Colony-forming unit

The purpose of plate counting is to estimate the number of cells present based on their ability to give rise to colonies under specific conditions of temperature, time, and nutrient medium.

This is because the counting of CFU assumes that every colony is separate and founded by a single viable microbial cell.

[3] The plate count is linear for E. coli over the range of 30 to 300 CFU on a standard sized Petri dish.

An advantage to this method is that different microbial species may give rise to colonies that are clearly different from each other, both microscopically and macroscopically.

Alternatively it is possible to decrease the average number of cells per CFU in some cases by vortexing the sample before conducting the dilution.

Colony-forming units are used to quantify results in many microbiological plating and counting methods, including: However, with the techniques that require the use of an agar plate, no fluid solution can be used because the purity of the specimen cannot be unidentified and it is not possible to count the cells one by one in the liquid.

[16] In addition to software based on traditional desktop computers, apps for both Android and iOS devices are available for semi-automated and automated colony counting.

Due to the fact that researchers regularly manually count the cells with the assistance of a transmitted light, this error prone technique can have a significant effect on the calculated concentration in the main liquid medium when the cells are in low numbers.

However, a disadvantage to these automated systems is that it is extremely difficult to differentiate between the microorganisms with dust or scratches on blood agar plates because both the dust and scratches can create a highly diverse combination of shapes and appearances.

A dilution made with bacteria and peptoned water is placed in an Agar plate ( Agar plate count for food samples or Trypticase soy agar for clinic samples) and spread over the plate by tipping in the pattern shown.
A solution of bacteria at an unknown concentration is often serially diluted in order to obtain at least one plate with a countable number of bacteria. In this figure, the "x10" plate is suitable for counting.
The traditional way of enumerating CFUs with a "click-counter" and a pen. When the colonies are too numerous, it is common practice to count CFUs only on a fraction of the dish.
An automated colony counter using image processing.