  We experimentally study density fluctuations and energy spectra of bulk
\textit{E. coli} suspensions of different concentrations. Our results verify
the predicted scaling law of giant number fluctuations in three-dimensional
(3D) wet active fluids. We find that such a scaling behavior persists at small
scales even in low concentration suspensions well below the transition
concentration to active turbulence. Our experiments also support the prediction
on the energy spectra of dilute pusher swimmers and illustrate the spectral
properties of the active turbulence of dense bacterial suspensions in the bulk
limit. By comparing density fluctuations and kinetic energy in active
turbulence, we identified a universal density-independent and scale-invariant
correlation between the two quantities across a wide range of length scales.
