Hydroxyapatite (HA) has many applications in medicine, dentistry, diagnosis, drug delivery systems, sewage treatment, bone remodeling, concentrating bacteria, covering implants, and antibacterial activity. Despite the numerous current applications of calcium phosphate compounds, particularly HA, their producing methods are being investigated to find the best processes. Several chemical and biological methods are used in calcium phosphate compounds synthesis. Researches have shown that compared to micrometer models, nanostructured HA has higher mechanical features and better biocompatibility in the human body. These properties optimize when nanometer components of HA are in similar size and shape with the least agglomerations. Biomineralization by microorganisms, which is a bacterial route, is a recent HA synthesis method. This paper is a review on the biosynthesis of HA emphasizing microbial methods. In this method, some bacteria and mold could be used in the nanometer production of HA. This type of bacterium commonly has a high amount of alkaline phosphatase enzymes. Desirable similarity to natural HA in the human body is the noticeable features of bacterial HA. Uniformity in the shape and size of synthesized particles that have the same crystallization is of other merits. Producing bacterial HA is easily reachable, one-step, inexpensive, harmless, and with high purity, and contrary to chemical synthesis, does not need heat treatment and precise pH adjustment.

Background: The unique physicochemical properties of nano-metals compared to non-nano materials has led to many investigations in recent decades. Nano-Titanium dioxide has been chiefly applied in novel implant forms. In addition, the structures of Selenium-containing compounds play a critical role in the biological activity of these elements. According to recent studies, nano-Selenium is not only less toxic but also has higher biological activity than Selenium ions such as Se4+ and Se6+. Methods: Researchers have turned to green nano-structure synthesis due to drawbacks of chemical techniques. This article presents a literature review on recent advances in microbial synthesis methods to produce nano-Selenium and nano-Titanium dioxide. Results: Despite numerous research articles on green nanoparticle production, little information has been provided on the microbial formation of Titanium and Selenium nanoparticles. This review article focuses on the possible mechanism of nano-Selenium production by Selenium respiration. Although there are reports of microbial synthesis of these particles, their production by probiotic bacteria is of great value. Conclusion: Considering the compatibility of probiotic bacteria with the immune system and their tremendous applications in medicine, it is suggested to use them in combination with nano-Titanium dioxide and nano-Selenium for various ends such as implant scaffolds and food additives, respectively.

In recent years’ green synthesis has received more attention than chemical synthesis due to its eco-friendly and compatibility. In this study Bacillus subtilis and Bacillus coagulans, two potential probiotics that produce antimicrobial agents were used to synthesize nano-hydroxyapatite and nano-calcite. Grown Bacillus in medium culture containing insoluble calcium phosphate produced nano-hydroxyapatite and nano-crystalline calcite. The nano-hydroxyapatite was purified from nano-calcite by heat treatment and washing with a 200 nm filter. The structures, characteristics, and elemental analysis of nano-sized material were studied by Fourier Transform Infrared Spectroscopy, X-Ray Diffraction, Scanning Electron Microscope, Ultraviolet-visible spectroscopy, Energy dispersive X-Ray, and X-Ray Fluorescence. The results showed that hydroxyapatite is made only in a medium containing insoluble calcium phosphate sublimated with urea which is induced phosphatase and urease. Here, for the first time, the braided bacterial nano-hydroxyapatite similar to the bone structure was made in the medium which induced the production of urease and phosphatase (Maximum 99 U/L) enzymes, and the particle size was less than 100 nm. The ratio of calcium to phosphate in crude hydroxyapatite and calcite crystal particles made by Bacillus coagulans was 2.9, however, this ratio for pure hydroxyapatite was 1.7. Since the particles are made by antibacterial potential probiotics, the biological production of these particles makes them a suitable candidate to be used in toothpaste, bone regeneration, and sanitation products.