Understanding and creating biocementing beachrocks via biostimulation of indigenous microbial communities

Bacterially induced precipitation of minerals leading to cementation of natural geological formations has been well recorded in a variety of environments. A range of microbial pathways and geochemical processes have been found to influence the cementation processes; but detailed formation mechanisms and biogeochemical relationships are still not very clear. There has been a growing demand for the application of bacterially driven biocementation in a number of geotechnical engineering applications recently. Here, we aimed to unpin the mechanisms behind the formation of actively mineralising beachrock sediments at Lucky Bay in Western Australia to understand the natural accretionary processes and potential of indigenous bacterial communities in biocementation. We observed ferruginous, aluminosilicate and carbonate cements along with extensive extra polymeric substances, borings with possible microbial activities in certain sections of native beachrock sediments. Cement precipitation under calcium- and iron-rich microenvironments sourced from seawater and iron creek seems to be driven by both biogenic and abiogenic processes in nature. Native microbial communities with a dominance of the genera Halococcus and Marinobacter were recorded. Enrichment of native bacterial communities under seawater media conditions was conducted which lead to successful biomineralisation of calcitic and ferruginous cements under in vitro conditions although the community composition changed significantly. Nanomechanical properties of natural and laboratory synthesised cement crystals showed that engineered biocement is highly promising. The results of this study clearly demonstrate biological influence in the formation of natural cements and hint significant potential of biostimulation which can be harnessed for different engineering applications including coastal erosion.

Microbial Diversity and Mineralogical-Mechanical Properties of Calcitic Cave Speleothems in Natural and in Vitro Biomineralization Conditions

Natural mineral formations are a window into important processes leading to carbon storage and mineralized carbonate structures formed through abiotic and biotic processes. In the current study, we made an attempt to undertake a comprehensive approach to characterize the mineralogical, mechanical, and microbial properties of different kinds of speleothems from karstic caves; with an aim to understand the bio-geo-chemical processes in speleothem structures and their impact on nanomechanical properties. We also investigated the biomineralization abilities of speleothem surface associated microbial communities in vitro. Mineralogical profiling using techniques such as X-ray powder Diffraction (XRD) and Tescan Integrated Mineral Analyzer (TIMA) demonstrated that calcite was the dominant mineral in the majority of speleothems with Energy Dispersive X-ray Analysis (EDS) indicating a few variations in the elemental components. Differing proportions of polymorphs of calcium carbonate such as aragonite and vaterite were also recorded. Significant variations in trace metal content were recorded through Inductively Coupled Plasma Mass Spectrometer (ICP-MS). Scanning Electron Microscopy (SEM) analysis revealed differences in morphological features of the crystals which varied from triangular prismatic shapes to etched spiky forms. Microbial imprints and associations were seen in a few sections. Analysis of the associated microbial diversity showed significant differences between various speleothems at Phylum level; although Proteobacteria and Actinobacteria were found to be the predominant groups. Genus level microbial associations showed a relationship with the geochemistry, mineralogical composition, and metal content of the speleothems. The assessment of nanomechanical properties measured by Nanoindentation revealed that the speleothems with a dominance of calcite were stronger than the speleothems with mixed calcium carbonate polymorphs and silica content. The in vitro metabolic activity of the microbial communities associated with the surfaces of the speleothems resulted in calcium carbonate crystal precipitation. Firmicutes and Proteobacteria dominated these populations, in contrast to the populations seen in natural systems. The precipitation of calcium carbonate crystals in vitro indicated that microbial metabolic activity may also play an important role in the synthesis and dissociation of biominerals in the natural environment. Our study provides novel evidence of the close relationship between mineralogy, microbial ecology, geochemistry, and nanomechanical properties of natural formations.

3D printing of hybrid biomaterials for bone tissue engineering: Calcium-polyphosphate microparticles encapsulated by polycaprolactone

