Hi, my name is Dr. Alice Lee and this lecture will talk about the growth of bacterial cultures. We will discuss the process of microbial growth, examine the different phases of microbial growth in batch culture, learn to calculate the generation time and how to measure growth. This image is a bacterial growth curve and is critical to our understanding of microbial population dynamics. Microbial cells have finite life span, and a species is maintained only as a result of continued growth of its population. Knowing how microbial populations can rapidly expand is useful for designing ways to control their growth. Growth is an orderly increase in the quantity of cellular constituents. It depends upon the ability of the cell to form new cellular material from nutrients available in the environment. Bacterial growth refers to an increase in bacterial numbers, not an increase in the size of the individual cells. In most bacteria, growth first involves increase in cell mass and number of ribosomes, then duplication of the bacterial chromosome, synthesis of new cell wall and plasma membrane, partitioning of the two chromosomes, septum formation, and cell division. This asexual process of reproduction is called binary fission and results in two daughter cells that are genetically identical. In this slide, the transmission electron micrograph (TEM) of a Bacillus licheniformis cell captured early in division. Two nucleoids can be distinguished on either side of the division plane, indicating that chromosome replication and segregation has already taken place. A nascent septum (cross wall) can be seen on both sides of the division plane, as evident by invaginated cell wall and membrane material. Binary is to express the fact that two cells have arisen from one. In a growing culture of E.coli, a rod, 1. cells elongate to approximately twice their original length and 2. then form a partition that constricts the cell into two daughter cells. This partition is called the septum and results from the inward growth of the cytoplasmic membrane and cell wall from opposing directions; 3. septum formation continues until the two daughter cells are 4. pinched off. By definition, when one cell divides to form two, one generation has occurred, and the time required for this process to occur is called generation time. The process for E. coli is about 20 minutes under the best conditions laboratory conditions. Most bacteria are slower than this (hours and days) for one generation! Most bacteria reproduce by binary fission, but a few species reproduce by budding, others produce chains of conidiospores, still other filamentous species simply fragment, initiating growth of new cells.