In the general exponential growth equation \( a_n = a_1 \cdot r^{n-1} \):
- \( a_n \) is the population at time \( n \),
- \( a_1 \) is the initial population (the value we're looking for),
- \( r \) is the growth factor (the rate at which the population increases),
- \( n \) is the number of time intervals (days, in this case).
Given that the bacteria quadruple daily, the growth factor \( r \) is 4 (since quadrupling means multiplying by 4). The initial population of bacteria, denoted as \( a_1 \), is given in the problem as 50.
Thus, \( a_1 = 50 \).