Here we describe the formulation of a morphogenetically active bio-ink consisting of amorphous microparticles (MP) prepared from Ca²⁺ and the physiological inorganic polymer, polyphosphate (polyP). Those MP had been fortified by mixing with poly-ε-caprolactone (PCL) to allow 3D-bioprinting. The resulting granular PCL/Ca-polyP-MP hybrid material, liquefied by short-time heating to 100 °C, was used for the 3D-printing of tissue-like scaffolds formed by strands with a thickness of 400 µm and a stacked architecture leaving ≈0.5 mm²-sized open holes enabling cell migration. The printed composite scaffold turned out to combine suitable biomechanical properties (Young's modulus of 1.60 ± 0.1 GPa; Martens hardness of 153 ± 28 MPa), matching those of cortical and trabecular bone, with morphogenetic activity. This scaffold was capable of attracting and promoting the growth of human bone-related SaOS-2 cells as demonstrated by staining for cell viability (Calcein AM), cell density (DRAQ5) and SEM studies. Furthermore, the hybrid material was demonstrated to upregulate the steady-state-expression of the cell migration-inducing chemokine SDF-1α. EDX analysis and FTIR measurements revealed the presence of hydroxyapatite in the mineral deposits formed on the scaffold surface. Based on the results we conclude that granular PCL/Ca-polyP-MP hybrid material is suitable for the fabrication of bioprintable scaffold which comprises not only biomechanical stability but also morphogenetic potential.

STATEMENT OF SIGNIFICANCE

In present-day regenerative engineering efforts, biomaterial- and cell-based strategies are proposed that meet the required functional and spatial characteristics and variations, especially in the transition regions between soft (cartilage, tendon or ligament) and hard (bone) tissues. In a biomimetic approach we succeeded to fabricate amorphous Ca-polyP nanoparticles/microparticles which are highly biocompatible. Together with polycaprolactone (PCL), polyP can be bio-printed. This hybrid material attracts the cells, as documented optically as well as by a gene-expression studies. Since PCL is already a FDA-approved organic and inert polymer and polyP a physiological biologically active component this new bio-hybrid material has the potential to restore physiological functions, including bone remodelling and regeneration if used as implant.

Rebalancing β-Amyloid-Induced Decrease of ATP Level by Amorphous Nano/Micro Polyphosphate: Suppression of the Neurotoxic Effect of Amyloid β-Protein Fragment 25-35

Morbus Alzheimer neuropathology is characterized by an impaired energy homeostasis of brain tissue. We present an approach towards a potential therapy of Alzheimer disease based on the high-energy polymer inorganic polyphosphate (polyP), which physiologically occurs both in the extracellular and in the intracellular space. Rat pheochromocytoma (PC) 12 cells, as well as rat primary cortical neurons were exposed to the Alzheimer peptide Aβ25-35. They were incubated in vitro with polyphosphate (polyP); ortho-phosphate was used as a control. The polymer remained as Na⁺ salt; or complexed in a stoichiometric ratio to Ca²⁺ (Na-polyP[Ca²⁺]); or was processed as amorphous Ca-polyP microparticles (Ca-polyP-MP). Ortho-phosphate was fabricated as crystalline Ca-phosphate nanoparticles (Ca-phosphate-NP). We show that the pre-incubation of PC12 cells and primary cortical neurons with polyP protects the cells against the neurotoxic effect of the Alzheimer peptide Aβ25-35. The strongest effect was observed with amorphous polyP microparticles (Ca-polyP-MP). The effect of the soluble sodium salt; Na-polyP (Na-polyP[Ca²⁺]) was lower; while crystalline orthophosphate nanoparticles (Ca-phosphate-NP) were ineffective. Ca-polyP-MP microparticles and Na-polyP[Ca²⁺] were found to markedly enhance the intracellular ATP level. Pre-incubation of Aβ25-35 during aggregate formation, with the polyP preparation before exposure of the cells, had a small effect on neurotoxicity. We conclude that recovery of the compromised energy status in neuronal cells by administration of nontoxic biodegradable Ca-salts of polyP reverse the β-amyloid-induced decrease of adenosine triphosphate (ATP) level. This study contributes to a new routes for a potential therapeutic intervention in Alzheimer’s disease pathophysiology.

Biocalcite and Carbonic Acid Activators

Based on evolution of biomineralizing systems and energetic considerations, there is now compelling evidence that enzymes play a driving role in the formation of the inorganic skeletons from the simplest animals, the sponges, up to humans. Focusing on skeletons based on calcium minerals, the principle enzymes involved are the carbonic anhydrase (formation of the calcium carbonate-based skeletons of many invertebrates like the calcareous sponges, as well as deposition of the calcium carbonate bioseeds during human bone formation) and the alkaline phosphatase (providing the phosphate for bone calcium phosphate-hydroxyapatite formation). These two enzymes, both being involved in human bone formation, open novel not yet exploited targets for pharmacological intervention of human bone diseases like osteoporosis, using compounds that act as activators of these enzymes. This chapter focuses on carbonic anhydrases of biomedical interest and the search for potential activators of these enzymes, was well as the interplay between carbonic anhydrase-mediated calcium carbonate bioseed synthesis and metabolism of energy-rich inorganic polyphosphates. Beyond that, the combination of the two metabolic products, calcium carbonate and calcium-polyphosphate, if applied in an amorphous form, turned out to provide the basis for a new generation of scaffold materials for bone tissue engineering and repair that are, for the first time, morphogenetically active.

