Final answer:
The highest concentration of endospores in a Bacillus cell culture would be observed during the stationary phase, where cells are stressed and begin to sporulate for survival.
Step-by-step explanation:
Most endospores in a Bacillus cell structure are expected to be observed during the stationary phase, which is the period where the rate of cell division and death are roughly equal. This phase is reached because of the depletion of a carbon source or the accumulation of end products. During the stationary phase, cells are under stress due to nutrient limitation or waste accumulation, which triggers some cells to form endospores as a survival strategy. In contrast, during the log phase, cells are actively growing and dividing, and antibiotics like penicillin would be most effective since they target processes essential for cell growth such as cell-wall synthesis.
The death phase also sees an increase in endospore formation as the culture medium's toxic waste accumulates and nutrients get exhausted leading to cell death. However, the stationary phase is usually where you would find the highest concentration of endospores due to the balance of living cells halting in growth and turning to sporulation for survival.