Urease-induced crystallization in synthetic urine

The Role of Urinary Modulators in the Development of Infectious Kidney Stones

Introduction: The pathogenesis of infectious kidney stones is poorly understood. Bacteria have been implicated in promoting infectious stones via urease production; however, there is mounting evidence indicating the relationship is more complex. The aim of our study was to characterize suspected biotic and abiotic extrinsic factors that may modulate the formation of infectious stones. Materials and Methods: A high-throughput experimental model with Griffith's artificial urine was used to test a wide variety of urinary modulators and cytoplasmic enzymes present in crude cell-free extracts (CFEs) from bacterial strains to investigate how they impact struvite and calcium (Ca) phosphate crystal production. Crystal formation was evaluated with spectrophotometry and growth curve analysis. Light microscopy and scanning electron microscopy/X-ray diffraction was used for crystal structure and composition identification. Results: The acidic urinary modulators used in this study prevented crystal formation, whereas osteopontin had a significant inhibitory effect. Addition of CFEs from Proteus mirabilis 175A and 177A resulted in Ca phosphate and struvite crystals. Of interest, Klebsiella pneumoniae and Klebsiella oxytoca produced crystals including Ca phosphate and Ca oxalate, respectively. Pseudomonas aeruginosa had no urease production detected and produced Ca phosphate crystals. Discussion: Urinary modulators can have a wide variety of effects on infectious stone formation and the role of pH is important but does not guarantee robust crystal formation. Bacterial strains can produce Ca oxalate, Ca phosphate, and struvite stones with and without urease activity. Conclusion: Various urinary modulators appear to influence the process and are worthy of further evaluation as a potential therapeutic strategy to prevent infection-related urinary stone formation. Stones formed from urinary tract infections may be a result of multiple encoded metabolic pathways and discovering these would improve our understanding of the stone-bacterial relationship.

Validation of the CDC biofilm reactor as a dynamic model for assessment of encrustation formation on urological device materials

Contemporary medical science is reliant upon the rational selection and utilization of devices, and therefore, an increasing need has developed for in vitro systems aimed at replicating the conditions to which urological devices will be subjected to during their use in vivo. We report the development and validation of a novel continuous flow encrustation model based on the commercially available CDC biofilm reactor. Proteus mirabilis-induced encrustation formation on test biomaterial sections under varying experimental parameters was analyzed by X-ray diffraction, infrared- and Raman spectroscopy and by scanning electron microscopy. The model system produced encrusted deposits similar to those observed in archived clinical samples. Results obtained for the system are highly reproducible with encrustation being rapidly deposited on test biomaterial sections. This model will have utility in the rapid screening of encrustation behavior of biomaterials for use in urological applications.

Cellular and molecular gateways to urolithiasis: a new insight

Urolithiasis is a relevant clinical problem in everyday practice with a subsequent burden for the health system. Urolithiasis is classically explained as the derangement in the process of biomineralization involving the equilibrium between promoters and inhibitors of crystallization: a deficit of one or several inhibitors or an excess of one or several promoters plays a pivotal role in the stone formation. The revolutionary introduction of the molecular biology in medicine has given a new insight in urolithiasis too. Genetic factors have also been postulated to play an important role. A review of the current knowledge on urolithiasis based upon a molecular and genetic approach is reported.

A model to quantify encrustation on ureteric stents, urethral catheters and polymers intended for urological use

OBJECTIVE

To validate an encrustation model and to quantify encrustation on currently used urological devices and polymers intended for urological use. Materials and methods An encrustation model was validated: (i) to measure the amount of calcium leaching from the glass model and from the polymer used; (ii) to determine whether the use of a single-source or pooled urine produced similar results; (iii) to determine in vitro encrustation; and (iv) to compare the results of in vivo implantation of the same materials into the bladders of rodents with the in vitro results. A test polymer (a ureteric stent, a urethral catheter or a biomaterial) and a control silicone polymer were housed separately but received human urine from the same reservoir and under the same conditions (pH 6.0 and 37 degrees C) for 5 days. The amount of calcium encrustation on each polymer was measured using atomic absorption spectroscopy. Each experiment was repeated at least four times and the results expressed as an encrustation index, defined as the ratio of encrustation of the test and reference polymers.