Insight into calcification of Synechocystis sp. enhanced by extracellular carbonic anhydrase

The mechanism of cyanobacterial calcification was proved to be related to extracellular carbonic anhydrase, which enhanced CaCO₃ precipitation through facilitating proton consumption during transformation of bicarbonate to carbon dioxide.

High biocompatibility and improved osteogenic potential of amorphous calcium carbonate/vaterite

In human bone, amorphous calcium carbonate (ACC) is formed as a precursor of the crystalline carbonated apatite/hydroxyapatite (HA). Here we describe that the metastable ACC phase can be stabilized by inorganic polyphosphate (polyP) that is also used as a phosphate source for the non-enzymatic carbonate/phosphate exchange during HA formation. This polymer was found to suppress the transformation of ACC into crystalline CaCO₃ at a percentage of 5% [w/w] ("CCP5") with respect to CaCO₃ and almost completely at 10% [w/w] ("CCP10"). Both preparations (CaCO₃/polyP) are amorphous, but also contain small amounts of vaterite, as revealed by XRD, FTIR and SEM analyses. They did not affect the growth/viability of SaOS-2 cells. Cell culture and Ca²⁺ release experiments revealed that the CaCO₃ particles formed in the presence of polyP (CaCO₃/polyP) are degradable and, unlike calcite, become disintegrated with time during the cell culture incubation. Again in contrast to calcite, "CCP5" and "CCP10" were found to exhibit osteogenic activity and induce the expression of alkaline phosphatase gene in SaOS-2 cells as well as in human mesenchymal stem cells (MSC). In vivo studies in rats, using PLGA microspheres inserted in the muscles of the back of the animals, revealed that the encapsulated "CCP10" is not only biocompatible but also supports the regeneration at the implant region. We conclude that ACC containing small amounts of vaterite has osteogenic potential and offers superior properties compared to the biologically inert calcite with respect to a potential application as a scaffold material for bone implants.

Biocalcite, a multifunctional inorganic polymer: Building block for calcareous sponge spicules and bioseed for the synthesis of calcium phosphate-based bone

Calcium carbonate is the material that builds up the spicules of the calcareous sponges. Recent results revealed that the calcium carbonate/biocalcite-based spicular skeleton of these animals is formed through an enzymatic mechanism, such as the skeleton of the siliceous sponges, evolutionarily the oldest animals that consist of biosilica. The enzyme that mediates the calcium carbonate deposition has been identified as a carbonic anhydrase (CA) and has been cloned from the calcareous sponge species Sycon raphanus. Calcium carbonate deposits are also found in vertebrate bones besides the main constituent, calcium phosphate/hydroxyapatite (HA). Evidence has been presented that during the initial phase of HA synthesis poorly crystalline carbonated apatite is deposited. Recent data summarized here indicate that during early bone formation calcium carbonate deposits enzymatically formed by CA, act as potential bioseeds for the precipitation of calcium phosphate mineral onto bone-forming osteoblasts. Two different calcium carbonate phases have been found during CA-driven enzymatic calcium carbonate deposition in in vitro assays: calcite crystals and round-shaped vaterite deposits. The CA provides a new target of potential anabolic agents for treatment of bone diseases; a first CA activator stimulating the CA-driven calcium carbonate deposition has been identified. In addition, the CA-driven calcium carbonate crystal formation can be frozen at the vaterite state in the presence of silintaphin-2, an aspartic acid/glutamic acid-rich sponge-specific protein. The discovery that calcium carbonate crystals act as bioseeds in human bone formation may allow the development of novel biomimetic scaffolds for bone tissue engineering. Na-alginate hydrogels, enriched with biosilica, have recently been demonstrated as a suitable matrix to embed bone forming cells for rapid prototyping bioprinting/3D cell printing applications.