A. In principle this should be possible, but bacteria have different stabilization mechanisms, so you need more information on the stabilization mechanism. The two main possibilities are:

(a) electrostatic stabilization: you have to suppress the electrical duoble layer around the particles.Preferably this should be done with a divalent salt. When possible you should first find out what the sign of the charge is by,e.g. an electrokinetic method (zeta potential). In general also the pH will be important, because H/OH are primary charge determining ions.

(b) steric stabilization by a polymer layer. In this case you have to change the solvent quality. Again some information on the system is required to make an appropriate choice.

(c) stabilized by a polyelectrolyte layer =a + b (solvent quality is in general decreased by suppressing the charge (pH; divalent salt)

B. The method will also depend on what you intend to do with the bacteria once concentrated. A coagulant/flocculant might be harmfull to living bacteria.