lecture 3 study guide

why is a colony considered a pure culture?

a colony grows from one original cell, so it contains only one type of microorganism

what types of media can you grow colonies on?

liquid, semi-solid, solid media

what is liquid media used for?

growing bacteria throughout a liquid

what is semi-solid media used for?

motility tests

what is solid media used for?

growing and isolating colonies

is nutrient broth/nutrient agar complex or synthetic?

complex medium

which enriched medium did we discuss?

enriched medium contains extra nutrients for fastidious organisms

what is a fastidious organism?

a microorganism with special nutritional requirements

which media are used to culture anaerobic organisms?

reducing media, such as FTM

what is the role of reducing agents?

they remove or slow oxygen so anaerobic bacteria can grow

what are the two common fermentation products produced during sugar fermentation?

acid and gas

how is acid detected?

a pH indicator detects acid production by a color change

how is gas detected?

a Durham tube traps gas, showing gas production

what is the purpose of carbohydrate fermentation media?

to test whether an organism can ferment a specific sugar

what organism is MSA used to isolate?

Staphylococcus aureus

what is the selective agent in MSA?

7.5% salt

which organisms does MSA select for?

Staphylococcus

what property allows Staphylococcus to grow on MSA?

salt tolerance

what is the differential agent in MSA?

mannitol

which species does MSA differentiate?

S. aureus and S. epidermidis

how does MSA differentiate them? (2)

1) S. aureus ferments mannitol → yellow

2) S. epidermidis usually does not → stays pink

what property is MSA differentiation based on?

the ability to ferment mannitol

what are macconkey and EMB agar used to detect?

coliforms/E. coli-like bacteria from water or food

which bacteria can grow on macconkey and EMB?

gram-negative bacteria

which bacteria are inhibited?

most gram-positive bacteria

what sugar is the differential agent in macconkey and EMB?

lactose

what two types of bacteria are differentiated?

lactose-fermenting (lac+) and non-lactose-fermenting (lac−) bacteria

what color are lac+ bacteria on macconkey?

dark/red colonies

what color are lac− bacteria on macconkey?

white colonies

what is the color of coliforms on EMB? (2)

dark colonies, E. coli can have a metallic green sheen

what are basic dyes (cationic)?

positively charged dyes that stain negatively charged cells

what are acidic dyes (anionic)?

negatively charged dyes that usually stain the background

which dyes stain microbes better?

basic

which dyes are used in positive stains? (2)

basic dyes, such as crystal violet

which dyes are used in negative stains? (2)

acidic dyes, such as india ink