What is the purpose of plating serial solutions?

The purpose of plating serial dilutions is to obtain an estimate of individual colonies or visible units on agar places. This allows for the quantification of the original concentration of microorganisms in a sample and to isolate individual colonies. Plating specific volumes from each dilution onto agar plates creates plates with varying concentrations of microorganisms. This technique ensures that one of the plates will have colonies that are countable. If there is an excess number of colonies on the plate, the colonies can merge and become almost identical, making them impossible to count accurately. This situation is referred to as confluent or “Too Numerous To Count”. In general, a countable plate should ideally contain between 30-300 colonies. Having more than 300 colonies makes counting harder, while fewer than 30 colonies is not enough to give an accurate representation of the original sample’s microbial content. The objective is to obtain plates with colonies which are easily countable, allowing for the calculation of the microorganism concentration in the original sample. This is typically expressed as colony-forming units per milliliter (CFU/mL).