1. What is a colony? How many ways are there to count colonies?
Colony of a bacterial species is defined as a cluster, visible to the naked eye, of the biomass of those bacteria when they grow on the surface of a solid substrate.
What is a colony
Colony counting unit
What is CFU
Colony forming unit CFU (abbreviation from the words Colony form units) is the unit used to estimate the number of bacteria or fungal cells in a given test sample.
When microorganisms in a test sample are checked by colony counting on petri dishes or ready-made medium plates (for example Compact Dry plates), the unit expressing colonies is usually CFU/mL, CFU/g, CFU/25g.
- The units CFU/g , CFU/25g are usually used for solid test samples (food, food ingredients…)
- The unit CFU/mL is used for test samples in liquid form such as aquaculture water samples, fruit juice…
What is MPN
MPN is the abbreviation of Most Probable Number, meaning the most probable number. MPN is usually used to express the number of bacteria in a given volume of a liquid sample.
What is the difference between CFU and MPN
- CFU is the colony forming unit present in a given test sample. CFU is usually calculated from the pour plate method or spread plating, whereas MPN is usually calculated by the fermentation method. For CFU, bacteria grow on a solid medium (for example agar), then the colonies are counted manually by eye or by modern application.
- For MPN determination, samples are cultured in a liquid medium (for example fermentation). The results do not use a counting method but comparison with a probability table.
- MPN and CFU are equivalent. Both are recognized by many regulatory and scientific organizations worldwide, including the United States Environmental Protection Agency (EPA) and the International Organization for Standardization (ISO).
Ways to count colonies
Manual colony counting method
With the manual colony counting, each colony spot represents the target bacterial species and is counted once. To make the counting process easier to control, petri dishes are usually placed on a grid. Then count the colonies in each square and mark the counted colonies on the underside of the petri dish. To ensure accuracy, each test sample must be cultured on at least 3 plates and the colony count suitable for manual counting is 30 to 300 colonies. Plates with too many or too few colonies are unsuitable and require dilution and re-culturing of the sample.
Automated colony counting method
Manual colony counting usually takes a lot of time and is subject to the observer's subjective error. To improve this situation, laboratories nowadays are equipped with additional automated colony counters.
The basic principle of an automated colony counter is a colony counter that takes a picture of the plate (petri or compact dry plate), and the application then uses an algorithm to separate and count the colonies on the plate. The algorithm can also have difficulty distinguishing colonies when there are too many overlapping colonies. In this case, an application with effective and powerful analysis capability is needed. This is also a challenge for experts developing this application.
Bactlab colony counting application
In order to reduce the time and errors when reading results manually. Together with the ready-made medium plate solution CompactDry™. Nissui has developed the bactLAB colony counting application on a smartphone, simple and effective.
This application software replaces the colony counters bulky ones of the past. You only need to download this app on CH PLay (on the Android operating system) or on Apple's iOS operating system with the keyword "@bactlab. Creating an account on the bactLAB app is simple by email and using it.Operating the colony countingbactLAB is also extremely simple in 3 steps with a smartphone
Take a photo -> Upload the image->The software will automatically count and calculate the number of colonies.
Colonies of some microorganisms cultured on CompactDry plates
BactLAB software, intelligent colony counting application
Reference document for the BactLAB application can be found at:
https://www.nissui-pharm.co.jp/english/pdf/products/global/bactlab_/user-manual_BactLAB_.pdf
