An E coli culture has a concentration of 8.5 times 10^8 cells/ml, the culture wa

Question

An E coli culture has a concentration of 8.5 times 10^8 cells/ml, the culture was diluted into the following serial dilutions: 1/100 rightarrow 1/100 rightarrow 1/10 rightarrow 1/8 The total volume of each tube was 1ml, from the last tube 100 ul were used to plate the bacteria. How many CFU do we expect to count in the plate? A laboratory hospital is trying to know a culture concentration. The culture was diluted a thousand times prior loading it to the hemocytometer. They got the next counts using the big squares from the hemocytometer 80, 72, 56, 95 What is the concentration of the initial culture? What is the concentration of the culture loaded in the h Propose a dilution series to plate 100 and 1000 colonies

Explanation / Answer

1.a

A Colony forming unit (cfu) refers to the rise of a bacterial colony from a single bacterial cell. Bacteria reproduce asexually ( by budding or sporulation) to give rise to daughter cells. The average time take by an bacterial cell to reproduce can be as short as 15 to 20 minutes.

Let us try to break down the problem by analysing the number of cells in each tube after each dilution -

Interpretation: The final tube contains 1 * 103 cells/mL. If we take 100uL of this solution, it will contain 100 cells. Each cell has the potential to form one colony.

Therefore, the potential number of CFU's that can be formed are 100.

Calculations:

For Dilution 1: Number of cells in original tube = 8 * 108 cells/ mL
In order to perform a 1: 100 dilution, we have to take 10uL from the original tube and dispense it into the second tube.
If 1000uL of original solution contains 8 * 108 cells, the the number of cells in 10uL is 8 * 106 cells.

In a similar manner, the dilutions for the remaining tubes have been calculated by substituting the original number of cells and the amount of solution that is dispensed into the proceeding test tube.

Test Tube 1 Test Tube 2 Test Tube 3 Test Tube 4 Test Tube 5 Dilution Original (1/100) (1/100) (1/10) (1/8) Amount of sample dispensed from previous tube NA 10uL 10uL 100uL 125uL Number of bacterial cells per test tube 8 * 108 8 * 106 8 * 104 8 * 103 1 * 103

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