Final answer:
Pour plates involve mixing the diluted sample with liquid agar before pouring into a dish, allowing for growth within and on top of the medium, while spread plates involve spreading the sample on solid agar, restricting growth to the surface.
Step-by-step explanation:
Pour plates and spread plates are two methodologies employed in microbiology to estimate the number of viable cells in a sample through serial dilution. In the pour plate method, the diluted sample is mixed with liquid warm agar, which is then poured into a Petri dish. This mixture is swirled to ensure even distribution of the sample before it solidifies. As the agar cools and solidifies, bacteria can grow both within and on the surface of the medium, leading to colonies that can be counted to estimate the original cell number.
Conversely, in the spread plate method, a diluted sample is directly pipetted onto the surface of solid agar and then spread uniformly across it using a sterile spreader. The primary difference here is that bacterial growth occurs only on the surface of the agar. Following incubation, colonies that form are counted to provide an estimate of the cell concentration in the original volume.
The key difference between these two methods is that pour plates allow for microbial growth within the agar as well as on its surface, which can lead to different colony morphologies, while spread plates limit growth to the surface, potentially making colonies easier to count.