RESULTS

The amount of calcium leaching from the glass model and polymers tested was insignificant. The use of a single-source or pooled urine gave the same results in the encrustation model. The in vitro results correlated with in vivo implantation of disks into the bladders of rats. Among the commonly used ureteric stents tested, the Cook C-Flex ureteric stents encrusted least. Hydrogel-coated ureteric stents encrusted more than uncoated stents. The Bard polytetrafluoroethylene short-term urethral catheter encrusted more than the Bard hydrogel-coated long-term catheter. A plasma-activated surface modification of a synthetic biomaterial with hyaluronic acid encrusted less than silicone, a long-term biomaterial widely regarded as the 'gold standard'.

CONCLUSION

This validated encrustation model is the first to quantify encrustation on currently available ureteric stents and urethral catheters. A novel coating for a biomaterial was identified using the encrustation model, and which encrusted less than silicone.

Effect of culture conditions on the formation of struvite by Myxococcus xanthus

The amount of struvite (MgNH4PO4 x 6H2O) produced by Myxococcus xanthus as well as the culture parameter values (pH, total phosphorus, total Kjeldahl nitrogen) were dependent on the culture medium used. Struvite formation started during the exponential phase and the maximum concentration was observed at the beginning of stationary growth phase. The addition of each medium component to the liquid culture influenced the amount of crystal produced. This amount did not depend on the pH increase during the culture period. The moment of the bacterial growth phase, at which each medium component was added, influenced the struvite formation.

Urease-induced crystallizations of calcium phosphate and magnesium ammonium phosphate in synthetic urine and human urine

An aggregometer technique was used to study urease-induced crystallizations in synthetic urine and human urine from healthy subjects and patients with chronic spinal cord injuries. The two different phases of crystallization, calcium phosphate and magnesium ammonium phosphate, were easily evaluated with a single assay using this technique. The crystallization of calcium phosphate and magnesium ammonium phosphate varied markedly among the different urine specimens after incubation with urease. The turbidity curves from human urine were divided into four patterns. We assumed that the variations in the patterns of the turbidity curves appeared to be mainly due to differences in the composition of the urine and in the original pH, and that the calcium and magnesium concentrations were very important in the urinary constituents.

Unique ability of the Proteus mirabilis capsule to enhance mineral growth in infectious urinary calculi

Struvite (MgNH4PO4.6H2O) calculi are a common complication of Proteus mirabilis urinary tract infections. Although urease is a major virulence factor in calculus formation, the polysaccharide capsule (CPS) of this organism also enhances struvite crystallization and growth in vitro (L. Clapham, R. J. C. McLean, J. C. Nickel, J. Downey, and J. W. Costerton, J. Crystal Growth 104:475-484, 1990). We obtained purified CPS, of known structure and varying anionic character, from P. mirabilis ATCC 49565 and several other organisms. Artificial urine was added to CPS, and the pH was elevated from 5.8 to 8.5 by the addition of urease or titration with 0.25 M NH4OH to induce struvite crystallization. Crystallization was measured by particle counting (Coulter counter), and the morphology (crystal habit) was examined by phase-contrast microscopy. In the presence of partially anionic P. mirabilis CPS, struvite formation occurred at a lower pH than in the absence of CPS or 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 the exception of one polymer (curdlan) which did not bind Mg2+, enhancement of struvite formation by CPS polymers was inversely proportional to their Mg2+ binding ability. We speculate that the structure and partial anionic nature of P. mirabilis CPS enable it to enhance struvite formation by weakly concentrating Mg2+ ions during struvite crystal formation. This illustrates a new virulence aspect of bacterial CPS during infection.

Citrate and urease-induced crystallization in synthetic and human urine

The effects of citrate on the different phases of urease-induced crystallization were studied using Coulter counter techniques and optical microscopy. Citrate increased urine pH and markedly delayed the initiation of the crystallization (nucleation) in both human and synthetic urine. In synthetic urine, particle aggregation and especially particle growth were delayed and inhibited by citrate. In human urine, aggregation was distinctly inhibited by citrate. It appears that the susceptibility of urine to form crystals in the presence of urease activity is influenced by its citrate concentration.

