Infectious urinary stones consist of an agglomerate of bacteria, highly crystalline struvite, and poorly crystalline and amorphous precipitate (PCaAP). The paper describes an experimental study on the agglomeration of PCaAP in the presence and absence of bacteria of the Proteus mirabilis species, in which blind studies are done with bacterial macromolecules with and without lipopolysaccharide. The formation of struvite is avoided by using Mg-free artificial urine. Optical microscopy methods and zeta potential measurements both confirm that the bacterial lipopolysaccharides are the key factors promoting agglomeration, besides the bacterially induced pH change which leads to precipitation in the first place.