Question

1) If you suspect your culture of bacteria has 125 x 106 cells per mL, what would you want the final dilution to be in order to end up with 50 cfu on a plate?

2) A 0.1 dilution is performed on a culture of bacteria in order to perform viable plate counts. From the dilution, *0.1 mL* of solution is plated on solid media, and 154 colony forming units grow on the plate. How many bacteria are in a single mL of the original culture?

3) How many microliters of original sample are required to produce a final dilution of 10^-3 in a total volume of 5 mL? 1 microliter is 10^-6 L or 10^-3 mL.

4) There are so many microbes in a single mL of culture, it is very difficult to perform one dilution to produce countable cells. Microbiologists need to perform a dilution series, where multiple dilutions are performed in sequence to arrive at the correct dilution.

Dilutions are cumulative. Multiply the series of dilutions together to find the final dilution value.

If 6 serial dilutions are performed, each with a dilution of 0.01, what is the cumulative dilution?

Answer 1

Answer:

1) If you suspect your culture of bacteria has 125 x 10⁶ cells per mL, what would you want the final dilution to be in order to end up with 50 cfu on a plate?

• Number of bacteria in culture =  125 x 10⁶ cells per mL
• Required number of CFU on a plate = 50
• If the volume of culture used on plate is example 100ul, then dilution factor =  250000 = 2.5 x 10⁵
• If the volume of culture used on plate is example 1ml or 1000ul, then Fold dilution required to achieve 50 CFU on plate (with 1 ml culture used) = Final CFU / number of bacteria in culture = 125 x 10⁶ / 50 = 2500000 = 2.5 x 10⁶ (Fold dilution)

2) A 0.1 dilution is performed on a culture of bacteria in order to perform viable plate counts. From the dilution, *0.1 mL* of solution is plated on solid media, and 154 colony forming units grow on the plate. How many bacteria are in a single mL of the original culture?

• Initial dilution = 0.1 = 1/10
• Dilution factor (fold dilution) = 10x
• Volume taken from first dilution = 0.1ml
• Colony forming units obtained = 154 per 0.1ml = 154 CFU / 100ul
• Number of bacteria in original 1ml of culture = 154*10 CFU/ml x DF = 1540 x 10 = 15400 cells
• = 1.54 x 10⁴ cells

3) How many microliters of original sample are required to produce a final dilution of 10^-3 in a total volume of 5 mL? 1 microliter is 10^-6 L or 10^-3 mL.

• Final dilution required = 10^-3
• Fold dilution = 1000x
• For this 5 ul (microliter) of the original sample will be required to be added to total volume of 5ml
• Total volume = 5ml = 5000ul
• Add 5ul sample + 4995ul buffer or solvent
• Total volume = 5000ul
• Dilution = 5/5000 = 1/1000 = 10^-3
• Therefore, 5ul (microliter) of the original sample is required.

4) There are so many microbes in a single mL of culture, it is very difficult to perform one dilution to produce countable cells. Microbiologists need to perform a dilution series, where multiple dilutions are performed in sequence to arrive at the correct dilution. Dilutions are cumulative. Multiply the series of dilutions together to find the final dilution value. If 6 serial dilutions are performed, each with a dilution of 0.01, what is the cumulative dilution?

• Six dilutions are performed with each dilution = 0.01
• Cumulative dilution = (0.01)*(0.01)*(0.01)*(0.01)*(0.01)*(0.01) = (0.01)⁶ = 10^-12