Lab Exercise 6 - Selective and Differential Media

general purpose media

media that support growth of a variety of organisms

good for maintaining cultures

3 examples of general purpose media

nutrient agar (NA)

tryptic soy agar (TSA)

brain heart infusion (BHI)

What does NA stand for? What type of medium is it?

nutrient agar

a general purpose medium

what does TSA stand for? What type of medium is it?

tryptic soy agar

a general purpose medium

what does BHI stand for? What type of medium is it?

brain heart infusion

a general purpose medium

selective media

media that use inhibitors to prevent growth of certain organisms (allowing only selective organisms to grow)

examples of selective media

phenyl ethyl agar (PEA)

eosin methylene blue

(EMB)

MacConkey agar

What does PEA stand for? What kind of medium is it?

phenyl ethyl agar

a selective medium

What does EMB stand for? What kind of medium is it?

eosin methylene blue

a selective medium

differential media

media that allow growth of many microbes, but have indicators to detect changes that have occurred

What are usual indicators in differential media?

dyes, reagents, blood cells, culture conditions

examples of differential media

EMB

blood agar

MacConkey agar

combination media

media that are both selective and differential

complex and defined media: What do they determine?

how nutrients are supplied

complex media

media in which the nutrient supply is undefined; exact chemical composition is not known

usually have nutrients supplied as extracts or digests from natural sources

examples of complex media

BHI

TSA

defined media

media in which the chemical composition for each component is known (nutrient supply is defined)

What kind of medium is PEA? General purpose, selective, differential, or combination?

selective only

What is the inhibitor in PEA?

phenylethyl alcohol

What does the inhibitor in PEA inhibit?

growth of Gram-negatives

What is PEA used for?

selecting for Gram-positive spp. in patient samples, e.g. Staphylococci, while getting rid of contaminants like E. coli

What kind of medium is BA? General purpose, selective, differential, or combination?

differential only

What is the indicator in BA?

red blood cells

5% sheep’s RBCs are added to a TSA base

What is BA useful for?

differentiating hemolysis patterns of microorganisms

different types of hemolysis detected by BA

beta

alpha

gamma

beta hemolysis

complete hemolysis detected by BA

“best” hemolysis

alpha hemolysis

partial/incomplete hemolysis detected by BA

“average” hemolysis

gamma hemolysis

no hemolysis detected by BA

“gross” hemolysis

What type of medium is MacConkey agar? General purpose, selective, differential, or combination?

combination: selective and differential

What is the indicator in MacConkey agar?

neutral red

What does the inhibitor in MacConkey agar differentiate?

differentiates lactose fermenters from non-fermenters

What are the inhibitors in MacConkey agar?

crystal violet and bile salts

What do the inhibitors in MacConkey agar inhibit?

growth of Gram positives

What is MacConkey agar used for?

used for identification of Gram-negative enterics, e.g. Enterobacteriaceae

What type of medium is EMB? General purpose, selective, differential, or combination?

combination: selective and differential

What are the inhibitors in EMB?

eosin

methylene blue

What do the inhibitors in EMB inhibit?

growth of Gram positives

What are the indicators for EMB?

eosin

methylene blue

What do the indicators in EMB indicate?

differentiate lactose fermenters from non-fermenters

What is EMB used for?

used to screen for coliforms, e.g. fecal coliforms

also used when testing for UTIs

motility media

media used to test the motility of an organism

What type of media are used for motility testing?

soft agar, with decreased agar concentration

dye indicators can also be used, but not necessarily (e.g. tetrazolium chloride)

How to test motility

inoculate medium with a single line stab

motile organisms swim out and spread away from the inoculation stab (causing turbidity)

non-motile organisms remain confined to the stab path

What kind of organisms are motile?

organisms with flagella