Acidification of urine is not a feasible method for preventing encrustation of indwelling urinary catheters

Encrusting deposits form on urinary catheters as a result of infection by bacteria which produce urease. Urease catalyses the hydrolysis of urea to produce ammonia, with the result that the pH of the urine increases and the deposits are precipitated. It has been suggested that acidification of urine may prevent this rise in pH and so prevent encrustation. Experiments were performed in vitro to determine whether acidification could be used to prevent the rise in pH when urease is present in urine. It was found that, in the presence of urease, the initial decrease in pH resulting from acidification was countered by further urea being converted into ammonia. Thus, although acidic washout solutions may dissolve encrusting deposits, acidification of urine is not a feasible method for preventing catheter encrustation.

In vitro inhibition of struvite crystal growth by acetohydroxamic acid

Struvite (MgNH4PO46H2O) crystals were produced by Proteus mirabilis growth in artificial urine, in the presence and absence of the urease inhibitor, acetohydroxamic acid (AHA). In the absence of AHA, struvite crystals assumed an "X-shaped" or dendritic crystal habit due to rapid growth along their 100 axis. When AHA was present, crystal growth, as monitored by phase contrast light microscopy, was greatly slowed, and the crystals assumed an octahedral crystal habit. Scanning electron microscopy revealed that crystals grown in the presence of AHA were pitted on their surface. This pitting was absent in control samples. While most of this inhibition by AHA was due to lowered urease activity, some crystal growth inhibition occurred in struvite produced in the absence of urease activity through NH4OH titration of artificial urine. We conclude that while AHA is primarily a urease inhibitor, it may also disrupt struvite growth and formation directly through interference with the molecular growth processes on crystal surfaces.

In vitro investigations into the formation and dissolution of infection-induced catheter encrustations

Encrustations are the most frequent complications occurring with indwelling catheters and urine drainage systems. The conditions for bacterial infections, using synthetic urine and controlled contamination by Proteus vulgaris, were standardised by using an in vitro model. Crystal deposits on catheters were analysed by infra-red spectroscopy and scanning electron microscopy. The main components of deposits in all investigations were struvite (MgNH4PO4.6H2O) 60-70% and carbonate apatite (Ca10(PO4,CO3)6 (OH,CO3)2) 30-40%. Investigations as to the quality and quantity of encrustations confirmed the analysis. Irrigation treatment was carried out with physiological saline solution and citric acid solution (Suby G) to study and quantify the dissolution of crystal deposits. Regular irrigation with citric solution resulted in a 70% dissolution of encrustations and ensured free flow as ascertained by measuring flow rates.

The influence of pH and urine composition on urease enzymatic activity in human urine

It is reasonable to assume that the rate of pH increase in urine induced by urease-producing microorganisms is one of the factors which determine whether crystallisation with subsequent stone formation will occur or not. To evaluate how the time needed to increase urine pH varies between different urine samples and how it depends on urine composition, a standardised amount of urease was added to different human urine samples. The incubations were performed in a pH-stat. This allowed simultaneous study of how urease enzymatic activity depends on urine pH and how it varies between different urines. The enzymatic activity was found to be negatively correlated to urine pH and to vary between different urines. The rate of the pH increase varied markedly between different urines. Small pH increases depended on the native urine pH and urease enzymatic activity. Higher pH increases up to the levels of phosphate crystallisation depended more on urine phosphate, the major urine buffer. The results presented show that urine composition influences the urease-induced pH increase. This might have clinical implications.

Studies of urease-induced crystallisation in undiluted human urine using the Coulter counter technique

Urease-induced crystallisation was studied in different human urine samples after urease incubation. The studies were performed using the Coulter counter technique, which enables determination of the number and size of particles in a solution and calculation of the total particle volume. The crystallization took place in three consecutive but overlapping steps: (1) nucleation, (2) growth and (3) aggregation. The maximal number of particles obtained in the different samples varied little, but there was a great variation in particle size and total particle volume. The variation in particle size appeared to be mainly due to differences in particle growth, a factor that might be of importance for stone formation.

