09 Sep

Differential and Selective Media

Media can be made with different components that will promote the growth of certain bacteria while inhibiting the growth of others.  This medium is known as a selective medium.  Antibiotics, dyes, and various inhibitory compounds are often incorporated into media to create selective conditions for growing specific organisms.  The dyes eosin and methylene blue when incororporatd into EMB media do not affect the growth of gram-negative bacteria however they do inhibit the growth of gram-positive bacteria.  When sodium chloride is placed in mannitol-salt agar it promotes the growth of Staphyloccus aureus but inhibits the growth of other bacteria that can’t tolerate high salt concentrations.  A differential medium has substances that cause bacteria to alter how they appear.  When S. aureus grows on mannitol-salt agar, it ferments mannitol, changing a ph indicator from red to yellow around the colonies.  Other staphylococci can’t ferment mannitol and their growth on this medium results in no change in the indicator.  Using selective and differential medium becomes extremely important in quality control and quality assurance techniques.