UNIQUE ABILITY OF THE PROTEUS-MIRABILIS CAPSULE TO ENHANCE MINERAL GROWTH IN INFECTIOUS URINARY CALCULI

Struvite (MgNH4PO4.6H(2)O) calculi are a common complication of Proteu s mirabilis urinary tract infections. Although urease is a major virul ence factor in calculus formation, the polysaccharide capsule (CPS) of this organism also enhances struvite crystallization and growth in vi tro (L. Clapham, R. J. C. McLean, J. C. Nickel, J. Downey, and J. W. C osterton, J. Crystal Growth 104:475-484, 1990). We obtained purified C PS, of known structure and varying anionic character, from P. mirabili s ATCC 49565 and several other organisms. Artificial urine was added t o CPS, and the pH was elevated from 5.8 to 8.5 by the addition of urea se or titration with 0.25 4 NH2OH to induce struvite crystallization. Crystallization was measured by particle counting (Coulter counter), a nd the morphology (crystal habit) was examined by phase contrast micro scopy. In the presence of partially anionic P. mirabilis CPS, struvite formation occurred at a lower pH than in the absence of CPS pr in the presence of other neutral, partially anionic, or anionic CPS. At pH 7 .5 to 8.0, significantly more struvite crystals formed in the presence of P. mirabilis CPS than under other experimental conditions. With th e exception of one polymer (curdlan) which did not bind Mg2+, enhancem ent of struvite formation by CPS polymers was inversely proportional t o their Mg2+ binding ability. We speculate that the structure and part ial anionic nature of P. mirabilis ICPS enable it to enhance struvite formation by weakly concentrating Mg2+ ions during struvite crystal fo rmation. This illustrates a new virulence aspect of bacterial CPS duri ng infection.