Final answer:
Bacteria experience the Death Phase when toxic waste builds up and nutrients are depleted, causing an exponential decline in cell numbers. Most endospores in a Bacillus culture are observed in the Death Phase, as well as in the Stationary Phase where cells may undergo sporulation.
Step-by-step explanation:
Bacteria enter the Death Phase due to an accumulation of toxic waste in their environment, exhaustion of nutrients, and a cessation of metabolic activity. During this phase, the rate at which cells die surpasses the rate of which new cells are formed, leading to an exponential decrease in the overall number of cells.
Many cells lyse, releasing nutrients back into the medium, which can be used by surviving cells to maintain viability and, in some cases, to form endospores. A distinct set of cells known as 'persisters' show a particularly slow metabolic rate and are of medical interest due to their association with chronic infections resistant to antibiotics.
In a Bacillus cell culture, one would expect to observe the most endospores during the Death Phase as well as the Stationary Phase, where many cells undergo sporulation, enabling them to endure harsh conditions.
Conversely, during the Log Phase, cells are most susceptible to antibiotics as this phase features constant growth and consistent metabolic activity. Therefore, cells in the Log Phase are not focused on forming endospores, but on growth and replication.