Relationship between urease-producing bacteria, urinary pH and encrustation on indwelling urinary catheters

In 11 patients with long-term indwelling catheters the amount of catheter encrustation and urinary pH were measured and the urine regularly cultured over a prolonged period of time (median of 7 periods of 3 weeks). The mean urinary pH was related to the persistent presence of urease-producing micro-organisms (P. mirabilis) and urinary pH governed the precipitation of catheter encrustation. The critical pH appeared to be around 6.8. In patients with a mean urinary pH below this level the encrustation was minute (less than or equal to 2.9 mg phosphate). In patients with a mean urinary pH above 6.8 it was considerable but with a marked interindividual variation (35.5-138.7 mg phosphate). The composition of the encrustation was also strongly pH-related, with a much higher proportion present as magnesium ammonium phosphate in patients with a mean urinary pH above 6.8. The persistent presence of urease producers was not associated with a high pH or a more pronounced precipitation of phosphate in all patients. The amount of encrustation thus appears to depend not only on the presence of urease-producing micro-organisms but also on individual factors such as urinary composition.

Influence of chondroitin sulfate, heparin sulfate, and citrate on Proteus mirabilis-induced struvite crystallization in vitro

Struvite crystals were produced by Proteus mirabilis growth in artificial urine, in the presence of a number of naturally occurring crystallization inhibitors. The use of phase contrast light microscopy enabled the effects of added chondroitin sulfate A, chondroitin sulfate C, heparin sulfate, or sodium citrate, on struvite crystal growth rates to be rapidly monitored as changes in crystal habit. Struvite crystals formed as a consequence of the urease activity of P. mirabilis under all chemical conditions. In the absence of inhibitor, early crystal development was marked by large quantities of amorphous precipitate, followed immediately by the appearance of rapidly growing X-shaped or planar crystals. Addition of the glycosaminoglycans, chondroitin sulfate A, chondroitin sulfate C, or heparin sulfate to the artificial urine mixture had no effect on the rate of crystal growth or appearance. When sodium citrate was present in elevated concentrations, crystal appearance was generally slowed, and the crystals assumed an octahedral, slow growing appearance. None of the added compounds had any influence on bacterial viability, pH, or urease activity. It is therefore likely that the inhibitory activity displayed by sodium citrate might be related to its ability to complex magnesium or to interfere with the crystal structure during struvite formation. From these experiments it would appear that citrate may be a factor in the natural resistance of whole urine to struvite crystallization.

A simple technique for studying struvite crystal growth in vitro

Struvite urolithiasis forms as a consequence of a urinary tract infection by urease-producing species of bacteria such as Proteus mirabilis. Ammonia, produced by the enzymatic hydrolysis of urea, elevates urine pH causing a supersaturation and precipitation of Mg++ as struvite (NH4MgPO4). Calcium often precipitates as well, forming the mineral carbonate-apatite (Ca10(PO4)6CO3). We have developed a procedure based on direct observation by light microscopy whereby struvite crystal growth can be quickly monitored in response to chemical changes in urine. As struvite crystals assume a characteristic shape or crystal habit based on their growth rate, the effect of urine chemistry and the action of various crystallization or urease inhibitors on struvite formation can be quickly shown. In addition preliminary effects of alkaline pH, or the presence of toxic compounds on bacteria can also be shown through their loss of motility.

Resistance of catheters coated with a modified hydrogel to encrustation during an in vitro test

Mid-shaft specimens were cut from latex catheters coated with a modified hydrogel, latex catheters coated with silicone elastomer, and 100% silicone catheters. These specimens were subjected to controlled in vitro encrustation conditions. During a test period of 11 weeks, there was no significant difference in the quantities of encrusting deposits formed on the three materials.

