Final answer:
Plate count methods are used for estimating viable bacterial populations by counting visible colonies on growth media, but this method does not apply to viruses since viruses do not form colonies and require host cells for replication.
Step-by-step explanation:
When evaluating the difference between plate count methods for bacterial and viral populations, the key distinction lies in the applicability of the method to the organism in question. Plate count methods are traditionally used to estimate the number of viable bacterial cells in a sample, as these cells can replicate and form visible colonies on a growth medium, which are counted as colony-forming units per milliliter (CFU/mL).
However, this method does not apply to viruses because viruses do not grow or form colonies on agar plates. Viruses require host cells to replicate, and hence their populations are typically measured by different criteria such as plaque assays, which count the areas of cell destruction caused by viral infection, rather than the traditional plate count used for bacteria.