= Solution
\b[TODO: user-deferred biology. The bacterium-specific numerical estimate and assumptions are deferred.]
The independent fluid-mechanical relation is
$$
\boxed{\mathrm{Re}=\frac{\rho UL}{\eta}=\frac{UL}{\nu},}
$$
where $\rho$ is <fluid> <mass density>, $\eta$ <dynamic viscosity> and $\nu$ <kinematic viscosity>. For a generic rigid <sphere>, the convention must specify whether $L$ is its diameter or radius; their <Reynolds numbers> differ by a factor of two. The ratio compares advective <inertia> $\rho U^2/L$ with viscous <stress> gradients $\eta U/L^2$. Small <Reynolds number> supports a creeping-flow approximation when unsteady <fluid> <inertia> is also negligible. No organism-specific size, speed or propulsion calculation is made here.
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