Microbiology Growth Media

Blood Agar Plate

• General purpose for variety of organisms
• - contains RBCs which provide nutrition

Differential pattern of hemolysis

Alpha-Hemolysis

• Microbe produces H2O2
• Leads to oxidation of Hb in RBCs
• Hb --> methemoglobinperoxide (green color)

Beta-Hemolysis

• Microbe completely digests RBCs, clearing the agar
• Organisms produce hemolysin which lyses RBCs

Gamma-Hemolysis

• No hemolysis of the RBCs in agar
• Microbes do not produce enzymes to digest RBCs

Chocolate Agar

• Blood agar with RBCs pre-lysed by heating
• - provides easier access to nutrients

Chocolate Agar

• Good for microbes with hard to meet needs that cannot hemolyze RBCs, like
• - Haemophilus sp.
• - Neisseria sp.

Thayer-Martin: selective version with added antibiotics to detect N. ghonorrhoeae

Mannitol-Salt Agar

• A selective media due to high [NaCl]
• Pink color is a pH indicator (turns yellow on pH drop)
• Mannitol is diagnostic nutrient

High [NaCl] selective for Staphylococcus species

- S. aureus: ferments mannitol, decreasing pH and changes agar from pink to yellow

- other Staphylococci: do not ferment mannitol, leaving agar pink; still capable of growth

MacConkey Agar

• Contains bile salts, crystal violet, and neutral red dye
• Lactose is sole carbohydrate
• Also contains peptones (source of amino acids, carbons)

Bile salts and crystal violet inhibit Gram+

• Selects for most Gram Neg bacteria
• - Fastidious G- (e.g. Neisseria, Hemophilus) do not grow well

• Color indicator
• - Lactose-fermenters pick up dye, turn pink or red
• - Non-fermenters remain clear

• Eosin-Methylene Blue Agar
• Gram Neg selective media like MacConkey, but with different dyes
• - Uses eosin and methylene blue

• Lactose Fermenters: turn blue/black, sometimes with a metallic sheen
• Non-fermenters remain clear

Bile-Esculin Agar

• A medium that contains high [bile salt] and Fe
• - Inhibits Gram+ organisms
• - Selects for Enterococcus and Group D Streptococci

Both hydrolyze esculin, which reacts with the Fe to form the black precipitate

Hektoen-Enteric Agar

• Contains bile salts and indicator dyes like MacConkey
• Contains lactose, sucrose, and salicin

Hektoen-Enteric Agar

Selective for Gram Negs (bile and dye inhibit G+)

• Differential indicators:
• - Sugar fermentors appear yellow-pink
• - Non-fermentors appear green or transparent
• - H₂S producers form black precipitate in the colony (e.g. Salmonella)

• Cetrimide Agar
• Cetrimide is a detergent that inhibits most bacteria
• - Selective and differential for Pseudomonas aeruginosa

• P. aeruginosa produces pyocyanin (blue-green) and fluorescein (yellow-green), which can be visualized

• A medium with potassium tellurite added to it.
• - inhibits Gram Neg and most upper respiratory flora

• It is selective and differential for Cornybacterium diptheriae
• - Colonies appear gray or black with a brown halo

Specialized agars selective and differential for various microbes

Usually differentiate specific species by change in agar color

e.g. Listeria monocytogenes, S. aureus, or phenotypes like MRSA, MSSA

A specialized media that selects for the growth of organisms with particular characteristics

A specialized media that can be used to differentiate particular organisms

Usually does this by color indicators; does not affect growth patterns (i.e. not inhibitory)

Aerobic: require O₂ for metabolism

Anaerobic: require reduced O₂ tension to grow, increased [CO₂]

Simple: organism grows under routine conditions

Fastidious: organism requires special conditions