbles form= + result

-If bubbles do not form= - result 

Biochemical/defferential Tests

1.) Phenol Red Broth

(Carbohydrates)

-Lactose -Glucose -Dextose

• Testing if fermentation occurs
• Gas bubbles will occur if fermentation took place
• Acid production= PH drop=media color change to yellow!!
• Media is Red to start

• Testing to see if citrate us used as a carbon source. Media is green to start
• Sodium citrate is present as only carbon source, PH indicator is bromthymol blue.
• If citrate is used up the PH rises and media turns blue. 
• Growth alone DOES INDICATE A POSITIVE!!!

• Tests for decarboxylase enzyme
• If the enzyme is present the it will turn the medica purple which is a ph indicator (alkaline-purple-acidic-yellow)
• If the enzyme is not present: Above doesn't occur but instead fermentation occurs and media turns yellow.

• Tests for sulfur reduction, indole production and motility.
• Media is a straw color to start

• Tests to see if the Urease enzyme is present
• Media is a yellow/orange to start

If urea is present in media along with phenol red as ph indicator. If urea can be used as carbon source than production of NH3 (ammonia) occurs, increasing PH to alkaline range.

• If Urea is used: NH3 is produced =alkaline=media turne a PINK COLOR
• 
  

Is a selective and differential medium containing lactose, bile salts, neutral red, and crystal violet. 

• Bile salts and crystal violet inhibit growth of gram (+) bacterial
• Neutral red dye is a PH indicator that is colorless above a PH of 6.8.
• Red at PH less than 6.8 (acid accumulating)
• Lactose fermentation turns the dye red.
• Lactose non-fermentators retain their normal color or the color of the medium.
• Formulations with out crystal violet allow the growth of entercoccus and some species of staphylcoccus, which ferment the lactose and appear pink on medium. 

Eosin Methylene Blue Agar

(EMB)

Is a complex, chemically undefined, selective, differential medium. It contains peptone, lactose, sucrose and the dyes eosin y and methylene blue. 

• Peptone provides a complex mixture of carbon, nitrogen, and other nutritional components. The sugars encourage the growth of enteric bacteria.

Purpose of the Dyes

• They inhibit the growth of gram (+) bacteria.
• They react with vigorous lactose fermenters and (in acidic environment) turn the growth DARK PURPLE or BLACK. Typical of escherichia coli. 
• Less aggressive lactose fermenters produce colonies that can range from pink to dark purple.
• Non-fermenters and sucrose fermenters typically retain their normal color or take on the coloration of the media. 

Mannitol Salts Agar 

(MSA)

Contains the carbohydrate mannitol, which is 7.5% sodium chloride (NaCl)

The PH indicator is phenol red. Phenol red turns yellow when the PH is below 6.8, turns red at PH 7.4 to 8.4 and turns pink at PH 8.4 and above. 

• mannitol provides the substratefor fermentation and makes the meduim differential. Nacl makes the medium selective because its concentrations are so high it kills (dehydrates) off most bacteria. 
• Staphlococci thrive in the media since they like salty environments.
• Most staphlococci are able to grow on media but do not ferment the mannitol so the growth appears red or pink. 
• Staphylococcus aureus ferments the mannitol, which produces acids and lowers the ph=yellow halo around colonies.