Bacterial Growth Curve - Microbiology

1. A researcher monitors a batch culture of E. coli by measuring both the viable cell count (using plate counts) and the total cell count (using a counting chamber and microscope). What relationship between these two counts would be expected as the culture progresses from the late log phase into the early death phase?
   Answer: The viable count and total count will be similar in the log phase, but the total count will remain high while the viable count drops in the death phase. The correct answer is C. In the log phase, most cells are alive and capable of reproduction, so the viable count (CFU) and total cell count are very similar. As the culture enters the stationary and then the death phase, cells begin to die. However, dead cells do not necessarily lyse immediately. A microscope and counting chamber will count all intact cells, whether they are live or dead. A viable plate count only detects live cells capable of forming a colony. Therefore, as cells die, the viable count will decrease, but the total count will decrease much more slowly (or remain high initially), leading to a growing discrepancy between the two measurements. A is incorrect because the counts do not decrease at the same rate. B is incorrect as it's impossible for the viable count (a subset) to be higher than the total count. D is incorrect; when growth stops, both counts should plateau, with the viable count beginning to fall first as death sets in.
2. A bacterial culture is initiated with a concentration of $4 \times 10^3$ cells/mL. The culture has a generation time of 30 minutes. After 2.5 hours of growth in the exponential phase, what is the approximate final cell concentration?
   Answer: $1.3 \times 10^5$ cells/mL The correct answer is B. This requires a multi-step calculation. 1. Calculate the number of generations (n). The total time is 2.5 hours, which is 150 minutes. The generation time (g) is 30 minutes. So, n = Total time / g = 150 min / 30 min/gen = 5 generations. 2. Use the formula for exponential growth: $N_f = N_0 \times 2^n$, where $N_f$ is the final cell concentration and $N_0$ is the initial concentration. 3. Substitute the values: $N_f = (4 \times 10^3) \times 2^5$. 4. Calculate $2^5 = 32$. 5. $N_f = (4 \times 10^3) \times 32 = 128 \times 10^3 = 1.28 \times 10^5$ cells/mL. This is approximately $1.3 \times 10^5$ cells/mL. A is incorrect, possibly from a miscalculation of $2^5$ as 25, leading to $4 \times 25 = 100 \rightarrow 1.0 \times 10^5$. C is incorrect, resulting from calculating $2^4$ instead of $2^5$ ($4 \times 10^3 \times 16 = 6.4 \times 10^4$) or some other error. D is incorrect, likely from incorrectly calculating the number of generations as 2.5 and multiplying the initial concentration by that number.
3. A batch culture of Bacillus anthracis is found to have a net growth rate of zero after 24 hours of incubation. Which of the following cellular processes is most likely to be significantly upregulated in a large portion of the population at this time?
   Answer: Expression of genes for sporulation The correct answer is B. A net growth rate of zero indicates the culture is in the stationary phase. For spore-forming bacteria like Bacillus species, entry into the stationary phase due to nutrient limitation or waste accumulation is a primary trigger for the complex genetic program of sporulation (endospore formation) as a survival mechanism. A and C are incorrect because the synthesis of ribosomal proteins and chromosomal replication are hallmarks of the exponential (log) growth phase, not the stationary phase. D is incorrect because while cells in stationary phase are stressed for nutrients, their overall metabolic activity, including active transport, is downregulated. The dominant survival strategy for Bacillus in this phase is sporulation, not a futile attempt to scavenge scarce nutrients for growth.

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