Question

A microbiology student was carrying out an independent research project in which he needed to know the number of infectious phage particles added during a particular step in an experimental protocol. In preparation for the experiment, he passed a crude mixture of phage plus infected bacteria through a 0.45 micrometer filter and collected the filtrate (portrayed below in the green-capped Eppendorf tube): 1:10 dilution steps (0.1 mL added to 0.9 mL at each step) filtrate 0.1 mL 0.1 ml 0.1 mL + host + STA P-200 plate X plate y plate z volume transfer to experiment

I got half credit for this question and not sure what I got wrong. Please do the math and explain.

Answer 1

The volume transferred by the student for the experiement he intended to do is 150 uL, as evident from the P200 micropipette picture.

The number of plaques in plate X = 244

Here, plate X is the second dilution of the original culture. The dilution used here is :

1 /10 * 1 / 10 = 1 / 100 or 10-2 dilution, and so the dilution factor used to calculate PFU in the original culture would be 102 or 100.

The volume plated to get 244 colonies = 0.1 mL

The concentration in the original culture can be calculated using the formula:

PFU / mL = No. of plaques * DF / volume plated

where DF is the dilution factor.

PFU / mL = 244 * 100 / 0.1 = 24400 / 0.1 = 244000 = 2.44 * 105.  

Therefore, the PFU / mL in the original culture is 2.44 * 105, not 2.44*106.

The student used 150 uL or 0.15 mL for the experiment, so the plaque concentration can be calculated from the above value of PFU /mL:

2.44 * 105 * 0.15 = 0.366 * 105 = 3.66 * 104 PFU/ 150 uL.

Therefore, the PFU used by the student for the experiment is 3.66 * 104/150 uL .

To represent values in scientific notation, the exponents should be represented as the number of decimals. In the case of 244000, it is represented as 2.44 * 10^5, as the number of decimals is 5 after the whole number 2.

In the case of the value obtained for 150 uL, 0.366 * 10^5, we are shifting the decimal point making 3 the whole number here, and so the exponent gets reduced to 10^4.