How to Count Bacterial Colonies

Counting Fun

     Once a dilution is completed and each sample is plated and incubated for 48 hours or more, colonies become visible. A colony is considered a visible mass of microorganisms, with each one originating from a single mother cell. In other words, a colony is a clone of bacteria that are all genetically alike. Colony counting is used to detect and count the number of microbes in soil, water, and food. In this case we used colony counting to identify the number of microbes found in yogurt. When counting colonies their are several ways of doing it and each way has a specific protocol, which allows for accuracy and efficiency. 
     Two ways of counting include manual and automated. Automated counting consists of placing the petri dish into a machine. The automated counting machine then takes a picture of the petri dish, separates out the colonies, and then uses algorithm to count the colonies on the plate. Although, automated counting takes away some work and stress from individuals, automated counting sometimes has trouble differentiating colonies when two or more colonies are touching edges. As a result, improvement must be done to the machine in order to have efficient counting. On the other hand, an approach that was used for our research project includes manual counting. Manual counting is done by counting each colony one dot at a time. When manually counting an individual can use the back of the petri dish and mark each colony that is visible. Generally, individuals should count 3 plates at the same dilution in order to receive accurate results. To make manually counting bearable, individuals can place a light board underneath to make the colonies easier to see. An individual can also put the petri dish on a grid pattern paper underneath the dish allowing them to move in a methodical way when counting. Similar to automated counting, manual counting can also cause errors because individuals may not be able to determine separate colonies when two or more are touching one another. An individual can also lose track of counting, which is why we suggest purchasing a counting device. 
     Once we were done counting all 3 plates at the same dilution factor, we began to calculate the CFU. CFU stands for, colony- forming unit, which is used to estimate the number of viable bacteria in a sample. When calculating the CFU, first the average number of colonies of the dilution factor should be found. This is done by adding up the number of colonies found in all three plates and dividing by 3. Once the average is found, individuals should multiple the average by the dilution factor and divide it by the volume in mls transferred to the dish. After the CFU is found for each brand of yogurt, we will be able to determine which yogurt has more bacteria and which yogurt does not. 

This is the CFU Formula the group followed: 

This is Theresa's counting method: