Question

A sample of ocean water is tested for its bacterial content in a plate count assay. A one-milliliter sample of the water is diluted in a 1:10 dilution series. 0.1 milliliter of the second dilution tube is plated in a pour plate. After incubation, the plate has 191 colonies, indicating that the original water sample contained______________

Answer 1

191000 cells per millilitre

Step-by-step explanation:

After incubation the plate has 191 colonies. Since each colony grows from a single cell, the plate had 191 cells.

Now only 0.1 millilitre was plated from 1 millilitre of sample hence number of cells in 1 millilitre of sample is 10 * 191 = 1910 cells.

It has been given that this sample is taken from second dilution tube of 1:10 dilution series. So total dilution here has been of 100 times. Hence, original water sample contained 1910 * 100 = 191000 cells per millilitre.