Yeast colonies are larger than bacteria colonies because individual yeast cells are considerably larger than bacterial cells. Both yeast and bacteria are unicellular microorganisms, but yeasts are eukaryotes, and bacteria are prokaryotes. A eukaryotic cell contains a nucleus and structural organelles enclosed within a double-membrane. Yeasts also have thick cell walls that encapsulate the organism. Prokaryotes, such as bacteria, are smaller because they lack a nucleus and membrane-bound organelles.
Yeast cells increase in number asexually by budding. As explained by Gary Kaiser at the Community College of Baltimore County, a bud forms on the outer surface of a parent cell, and the cell’s nucleus divides. One of the divided nuclei enters the forming bud, and cell wall material is produced between the parent cell and the bud. The bud breaks away to become a new daughter cell. This multiplication of genetically identical yeast cells populates into a colony. Bacteria divide and populate colonies via binary fission, another form of asexual reproduction. The circular DNA of the bacterium replicates, and the copies are separated to opposite ends of the cell. The bacterium’s cell wall grows, while forming a cleft at the center of the cell. The cleft eventually pinches the splitting cell into two separate but identical daughter cells. Binary fission continues within each daughter cell, populating the bacterial colony.