*3.3. Model Solutions with Maximum Concentrations of Additional Ions Characteristic of a Healthy Person's Urine Composition*

In the phosphate syntheses with maximum concentrations of inorganic impurities without additives, the formation of brushite and struvite was observed (Figure 4). Brushite was formed in a wide range of pH values of the initial solution from 5.81 to 7.63. Struvite growth occurred at higher pH values (from 7.23 to 7.73) and usually along with brushite.

When MHB medium was added to the model solution, hydroxylapatite was clearly observed in the precipitate composition, in addition to common brushite and struvite. Brushite and struvite phases also formed when various bacteria were added to the solution, while apatite was detected only in syntheses with *E. coli* and *Pseudomonas aeruginosa*. In all the systems, except for the synthesis in the presence of *Klebsiella pneumoniae*, there was a significant shift in the beginning of the precipitate formation toward higher pH values. Thus, brushite was obtained in syntheses within the pH range 7.0–7.03 (in the system with "kl" at a pH of 6.10), apatite was observed only at a pH of 7.0, and struvite at a pH of 7.0 or higher.

Addition of MPB medium to the model solution did not lead to changes in the phase composition of the sediment. Brushite and struvite were formed when various bacteria were added to the solution (Figure 4). The brushite phase was found in all systems, but at different pH values: Between 6.10 and 7.06 (syntheses with "st", "kl", and "ps") or 6.96–7.06 (syntheses with "e"). Struvite was formed in all the systems at a pH of about 7 and higher.

**Figure 4.** Phase composition of synthesized products from model solutions with maximum concentrations of additional ions characteristic of a healthy person's urine composition. Legend as in Figure 3.

## *3.4. Crystallization in the Oxalate System*

As the result of the biomimetic syntheses, it was found that the presence of bacteria accelerates nucleation within the oxalate system (Table 3). Perhaps, bacteria can act as nucleation centers. The greatest effect (more than twice) in accelerating the crystallization rates of the calcium oxalates was observed in the presence of *Pseudomonas aeruginosa*.

**Table 3.** Nucleation of calcium oxalates in the presence of bacteria at various supersaturations (γ).


PXRD analyses of the precipitates obtained in the presence of bacteria within the oxalate system showed formation of calcium oxalate mono and dihydrate (whewellite and weddellite, respectively), while in the syntheses without bacteria only whewellite was formed. According to the PXRD data, the whewellite/weddellite ratio in precipitants was determined along with unit cell parameters, from which the content of "zeolite" water, x, in the structure of weddellite (CaC2O4·(2 + x)H2O) was calculated (Table 4) [29]. The presence of bacteria did not practically affect the whewellite/weddellite ratio, as well as the content of "zeolite" water (x). Moreover, the results obtained for the syntheses with bacteria were close to the effect of proteins that stabilize calcium oxalate dihydrate crystallization [1,30].

**Table 4.** Characteristics of phases synthesized within the oxalate system in the presence of bacteria and protein additives.


**\*** Per formula unit; calculations were made with regard to the *a* unit cell parameter, using the regression equation reported in [29].
