What is a colony forming unit?

Synbiosis, a division of Synoptics, is a stakeholder in the colony-counter market. It offers a range of devices that can help users measure inhibition zones and spiral plates. The primary use of Synbiosis products is the simple and rapid counting of colony-forming units.

Creating a colony-forming unit commonly starts with adding around 15 ml of molten agar to around 1 ml of bacterial culture in a lidded petri dish, which is then swirled gently to mix the substances thoroughly in a technique called the pour plate method.

When the plate has solidified, it is inverted and incubated at around 37 ⁰C for between 24 and 48 hours. Although this is just one method for culturing bacteria, colonies will form on the plate after the elapsed time.

Various microbial organisms, including mold, bacteria, and fungi, can form colonies. Each colony starts as a single bacterium. The single bacteria will divide enough to become visible to the human eye as a colony by leaving a bacterium to incubate in ideal conditions for at least a day.

The colony is a clonal population descended from a single viable organism known as one colony-forming unit.

Significantly, only viable colonies will grow on such plates – any non-dividing or dead bacteria will not be able to form colonies. As such, only viable colonies will be tallied in the final count.

Automated colony counters

Traditionally, viable colonies had to be counted by hand for each plate, and the total was written on the petri dish with a marker pen. This process is time-consuming and costly. However, the Synbiosis automated colony counter uses bespoke imaging software to detect the number of colony-forming units on a plate within just a few seconds.

The automated machine can calculate the total with high accuracy regardless of the colony-forming units' size, shape, color, or morphology. The machine’s speed and review function means the user can quickly add or remove colonies at will, saving time and allowing users to focus on results.

What is a colony forming unit?

Image Credit: Synoptics Ltd

At the start of the process, the sample is diluted culture, meaning that only a small amount is used before it is mixed with agar. A common way to report the number of viable colonies is to express the result in colony-forming units per ml.

The sample begins as a diluted culture at the start of the process, meaning only a tiny portion is used before being mixed with the agar in the pour plate. A common way to report the number of viable colonies in a sample is to express the result in CFU per ml.

This method works the final count on the plate backward and factors in dilutions and the amount used on the plate, giving a value of how many viable colonies should be expected in the sample. A simple formula to calculate this is as follows:

Colony forming units / ml = (number of colonies x dilution factor) / volume of culture on a plate

For example, a starting culture contains 10 ml of the sample; 1 ml of this sample is diluted with 9 ml of diluent. From this dilution, 1 ml is spread on the plate. After incubation, the colony counter finds that 38 viable colonies have grown. We can then use the formula to see that:

Colony forming units / ml = (38 x 10) / 1

Colony forming units / ml = 380

What is a colony forming unit?

Image Credit: Synoptics Ltd

Our reports, when combined with the ProtoCOL and other Synbiosis colony counters, can produce a range of data. These include dilution factors, colony name, mean count per ml, plate area, count per frame, and many more customizable possibilities. We also offer a statistical package with countless options for data analysis.

About Synoptics Ltd

Synoptics is a prominent Cambridge-based company that has been at the forefront of manufacturing scientific-grade digital imaging systems, water purification systems, and vacuum ovens for over 30 years. The company is comprised of three divisions: Syngene, Synbiosis, and Fistreem, each specializing in specific areas of expertise. With a strong commitment to innovation and quality, Synoptics has established itself as a trusted provider of cutting-edge solutions in the scientific research and laboratory equipment industry.


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Last updated: Jun 20, 2024 at 4:34 AM

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