Exercise 6-Skin bacteria: Differential and Selective Media

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Name a typical resident organism of the skin.

Staphylococcus epidermis

Where are gram-negative bacilli usually found on the skin?

In moister areas, such as the axillae and inguinal folds

Name some transient skin flora

Enterococcus pyogenes, coliform organisms, Proteus and diptheriods (non-pathogenic Corynebacteria)

Systemic infectious following severe burns are often caused by which microorganisms?

Staph. Aureus, Pseudomonas aeruginosa and Herellea vaginicola (resident)

Differential media

Media used to distinguish between different bacteria based on appearance of the colonies on the plate.

Selective media

Selects out members of a mixed bacterial inoculum by permitting certain organisms to grow while delaying or completely inhibiting the growth of other bacteria

What makes Mannitol-Salt Agar selective?

High salt concentration that inhibits the grown of organisms other than Staphylococci

What makes Mannitol-Salt Agar differential?

Mannitol fermentation (by Staph. Aureus) turns agar yellow

Should Staph. Epidermis turn Mannitol-Salt Agar yellow?

No

How is Tellurite-Glycine selective?

Potassium tellurite, lithium chloride and glycine make it selective for Staph. aureus

How is Tellurite-Glycine differential?

The reduction of tellurite to black reduction compounds (Staph. Aureus black, non-pathogenic coagulase-negative species grey, proteus brown)

What happens if you leave a tellurite-glycine plate in the incubator for more than 24 hours?

Other organisms may give rise to black colonies

What is Brain Heart Infusion Agar?

A general-purpose medium suitable for the cultivation of a wide variety of microorganisms, including bacteria, yeasts and molds

What should happen in the hand washing experiment?

There should be fewer colonies after more washes have been done

Why is there a sodium thiosulfate tablet in the water collection container?

To neutralize any chlorine in the sample

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