Lab 3: Microscopic Measurements and Pure Culture Techniques

1 stage division from a stage micrometer is equivalent to how many microns?

1SD = 10 microns

Where is the ocular micrometer located?

in the eye piece of the microscope

What is a stage micrometer and its purpose?

it is a ruler on a slide used to calibrate a microscopes ocular micrometer

What is a simple stain?

Simple stain refers to the use of only 1 dye to stain a slide

Name popular dyes used to stain slide

Methylene blue

crystal violet

safranin

How does the dye attach to the cell membrane of bacterial cell walls?

The dye is positively charged, and the bacterial cell walls are negatively charged, so they dye is attracted to the cell wall and sticks

What is the purpose of heat-fixing a slide?

Heat-fixing prevents the microbe from being washed off the slide during the staining process

What are the most common bacterial shapes?

-Cocci

-Bacilli

-Spirilla

In microscope calibration, what information is needed to calculate the ocular division (OD)?

-stage divisions (SD)

-ocular divisions (OD)

What is the formula used to calculate the ocular division in microscope calibration?

xOD = xSD x 10 microns

At which objective are must bacteria be examined under?

Oil immersion (1000x)

Name 2 ways of determining a pure culture

-Streak plate technique

-Microscopy (staining)

Purpose of streak plating technique

streak bacteria onto a nutrient plate to separate out individual colonies

What is selective media?

Media that contains substances that inhibit growth of unwanted microorganisms while allowing others to grow

What is differential media?

Media that permits a visual distinction between different types of bacteria (i.e. color change)

What is hemolytic bacteria?

bacteria that have the ability to lyse red blood cells

Beta hemolysis is ?

complete breakdown of RBC’s

Alpha hemolysis is?

partial breakdown of RBC’s

Gamma hemolysis is?

no breakdown of RBC’s

Which type of media are EMB, MacConkey, and Mannitol salt?

All are selective and differential medias

What type of medium is blood agar?

differential medium

Why is EMB a selective medium?

it inhibits the growth of gram-positive organisms

Why is EMB a differential medium?

-EMB contains lactose so organisms that can ferment lactose produce acid that cause a color change in the colony

-strong acid producers = metallic green sheen

-weaker acid producers = pinkish-purple colonies

What makes MacConkey a selective medium?

-selective for gram-negative bacteria

-bile salts and crystal inhibit growth of gram-positive bacteria

What makes MacConkey a differential medium?

-The lactose in the medium leads to color change if pH changes due to fermentation of the lactose, which creates acid

What makes Mannitol salt a selective medium?

-very few organisms can withstand high concentrations of salt

-selective for salt-tolerant bacteria

What make Mannitol salt a differential medium?

-the medium can change color depending on the pH, that is if acids are produced

-medium changes from red to yellow