Question

1. You are given the task to quantify the bacteria in your soil sample. You do a 10-fold serial dilution of your soil sample in sterile water and plate 1/10 of each subsequent (serial) dilution in LB agar plates with cycloheximide – an antifungal and eukaryotic cell inhibitor. If your 10–6 (ten to the minus sixth) dilution plate contains 97 bacterial colonies after two days of incubation, how many bacteria can you estimate were present in your initial soil sample (1 g of soil)? Show your calculation and briefly describe each step.

Answer 1

Answer:

A serial dilution and the following plating experiment is done in order to determine the actual amount of bacteria/microbes in a specific volume of soil sample. As a standard test, a set volume of the lower dilution is obtained and positioned on a median plate and permitted to increase for the needed amount of time. The amount of colonies is evaluated and thus the overall Colony Forming Units (CFU) is determined by unit volume of the sample plated and therefore the volume of soil sample employed.

Therefore,

The calculation is done using:

CFU/ml = Number of colonies appeared × dilution factor / volume plated

Given,

Number of colonies appeared = 97

dilution factor = 10^(-6)

volume plated = 1/10 = 0.1

This will help us calculate it as:

CFU/ml = 97 * 10^(-6) / 0.1

= 97 * 10^(-7) CFU/ml

This CFU/ml helps determine the amount of bacterial colonies per unit volume of sample plated.

Given,

The original sample as 1g or 1000 mg of soil in its total volume.

Therefore,

The number of bacteria can be calculated using:

Amount of bacteria in original sample

= 97 * 10^(-7) CFU/ml × 1/ 1000 mg

= 9.7 * 10^(-3) CFU/mg