Neco Model Questions Vol1 2021 Biology Question 46

A water sample is tested for bacterial contamination. The procedure is as follows: 10\,mL of the sample is added to 90\,mL sterile water (first 1\!:\!10 dilution). From this mixture, 1\,mL is transferred to 9\,mL sterile water (second 1\!:\!10 dilution). Then, 0.1\,mL of the final dilution is filtered through a membrane and the filter is placed on nutrient agar. After incubation, 250 colonies are counted.\n\n(a) Calculate the original concentration of bacteria in the water sample in colony‑forming units per millilitre (CFU\,mL\(^{-1}\)).\n(b) If the laboratory needs a suspension containing exactly 1\times10^{3}\,CFU\,mL\(^{-1}\) for a downstream test, what overall dilution factor should be applied to the original sample (expressed as a single dilution, e.g., 1\!:\!X)?