4: Dilution Worksheet and Problems - Biology LibreTexts - Free Printable

I'll solve this step by step, analyzing the serial dilution process shown in the image.

First, look closely:

The image shows a serial dilution experiment where:
- We start with 1 mL of original culture
- We perform a series of 1/10 dilutions (adding 1 mL to 9 mL of diluent each time)
- After each dilution, we plate 1 mL onto agar plates
- The results show colony counts on each plate after incubation

The dilution factors are:
- 1st tube: 1/10 or 10⁻¹
- 2nd tube: 1/100 or 10⁻²
- 3rd tube: 1/1,000 or 10⁻³
- 4th tube: 1/10,000 or 10⁻⁴
- 5th tube: 1/100,000 or 10⁻⁵

The colony counts are:
- Plate 1 (10⁻¹): Too many to count
- Plate 2 (10⁻²): >500 colonies
- Plate 3 (10⁻³): 71 colonies
- Plate 4 (10⁻⁴): 6 colonies
- Plate 5 (10⁻⁵): 0 colonies

Next, find information:

To calculate the original concentration of bacteria in the culture, I need to use the plate that has a countable number of colonies (typically between 30-300 colonies). Plate 3 with 71 colonies is in this ideal range.

The formula for calculating CFU/mL (Colony Forming Units per milliliter) is:
CFU/mL = (Number of colonies) / (Volume plated × Dilution factor)

For plate 3:
- Number of colonies = 71
- Volume plated = 1 mL
- Dilution factor = 10⁻³ (or 1/1,000)

Then, review the findings:

Let me calculate the CFU/mL:

CFU/mL = 71 / (1 mL × 10⁻³)
CFU/mL = 71 / 0.001
CFU/mL = 71,000

This means there were 71,000 colony forming units per milliliter in the original culture.

I should also check if other plates give reasonable results:
- Plate 4 (10⁻⁴) has only 6 colonies, which is too low for accurate counting (below 30)
- Plate 2 (10⁻²) has >500 colonies, which is too high for accurate counting (above 300)

Therefore, plate 3 is the most appropriate for calculation.

Final Answer:

The original culture contains approximately 71,000 CFU/mL (Colony Forming Units per milliliter).

This was calculated using the plate with 71 colonies, which corresponds to the 10⁻³ dilution. Since 1 mL was plated from this dilution, we multiply the colony count by the reciprocal of the dilution factor (10³) to get the concentration in the original culture: 71 × 1,000 = 71,000 CFU/mL.