A student adds 5 bacterial cells to a test tube containing fresh medium and incubates it for 3 hours at 37°C. If the bacterial population shows an initial lag phase of 20 minutes followed by a doubling time of 40 minutes, what is the approximate number of bacterial cells present at the end of the incubation period? (Note: Assume unrestricted growth during incubation.)

Question

A student adds 5 bacterial cells to a test tube containing fresh medium and incubates it for 3 hours at 37°C.  If the bacterial population shows an initial lag phase of 20 minutes followed by a doubling time of 40 minutes, what is the approximate number of bacterial cells present at the end of the incubation period?  (Note: Assume unrestricted growth during incubation.)

Quizplus · Accepted Answer

11ecba2d_0fd5_3bf2_a3bf_aded2839bed9_MD0008_00 Binary fission is the process by which unicellular organisms such as bacteria reproduce asexually. During this process, the parental bacterial cell doubles in size and then divides into two identical daughter cells. Subsequently, each daughter cell also divides into two, yielding four cells that then continue to divide in a similar manner. This type of growth is called exponential; the generation time (doubling time) is the time needed for a bacterial population to double, or the time needed for a single bacterium to divide into two. When a single cell divides by binary fission, the number of cells that result after a specified time is calculated by multiplying the original number of bacterial cells by 2ⁿ, where n equals the number of generations. The rate at which a bacterial population grows can be demonstrated by plotting a typical growth curve, which has a lag, log, stationary, and death phase. In the lag phase, bacteria become accustomed to the local conditions and synthesize the components (eg, enzymes, RNA) required for metabolism but do not divide. In the subsequent log phase, the bacterial population grows exponentially as bacteria are able to metabolize nutrients. According to the question, 5 bacterial cells were incubated for 3 hours (180 minutes); these exhibited a 20-minute lag phase and a 40-minute generation time. Assuming unrestricted growth during incubation, the bacterium could only have had a log phase and a lag phase. Therefore, the number of cells that result after incubation can be calculated as follows: Calculate the amount of time in which the bacterial population was in log phase: 180 min incubation time − 20 min lag phase = 160 min log phase Find the number of generations in the log phase period: 160 min log phase / 40 min generation time = 4 generations Multiply the original number of bacterial cells by 2ⁿ to get the final population size: 5 × 2⁴ = 80 cells Educational objective: Binary fission is the process by which a unicellular organism (eg, bacterium) divides into two identical daughter cells. The generation time refers to the time in which a bacterial population doubles. When a single cell divides by binary fission, the number of cells that result is calculated by multiplying the original number of bacterial cells by 2ⁿ, where n equals the number of generations.

A Student Adds 5 Bacterial Cells to a Test Tube