  In this work we report on the simultaneous measurement of the hydrodynamic
coefficient and the electric charge of single {\it Bacillus subtilis} spores.
The latter has great importance in protein binding to spores and in the
adhesion of spores onto surfaces. The charge and the hydrodynamic coefficient
were measured by an accurate procedure based on the analysis of the motion of
single spores confined by an optical trap. The technique has been validated
using charged spherical polystyrene beads. The excellent agreement of our
results with the expected values demonstrates the quality of our procedure. We
measured the charge of spores of {\it B. subtilis} purified from a wild type
strain and from two isogenic mutants characterized by an altered spore surface.
Our technique is able to discriminate the three spore types used, by their
charge and by their hydrodynamic coefficient which is related to the
hydrophobic properties of the spore surface.
