Colony Forming Unit (CFU) in Microbiology


In microbiology, a CFU stands for colony-forming units. It is a unit that we use for estimating the number of viable bacteria or the fungal cells in a sample.

Counting with the CFU colony-forming units needs culturing the microbes. It counts only the viable cells. It is opposite to the microscopic examination which counts each and every cell, either living or dead.

The visibility of a colony in a cell culture needs significant growth. When counting the colonies it is not certain if the colony arises from one cell or a cell group. Expressing the outcome as colony-forming units reflect this uncertainty.


Definition of CFU

The purpose of plate counting is to estimate the number of cells that are present based on their ability to give rise to the colonies.

It occurs under the specific conditions of the nutrient medium, time and temperature. However, one viable cell can absolutely give rise to a colony by replication. However, the solitary cells are the exception in our nature.

Most likely these are the progenitor of the colony was a mass of cells that deposits with each other. In addition, some bacteria grow in chains or clumps.

In many cases, the estimation of the microbial numbers by CFU will undercount the number of living cells that are present in a sample for these reasons.

This is because the counting of the CFU assumes that all the colonies are separate and founded by a viable microbial cell.

The plate count is linear for E. coli above the range of 30 to 300 CFU on a petri dish of standard size. Therefore, for ensuring that a sample will yield CFU in this range needs dilution of the sample and plating for various dilutions.

We use typically 10 fold dilutions. The CFU/Plate is read from a plate present in the linear range, and then the CFU/g (or CFU/mL) of the original is deduced through the mathematics. It factors in the amount that is plated in the dilution factor.

The Log Notation

Concentrations of the CFU (colony-forming units) are expressible by using logarithm notation, where the visible value is the base 10 logarithm of the concentration. In addition, this permits the log reduction of a decontamination process that we calculate as a simple subtraction.

Uses of CFU

We use CFU (colony-forming units) for quantifying the results in various biological planting. In addition to in the counting methods that include:

The method of pour plate where it suspends the sample in a petri dish using the molten agar. We cool it to approximately 40 to 45 degrees Celsius. After the nutrient agar solidifies, as a result, the plate is incubated.

The method of spread plate where the sample is spread over the surface of a nutrient agar plate and allows it to dry before the incubation for the counting.

The membrane filter method where it filters the sample through a membrane filter, as a result, the filter is present at the surface of a nutrient agar plate. Whereas, the surface area of various filters is less than that of a standard petri dish.

However, with the help of the techniques that need the use of an agar plate, no fluid solution is useful. This is because we cannot identify the purity of the specimen. Further, it is impossible to count the cells individually in the liquid.

Solved Question For You

Ques. How many cells are present per millilitre in bacterial culture is capable of making the culture turbid?

a) 1 cell.
b) 1000 cells.
c) 1 lakh cells.
d) 107-108 cells.

Ans. d) 107-108 cells.

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