The Link Between the Number of Bacterial Colonies on the Agar and the Number of Bacteria in the Original Sample

The agar medium is a nutrient-filled gel used to grow microbes in a laboratory setting. These microbes include bacteria, fungi and others.
The bacterial colonies grown on the agar medium can be clearly seen as dots or patches. Each colony represents a population of bacteria derived from an individual bacterium from the original sample.
Therefore, if a bacterium from the original sample lands on the nutritive agar and finds the conditions appropriate, it will reproduce and multiply, forming a visible colony.
The greater the number of bacteria in the original sample, the more colonies will be seen on the agar plate, all else being equal. Likewise, fewer bacteria in the initial sample will usually lead to fewer observable colonies.
It’s important to note, though, that not all bacteria present in a sample will grow on agar – either because they might not thrive in those conditions or due to competition with other bacteria. This means that the visible colonies might not represent the total quantity of bacteria in the initial sample.
Technicians can use a dilution method to make a series of solutions with lower bacterial concentrations from the original sample. By inoculating agar plates with these diluted solutions, they can achieve a more accurate estimation of the total bacterial count in the starting sample.
While the count of visible colonies provides an estimate (the colony-forming units or CFUs), the actual number of bacteria in the original sample is likely to be higher.
It’s vital to maintain sterile conditions during bacterial culturing to avoid contamination that could impact the validity of results. Standards and controls help to maintain reliability.
Remember: Agar plate results can only provide data about the types of bacteria that can grow under the specific conditions provided. Different types of agar and different conditions such as temperature and oxygen levels can favour different bacteria.
In conclusion, although there are potential sources of error, the number of cultured colonies on an agar plate is generally taken as a good indicator of the original number of viable (live and able to reproduce) bacteria in the sample.