  We perform a QCD sum rule analysis for the $f_0$ light tetraquark taking into
account the contribution arising from the two pion intermediate state. With the
interpolating currents of the different chiral combinations of scalar and
pseudoscalar diquarks, it is demonstrated that the interpolating current with
maximum chirality has a large coupling to the two pion state, but the current
with zero chirality interacts only weekly with this state. Taking into account
the form factor in the $f_0$--two pion vertex, it is shown that the
$f_0$--coupling to the two pion state leads to an increase of the lightest
tetraquark mass by a value of about 100 MeV. The analysis of the resulting sum
rule shows that the $\sigma(f_0(600))$--meson state might be treated as the
four--quark bound state in the instanton field which has a rather strong
coupling to the two pion state.
