a Describe how you can estimate the number of bacteria in a

a) Describe how you can estimate the number of bacteria in a sample by plating.

b) Does this technique work for all bacteria? Why or why not?

c) Why does this method tend to underestimate the number of bacteria in your sample?

Explain how you might test for (a) the presence polyphosphate accumulating bacteria, and (b) the activity of these bacteria in a wastewater treatment system.

Solution

Bacterial Cell plating

To quantify the number of cells in a culture, the cells can be simply plated on a petridish with growth medium. If the cells are efficiently distributed on the plates it can be generally assumed that each cell will give rise to a single colony or colony formin unit (CFU). The colonies then can be counted and based on the known volume of culture that was spread on the plate, teh cell concnetration can be calculated.

Procedure: Transfer 0.1 mL from the bacterial culture into tube1. Tube1 which contains 9.9 mL of Diluting Fluid (DF). Note that 0.1 mL + 9.9 mL = 10 mL. Therefore, Tube 1 contains a 100-fold dilution of the original liquid culture. Tube 2 is handled exactly the same way. For tube 3, 0.1 mL was diluted into 1.9 mL making this a 20 fold dilution. Notice I plated 0.1 mL from Tube3 which is a 10 fold factor. Suppose I count 10⁴ bacteria on the plant. Then, the calculation of titer of the original bacteria culture is 100 x 100 x 20 x 10 x 104= 2.08 x 10⁸ cells/mL.

2. This technique does not work for all bacterial cells. The main disadvantages are: