Struvite (MgNH₄PO₄·6H₂O), is not only an important biomineral, but also a major component of the so called infectious urinary stones. It is formed when urinary tract becomes colonised by bacteria producing urease. Urease is a characteristic bacterial enzyme splitting urea [1] into carbon dioxide and ammonia. These products alkalinize urine. Under alkaline conditions, an increase in the concentration of the ammonium, carbonate and phosphate ions occurs. These ions together with the ions of calcium and magnesium present in the urine lead to the crystallization of struvite, according to the following reaction [2]:

Mg²⁺+NH₄⁺+PO₄^3–+6H₂O→MgNH₄PO₄·6H₂O.                      (1)

The minor component which crystallizes as a result of the infection is carbonate apatite (Ca₁₀(PO₄)₆CO₃; CA) formed according to the general reaction [2]:

CO₃^2–+10Ca²⁺+6PO₄^3–→Ca₁₀(PO₄)₆CO₃.                                 (2)

This component is present as an amorphous precipitate and does not form crystals of defined morphology typical to struvite. Struvite together with small amount of CA forms the so-called infectious stones.

In the present study we describe mineralization of CA and struvite crystals in the environment of artificial urine in the absence and presence of Proteus  mirabilis and the comparative study of the crystal grown. The obtained crystals take habits typical for crystals growing in living animal and human organisms. Struvite grown in the presence of Proteus  mirabilis, at first stage of growth (low pH), takes coffin-like habit and the crystals are mainly composed of the (001) and (00–1) pedions and the {101}, {10–1}, {011} and {01–1} forms [3]. At higher pH single crystals are observed rarely, and instead twins and dendritic structures appear. Twins may be divided into two groups. First group of twins is composed of two (or three) crystals one rotated 60 or 90 degrees relative to the other [4]. Second kind of twins is typical penetration twin with the (001) pedion as penetration plane. Dendrites are X-shaped or they are composed of one trunk and branches symmetrically distributed on two sides of the trunk. The evolution of dendrites is mainly influenced by the rate of change in pH with minor influence of the value of pH. The absence of dendrites is a key indication that sample did not experience a rapid change in pH value. The virtual boundary separating these two kinds of crystals (single and dendrites) can be considered as the limit between the slow and rapid growth.

From the comparison of this result with the results of the control experiments without bacteria, it can be concluded that in the case of absence of bacteria the crystals in most cases take also the coffin-like habits, but instead of the {011} and {01–1} forms, the {012} and {010} forms appear. Additionally, in the case of the absence of bacteria, it is observed very frequently, that the (001) pedion disappears totally and is not represented in the morphology. It is known that the changes in crystal habit and morphology are due to differences in relative growth rates of faces of which the crystal is composed. In this case, it is suggested that the changes in morphology are induced by the presence of bacteria. This may happen because the surface of bacterial cells usually demonstrates anionic character and therefore they have ability to trap positive ions from the surrounding environment. In the case of Proteus mirabilis, bacterial polysaccharides contain negatively charged residues, which are able to bind the Ca²⁺ and Mg²⁺ ions [5]. These ions accumulate around bacterial cells and the crystallization process appears to be mediated by specific molecular interactions between molecular structures of the crystal surface and molecular arrays around bacterial cells.

References

[1] Bichler, K.H.; Eipper, E.; Naber, K.; Braun, V.; Zimmermann, R.; Lahme, S. Int. J. Antimicrob. Agents 2002, 19, 488–498.

[2] McLean, R.J.C.; Nickel, J. C.; Cheng, K. J.; Costerton, J.W. Crit. Rev. Microbiol. 1988,16 , 37–79.

[3] Prywer, J.; Torzewska, A. Crystal Growth & Design 2009, 9, 3538–3543.

[4] Prywer, J.; Torzewska, A.; Płociński, T. Urological Research 2012, 40, 699–707.

[5] Torzewska, A.; Stączek, P.; Różalski, A. J. Medical Microbiol. 2003, 52, 471–477.

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