This thesis focuses on experimental studies on collective motion using
swimming bacteria as model active-matter systems. It offers
comprehensive reviews of state-of-the-art theories and experiments on
collective motion from the viewpoint of nonequilibrium statistical
physics. The author presents his experimental studies on two major
classes of collective motion that had been well studied theoretically.
Firstly, swimming filamentous bacteria in a thin fluid layer are shown
to exhibit true, long-range orientational order and anomalously strong
giant density fluctuations, which are considered universal and landmark
signatures of collective motion by many numerical and theoretical works
but have never been observed in real systems. Secondly, chaotic
bacterial turbulence in a three-dimensional dense suspension without any
long-range order as described in the first half is demonstrated to be
capable of achieving antiferromagnetic vortex order by imposing a small
number of constraints with appropriate periodicity. The experimental
results presented significantly advance our fundamental understanding of
order and fluctuations in collective motion of motile elements and their
future applications.