Lab 9: Pure Culture and Pipetting Techniques

What do direct bacterial counts measure?

Live cells that form colonies (CFU)

What does CFU stand for?

Colony Forming Units

What is serial dilution used for?

To estimate the number of bacteria in a sample

What do indirect bacterial counts measure?

Cell presence or metabolic activity without counting individual cells

What are two quantitative plate counting methods?

Spread plate and pour plate

Which method was used in this lab?

Spread plate method

What happens in the spread plate method?

A sample is spread across solid agar and colonies are counted

Why is serial dilution done before plating?

To get a countable number of colonies

How much diluent is added to each tube?

900 µL

How much sample is added to start the dilution?

100 µL

What type of dilution is used in this lab?

10-fold serial dilution

How is the serial dilution continued?

Transfer 100 µL from one tube to the next

Which dilutions were plated in this lab?

10⁻², 10⁻⁴, and 10⁻⁶

How much sample is spread on each plate?

100 µL

What colony range is considered countable?

30-300 colonies

What does TFTC mean?

Too Few To Count (less than 30 colonies)

What does TNTC mean?

Too Numerous To Count (more than 300 colonies)

What formula is used to calculate bacterial concentration?

CFU/mL = colonies ÷ (dilution × amount plated)

What is a pure culture?

A culture containing only one species of microorganism

What is a colony?

A population of cells from a single cell or group of cells

What method is used to isolate pure cultures?

Streak plate method

What streaking technique was used?

Quadrant streak method

What must you do to the loop between streak sections?

Flame the loop

At what temperature are the plates incubated?

30°C