  We propose a simple form for the superalgebra of M2 and M5-brane probes in
arbitrary supersymmetric backgrounds of 11D supergravity, extending previous
results in the literature. In particular, we identify the topological charges
in the algebras and find BPS bounds for the energies. The charges are given by
the integral over a brane's spatial worldvolume of a certain closed form built
out of the Killing spinors and background fields. The existence of such closed
forms for arbitrary supersymmetric backgrounds generalises the existence of
calibration forms for special holonomy manifolds.
