  We experimentally investigate the effects of polymer additives on the
collective dynamics of swarming Serratia marcescens in quasi two-dimensional
(2D) liquid films. We find that even minute amounts of polymers (< 20 ppm) can
significantly enhance swimming speed and promote largescale coherent
structures. Velocity statistics show that polymers suppress large velocity
fluctuation, transforming the velocity distributions from super-Gaussian to
Gaussian. Spatial and temporal correlation functions suggest that polymers
increase both the size and lifetime of flow structures. The energy spectra show
an exponential decay at low wavenumbers, with a characteristic length scale
increasing with polymer concentration. Overall, these result show polymers can
mediate bacteria interaction and promote large-scale coherence in dense active
suspensions.
