Based on the data above calculate the concentration of bacte

Based on the data above calculate the concentration of bacteria (cells/mL) in Tube 3D. You are given a culture that has 1 times 10^6 CFU/ml. Write out a dilution scheme with a final volume of 1 ml that will allow you to obtain 75 colonies on a plate. Assume that you\'ll be plating 100 uL of the final volume on the plate, as done in your experiment. This is primarily a mathematical exercise, and you should recognize that if you were actually doing this experiment you would not be able to get exactly the number of colonies you want.

Solution

ii) This part of the question cannot be answered without the associated data.

iii) It is difficult for me to read properly in the given image whether the given value is 1×10⁵ CFU/mL* or 1×10⁶ CFU/mL. I will solve the question assuming the value to be 1×10⁶ CFU/mL.

So, the culture has 1×10⁶ CFU/mL. We need it to be diluted to give 75 colonies per 100 L (i.e. 7.5×10² colonies per mL). Hence, the culture must be diluted to (7.5×10²)/(1×10⁶) = 0.00075* (i.e. 1000×3/4) times.

So, first we take 1 L of the culture and add 999 L of diluent (normal saline, sterile media, etc.) to it, and mix properly [1:(1+999) = 1:1000 dilution]*. Then, we take 750 L of this diluted suspension, add 250 L of diluent to it and mix it properly [750:(750+250) = 3:4 dilution]. This gives 1 mL of inoculum, which should, theoretically, yield 75 colonies per 100 L upon plating.

* If the culture has 1×10⁵ CFU/mL, it would need to be diluted 0.0075 times, and in the first step of serial dilution, you may use 10 L of culture and 990 L of diluent to get [10:(10+990) = 1:100 dilution]. Rest of the calculations would remain the same.