EX. 12 | Special Media for Isolating Bacteria

Selective culturing media

These types of media help isolate organisms found in the minority; some can either enhance or inhibit the growth of some organisms

Selective media

Enrichment media

Differential media

Selective media

Contains chemicals that prevent the growth of unwanted bacteria without inhibiting the growth of the desired organism

Enrichment media

Contains chemicals that enhance the growth of the desired bacteria

• Other bacteria will grow as well, but the desired bacteria will be increased

Differential media

Able to distinguish one bacterium from another based on how they metabolize or change the media

3 Types of Media / Agar

Nutrient Agar

Mannitol Salt Agar

EMB Agar

Nutrient agar

Purpose

• To grow and maintain a wide variety of bacteria without favoring or inhibiting any particular organism

Mannitol salt agar

Purpose

• To isolate and identify Staphylococcus species, particularly distinguishing S. aureus from other bacteria

• High salt concentration

• Both selective and differential media
  
  

How it works (Selective)

• Contains 7.5% NaCl (very high salt)

• Most bacteria cannot survive this high salt concentration

• Only salt-tolerant bacteria (Staphylococcus) can grow
  
  

How it works (Differential)

• Contains mannitol (a sugar) and phenol red (a pH indicator dye)

• If bacteria ferment mannitol, → produces acid → phenol red turns yellow

• If bacteria don't ferment mannitol → no acid → phenol red stays pink/red

EMB agar

Purpose

• To isolate Gram-negative bacteria and differentiate between lactose fermenters and non-fermenters

• Both selective and differential media
  
  

How it Works (Selective):

• Contains eosin and methylene blue dyes

• These dyes inhibit Gram-positive bacteria

• Only Gram-negative bacteria can grow
  
  

How it Works (Differential):

• Contains lactose (a sugar)

• If bacteria ferment lactose rapidly → produces acid → reacts with eosin/methylene blue → metallic green sheen

• If bacteria ferment lactose slowly → pink/dark colonies

• If bacteria don't ferment lactose → colorless/translucent colonies