High School Reading Grades 11 12 Quiz
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Procedure: Serial Dilution for Bacterial Enumeration
1. Label seven sterile tubes 10⁻¹ through 10⁻⁷. Aseptically dispense 9.0 mL of sterile saline into each tube.
2. Vortex the original bacterial culture. Transfer 1.0 mL into the 10⁻¹ tube and vortex thoroughly. This produces a 1:10 dilution of the original.
3. Using a fresh sterile pipette tip, transfer 1.0 mL from 10⁻¹ into 10⁻². Vortex. Continue serially through 10⁻⁷, changing tips at each step.
4. From each of the last four tubes (10⁻⁴ through 10⁻⁷), pipette 0.1 mL onto a labeled nutrient-agar plate and spread evenly.
5. Invert plates and incubate at 37 °C for 24 hours.
6. Count only plates containing between 30 and 300 colonies. Plates outside that range are statistically unreliable.
7. Compute colony-forming units per mL of original culture as:
CFU/mL=dilution factor×volume plated (mL)colonies counted.
A microbiologist counts 240 colonies on the 10⁻⁶ plate (all other plates being outside the countable range). Applying step 7 precisely, what is the correctly computed CFU/mL in the original culture?
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