  M. Kontsevich constructed universal characteristic classes of smooth bundles
with fiber a framed odd-dimensional integral homology sphere. In dimension 3,
they are known to give a universal finite type invariants of homology
3-spheres. However, they have not been well understood for higher fiber
dimensions. The purpose of the present paper is twofold. First, we obtain a
bordism invariant of smooth unframed bundles with fiber a 5-dimensional
homology sphere, which is defined as a sum of the simplest Kontsevich class and
the second signature defect. It may be in some sense a higher dimensional
analogue of the Casson invariant. Second, we construct a family of M-bundles.
By evaluating on those M-bundles, we show that Kontsevich's universal
characteristic classes are highly non-trivial in the case of fiber dimension 7.
As a corollary, new estimates for unstable rational homotopy groups of
Diff(D^7,\partial D^7) are obtained.
