Dilutions, Plate Counts, and Spread Plating

What is the dilution equation?

dilution = x / (x + y)

In the dilution formula, x / (x + y), what does x represent?

The substance that is being diluted

In the dilution formula, x / (x + y), what does y represent?

The substance that is being used to dilute

Serial dilution

Calculates the total amount a substance has been diluted over multiple passages

In microbiology, what can a serial dilution calculate what?

The size of a bacterial population

What is the first step in calculating a serial dilution?

Calculate each dilution being performed

What is the second step in calculating a serial dilution?

Multiply the two numerators together and multiply the two denominators together

What is the third step (and final step) in calculating a serial dilution?

Record the total dilution and continue to the next tube

Colony forming units (CFU)

The number of colonies

What assumption do microbiologists make when using Colony Forming Units?

1 cell gives rise to one colony

What is the most statistically accurate Colony Forming Unit value on any given plate?

30 - 300 CFU

A student counts 10 growing microbe colonies growing on their petri dish. What should the student be concerned about?

Their count may not be a true representation of their sample

A student counts 310 growing microbe colonies growing on their petri dish. What should the student be concerned about?

They likely made errors while counting their colonies

One error that can be made while counting Colony Forming Units is counting a colony _____

more than once

One error that can be made while measuring Colony Forming Units is counting two colonies as _____

one

To find the dilution factor, count how many ________ are being performed.

dilutions

To find CFU/mL, what equation is used?

The # of counted colonies x Dilution Factor

What is the purpose of a spread plate?

Provides an even lawn of microbial growth on an agar plate

A microbiologist can use a spread plate to test how effective ________ are.

cleansers

A microbiologist can use a spread plate to test the ______ of a microbe.

antibiotic resistance

A student attempts to make a spread plate. All the steps are correctly followed, but the student uses a pipette with a soiled tip. What will happen?

A variety of microbes that aren’t supposed to be studied may grow on the plate

A student attempts to make a spread plate. All the steps are correctly followed, but the student forgets to flame the spreader tool. What will happen?

Microbe growth might not occur