Selective vs differintial media

Selective media

Contains certain ingredients that inhibit the growth of unwanted microorganisms while allowing the growth of target pathogens. (Example mannitol salt agar)

Differential media

Media that allows multiple organisms to growth but contains indicators that show visible differences between organisms based on biochemical reactions (Ex, MacConkey agar)

What type of media is Mannitol Salt Agar?

Selective and differential media

What makes Mannitol Salt Agar selective

The high concentration of salt (7.5%) inhibits most bacteria but allow salt tolerant bacteria to grow (Staphlococcus species)

What makes Mannitol Salt Agar differential?

It contains Mannitol sugar and a phenol red pH indicator to detect mannitol fermentation

What does a positive mannitol fermentation result look like on MSA?

The agar turns yellow because acid is produced from fermentation, lowering pH

What does a negative mannitol fermentation result look like on MSA?

The agar remains red/pink becase no acid is produced

What organism is commonly identified using MSA?

S. Aureus

What organism is commonly mannitol negative on MSA?

S.epidermis

What type of media is EMB agar?

Selective and differential media used to identify gram negative bacteria

What makes EMB agar selective?

The dyes eosin and methylene blue inhibit most gram positive bacteria

What makes EMB agar differential?

It differentiates bacteria based on their ability to ferment lactose

What does a positive lactose fermentation result look like on EMB agar?

Dark purple/black colonies, often with a metallic green sheen (strong fermenters like e.coli)

What does a negative lactose fermentation result look like on EMB agar?

Colorless or light colonies because no acid is produced from lactose fermentation

What does EMB agar test for?

Enteric bacteria like e.coli

What type of media is MacConkey agar?

Selective and differential media used to isolate and identify gram negative enteric bacteria

What makes MacConkey agar selective?

Bile salts and crystal violet inhibit most gram positive bacteria

What makes MacConkey agar differential?

It differentiates bacteria based on their ability to ferment lactose

What does a positive lactose fermentation result look like on MacConkey agar?

Pink/Red colonies

What does a negative lactose fermentation result look like on MacConkey agar?

Colorless or pale colonies

MacConkey selects for?

Gram negative bacteria (e.coli turns pink, salmonella is colorless)

What type of media is blood agar

Enriched and differential media used to grow many types of bacteria and differentiate them by hemolysis

What makes blood agar enriched?

It contains red blood cells (usually sheep blood) that provide extra nutrients for bacterial growth

What are the types of hemolysis on blood agar?

Alpha- partial breakdown

Beta- Complete breakdown

Gamma- No hemolysis

What does beta hemolysis indicate on blood agar?

Complete destruction of RBC’s creating a clear zone around bacterial growth

What bacteria are commonly identified using blood agar?

S.pyogenes and S.pneumoniae