Types of Agar

Culture Medium

a nutrient material prepared for the growth of a microorganisms in laboratory setting

Agar Basics

• is a seaweed polysaccharide

• not metabolized by bacteria

• commercially powdered and needs to be prepared prior to use

• becomes liquid at ~ 40^o

• does not bind to water, allows bacteria to use it for growth

Preparing Agar

1. add liquid

2. melt with autoclave

   • sterilize

3. pour into required glassware

4. allow it to cool and harden

Mannitol Salt Agar (MSA)

• a selective media

• high NaCl

  • inhibits gram negative bacteria

• fermentation of mannitol produces acid creating a yellow colour

MacConkey’s Agar

• has selective media traits

  • inhibits gram positive

• has differential media traits

  • allows for different types of growth

    • Lactose Fermenters = Pink Precipitate

    • Bile Salts = Precipitate with Acid

Blood Agar

• contains blood/tissue from animal (sheep)

• if bacteria produces Haemolysis the RBCs burst = clearing in agar

Citrate Slant Agar

this medium becomes alkaline as the carbon dioxide combines with sodium and water

• produces alkaline carbonates and bicarbonates

the presence of sodium carbonate changes the bromothymol to blue

• Blue = alkaline agar = bacteria uses citrate

• Green = no change = bacteria does not use citrate

Starch Agar

contain starch

used to test for amylase enzyme activity

Trypticase Soy Agar (TSA)

• a general-purpose medium

• contains:

  • pancreatic digest of casein

  • papain digest of soybean meal

  • sodium chloride