A water sample is examined for coliform bacteria using a serial‑dilution method. 1 mL of the original sample is diluted 1 : 10 three times, giving tubes A (undiluted), B ($10^{-1}$), C ($10^{-2}$) and D ($10^{-3}$). From each tube, 0.1 mL is spread on an agar plate. After incubation the colony counts are: A - too numerous to count, B - 150 colonies, C - 12 colonies, D - no colonies.

(a) Estimate the concentration of bacteria (colonies mL⁻¹) in the original water sample.

(b) A disinfectant is then applied that kills 99.5 % of the bacteria. If a plate is prepared from 0.1 mL of the treated, undiluted sample, how many colonies would you expect to count (round to the nearest whole number)?

Question

A water sample is examined for coliform bacteria using a serial‑dilution method. 1 mL of the original sample is diluted 1 : 10 three times, giving tubes A (undiluted), B ($10^{-1}$), C ($10^{-2}$) and D ($10^{-3}$). From each tube, 0.1 mL is spread on an agar plate. After incubation the colony counts are: A - too numerous to count, B - 150 colonies, C - 12 colonies, D - no colonies.

(a) Estimate the concentration of bacteria (colonies mL⁻¹) in the original water sample.

(b) A disinfectant is then applied that kills 99.5 % of the bacteria. If a plate is prepared from 0.1 mL of the treated, undiluted sample, how many colonies would you expect to count (round to the nearest whole number)?

Accepted Answer

15 000 colonies mL⁻¹; 8 colonies

Igcse Model Questions Vol1 2021 Biology Question 36

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