  We use the mean-bacterial-velocity model to investigate the
\textit{irreversibility} of two-dimensional (2D) \textit{bacterial turbulence}
and to compare it with its 2D fluid-turbulence counterpart. We carry out
extensive direct numerical simulations of Lagrangian tracer particles that are
advected by the velocity field in this model. Our work uncovers an important,
qualitative way in which irreversibility in bacterial turbulence is different
from its fluid-turbulence counterpart: For large positive (or large but
negative) values of the \textit{friction} (or \textit{activity}) parameter, the
probability distribution functions of energy increments, along tracer
trajectories, or the power are \textit{positively} skewed; so irreversibility
in bacterial turbulence can lead, on average, to \textit{particles gaining
energy faster than they lose it}, which is the exact opposite of what is
observed for tracers in 2D fluid turbulence.
