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Counter colony

Colony counting is a common method in clinical, food, dairy, and pharmaceutical microbiology to obtain a total microbial count or total viable count. Traditional manual methods for counting bacterial colonies (or colony-forming units) are labor-intensive and tedious processes that require high concentration levels. Colony counter is one of the laboratory equipment that provides an alternative sustainable solution to this problem by helping to count colonies quickly and accurately.

Biological methods often rely on accurate counting of bacterial colonies and cells. Colony counters are used to estimate the density of liquid culture microorganisms by counting individual colonies on an agar plate, slide, mini-gel, or petri dish.

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Colony counter application

Typical applications include the Ames test, the bacterial mutation assay, and the E. coli colony assay. Instruments come in a variety of sizes and formats, including units that can scan plates up to 300 x 300 mm and are optimized for use with UV light, white light, fluorescent, and/or green fluorescent protein colonies.

 Counting can be done manually, often with touch pressure and a digital counter, or it can be semi-automatic or fully automatic. With automated counters, improvements have focused on increasing precision and accuracy, such as the ability to detect smaller colonies in low-contrast media.

Types of counter colonies

A colony counter is an instrument used to count colonies of bacteria or other microorganisms growing on an agar plate. A variety of colony counters are available to aid in the rapid and accurate enumeration of bacterial and yeast colonies. Some of these colony counters are manual, while others are automated.

 Colony Counter manual counters

Manual/semi-automatic counters depend on the technician’s ability to see the colonies clearly and mark them with a special type of pen on the outer surface of the plate. The device keeps track of the number of such marked colonies. Although this method is somewhat helpful, it is still time-consuming, confusing, and error-prone.

The principle of the manual counter colony counter 

One model of the hand-held counter colony counter works by placing a Petri plate on a brightly lit electronic pressure pad and marking each colony by touching the plate with the tip of a pen. Touch pressure registers the count on the digital display. The pressure can be adjusted according to the need.

 This model helps to avoid mistakenly missing or double counting colonies. A Wolfhuegal graticule, dividing disc and center adapters for 50-90mm plates are also provided.

 There are some added features such as the possibility of a dark background for transparent colonies, optimal viewing of colonies with ambient light, without glare, and built-in facilities for counting multiple plates. The software generates an average number of colonies and changes using a background switch.

Colonies may be crowded and small, making counting difficult. Therefore, dividing the count into small squares and magnifying the colonies with the help of a magnifying glass mounted on a flexible arm makes the work somewhat easier. 

This model for counting colonies allows for a significantly low and time-consuming output. Also, when counted by more than one technician, the total number of colonies may vary.

Automatic counter colony

Bacteria Colony Counter automatically uses image processing algorithms such as gray scaling, thresholding, filtering, etc. to efficiently count colonies.

The principle of automatic counter colony

A fully automated colony counter collects images using digital image capture devices such as document scanners, digital cameras, webcams, charge-coupled device (CCD) or video equipment.

. Colony counting starts automatically by taking an image of the Petri dish with colonies. Then it digitizes the image using software packages. 

Finally, the last step is digital image processing using single/multi-threshold segmentation method to isolate and identify colonies. Automating the colony counting method helps save time.

 Those laboratories that do not have a colony counter can send the images to a laboratory with analysis software via the Internet. 

The contrast of objects and the transparency of their background are very different in an automated system. One of the following lighting methods has been chosen to increase visibility and accuracy in colony counting.

  • Transfer method: For normal high-contrast objects with a relatively transparent background.
  • Dark field method: For high-contrast objects with opaque backgrounds.
  • Reflection: for objects with low contrast and relatively transparent background.

A modern instrument with a 1.4 megapixel CCD can capture HD (high quality) color images. Likewise, a typical automatic colony counter consists of long-life red, green, and blue LEDs, which help obtain good color images with brilliant contrast without chromatic aberrations.

 Black, white, and transparent backgrounds are available for effective brightfield and darkfield exposure for the sample platform.

Both 55-150 mm round plate and 150 mm x 150 mm square plate can be used. It can measure colonies as small as 43 µm or regions with an accuracy of 0.1 mm. 

Petri dishes without reflections and shadows are difficult to light because they are transparent and reflective. Multiple exposures can produce artifacts at the edge of the agar that are likely to count as a colony.

 It leads to inaccuracy in counting and affects the accuracy of the instrument. Interscience for microbiology has developed a white diffuser dome for 360 degree light without reflections and shadows to overcome the problem. Their Scan® 4000 model has this feature. 

Advantages of using the automatic counter colony

Automatic colony counter has many advantages. Some of them are:

  • It takes less than 10 seconds to photograph a petri dish with colonies and the software analyzes it quickly. Therefore, the time required for counting is reduced. 
  • As the software counts colonies, it increases accuracy and reduces manual labor.
  • Counting multiple pages becomes effortless.


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