Final answer:
The concentration of viable cells in the original culture is 5 million cells per mL.
Step-by-step explanation:
The concentration of viable cells in the original culture can be calculated using the number of colonies observed on the plate and the dilution factor. In this case, 100 microlitres of a 0.001 dilution of the culture was plated and 50 colonies were observed. The dilution factor for this plate is 10,000 (1:10,000), meaning that the original concentration of viable cells is 5 million cells per mL.