Adherence of urease-induced crystals to rat bladder epithelium following acute infection with different uropathogenic microorganisms

Apart from urine supersaturation with respect to struvite and calcium phosphate, crystal retention is considered to be necessary for the formation of infection stones. This study was performed to investigate the role of the mucous coat in rat bladders in the adhesion of sterile urease-induced crystals and to determine to what extent the adhesion was influenced by infection. Elimination of the mucous coat with 0.1 M HCl increased the adherence of crystals six times compared to that in bladders with an intact mucous coat. Infection with Proteus mirabilis, Escherichia coli, enterococci and Ureaplasma urealyticum increased the adherence six, five, four and two times, respectively. Injury to the mucous coat may thus be one mechanism by which microorganisms can contribute to the formation of infection stones in the urinary tract.

The ecology and pathogenicity of urease-producing bacteria in the urinary tract

Urease activity is a physiological function of many bacteria that enables these organisms to utilize urea as a source of nitrogen. The association of ureolytic bacteria with human or animal hosts varies widely from a commensal relationship as demonstrated with skin microflora, a symbiotic relationship in the gastrointestinal tract, to a pathogenic relationship in the urinary tract. Since similar or identical species of bacteria such as Staphylococcus aureus are found in all three environments, the effect of urease activity on the host must be solely a function of the environment of these organisms. In this review, the importance of urease to bacteria is discussed, identifying the gastrointestinal tract as a major reservoir of ureolytic bacteria and investigating the urinary tract environment and the infectious struvite stone production that often accompanies urease-producing bacteria there. Finally, an infection model is presented which explains the development and growth of these urinary calculi and their remarkable persistence in spite of modern urological treatments.

The effects of glycosaminoglycans, pyrophosphate and allopurinol treatment on urease-induced crystallization in vitro

Previous studies have found that human urine inhibits urease-induced crystallization in synthetic urine. To identify the inhibitory components the effects of chondroitin sulphate, heparin and pyrophosphate on urease-induced crystallization were studied and the inhibitory capacity of human urine collected during and before/after allopurinol treatment compared. None of the substances studied nor allopurinol treatment was found to influence the urease-induced crystallization. These findings do not support the idea that glycosaminoglycans or pyrophosphate are responsible for the inhibitory effect detected in human urine.

The effects of urease in undiluted human urine

Undiluted human urine and synthetic urine were inoculated with urease. No inhibitory activity against urease enzymatic activity could be detected in human urine. The urease-induced crystallization of both calcium phosphate and magnesium ammonium phosphate differed markedly, however, between the individuals studied, and it was less pronounced in human urine than in synthetic urine. This supports the observation made in experiments using diluted urine that human urine possesses an inhibitory activity against urease-induced crystallization and suggests that it has a large interindividual variation.

The inhibitory effect of human urine on urease-induced crystallization in vitro

To study whether human urine contains inhibitors against urease-induced crystallization, Jackbean urease and human urine, in amounts small enough (0.5 to 10 per cent) not to influence the ion concentration, buffering capacity or pH, were added to synthetic urine. The ammonia production and alkalinization that followed were independent of the amounts of human urine added. The addition of human urine gave a dose-related decrease in the amount of calcium phosphate and struvite precipitated on glass rods immersed in the synthetic urine, however. Addition of only 0.5 per cent human urine gave a reproducible decrease and when 10 per cent human urine was added to the synthetic urine the precipitation of calcium phosphate was reduced by 50 per cent and that of struvite by 75 per cent. The results thus indicate that human urine contains components with the ability to reduce the urease-induced crystallization.

Etiopathogenesis of canine struvite urolithiasis

Urine must be oversaturated with magnesium ammonium phosphate for struvite uroliths to form. Oversaturation of urine with magnesium ammonium phosphate may be associated with several factors, including urinary tract infections with urease-producing microbes, alkaline urine, diet, and genetic predisposition. Of the urease-producing microbes, staphylococci are most struvitogenic in dogs. The precise mechanisms resulting in formation of sterile struvite uroliths in dogs have not been determined.

Crystalluria. Observations, interpretations, and misinterpretations

Crystalluria results from oversaturation of urine with crystallogenic substance. However, oversaturation may occur as a result of in vitro as well as in vivo events. The microscopic appearance of crystals only represents a tentative identification of their composition because variable conditions associated with their formation, growth, and dissolution may alter their appearance. Definitive identification is dependent on physical methods such as optical crystallography, x-ray diffraction, and electron microscopic analysis.