  We study the bulk and shear viscosity and the electrical conductivity in a
quasiparticle approach to Yang-Mills theory and QCD with light and strange
quarks to assess the dynamical role of quarks in transport properties at finite
temperature. The interactions with a hot medium are embodied in effective
masses of the constituents through a temperature-dependent running coupling
extracted from the lattice QCD thermodynamics. In Yang-Mills theory, the bulk
viscosity to entropy density ratio exhibits a non-monotonous structure around
the phase transition temperature. In QCD, this is totally dissolved because of
a substantial contribution from quark quasiparticles. The bulk to shear
viscosity ratio near the phase transition behaves consistently to the scaling
with the speed of sound derived in the AdS/CFT approach, whereas at high
temperature it obeys the same parametric dependence as in perturbation theory.
Thus, the employed quasiparticle model is adequate to capture the transport
properties in the weak and strong coupling regimes of the theory. This feature
is not altered by including dynamical quarks which, however, retards the system
from restoring conformal invariance. We also examine the individual flavor
contributions to the electrical conductivity and show that the obtained
behavior agrees qualitatively well with the recent results of lattice
simulations and with a class of phenomenological approaches.
