  We derive a sufficient condition for the linear stability of plasma
equilibria with incompressible flow parallel to the magnetic field, $\bf B$,
constant mass density and anisotropic pressure such that the quantity
$\sigma_d= \mu_0(P_\parallel -P_\perp)/B^2$, where $P_\parallel$ ($P_\perp$) is
the pressure tensor element parallel (perpendicular) to $\bf B$, remains
constant. This condition is applicable to any steady state without geometrical
restriction. The condition, generalising the respective condition for MHD
equilibria with isotropic pressure and constant density derived in
Throumoulopoulos & Tasso (2007), involves physically interpretable terms
related to the magnetic shear, the flow shear and the variation of total
pressure perpendicular to the magnetic surfaces. On the basis of this condition
we prove that if a given equilibrium is linearly stable, then the ones
resulting from the application of Bogoyavlenskij symmetry transformations are
linearly stable too, provided that a parameter involved in those
transformations is positive. In addition, we examine the impact of pressure
anisotropy, flow, and torsion of a helical magnetic axis, for a specific class
of analytic equilibria. In this case we find that the pressure anisotropy and
the flow may have either stabilising or destabilising effects. Also, helical
configurations with small torsion and large pitch seem to have more favorable
stability properties.
