Final answer:
The most rapid bacterial growth occurs in the log phase, where the number of cells increases exponentially and the bacteria are most susceptible to antibiotics.
Step-by-step explanation:
The most rapid growth of bacteria takes place in the log phase, which is option 2 on the list provided. During the log phase, bacterial cells divide by binary fission and the number of cells increases exponentially. This phase is characterized by constant growth rate and uniform metabolic activity. It is also the phase where bacteria are most susceptible to antibiotics, like penicillin, that inhibit cell-wall synthesis.
To give a bit more context, the growth curve of a bacterial culture is illustrated with the logarithm of the number of live cells plotted as a function of time. The curve can be divided into the lag phase, log phase, stationary phase, and death phase. The log phase is preferred for the production of microbial mass due to the rapid growth and high cell numbers.