In the general exponential equation \( a_n = a_1 \cdot r^{n-1} \), the term \( a_1 \) represents the initial value of the sequence.
In this context, since the population of bacteria starts with 50, we can identify \( a_1 \) as the initial population.
Therefore, the value of \( a_1 \) is:
\[ \boxed{50} \]