Date Published: September 19, 2019
Publisher: Public Library of Science
Author(s): Hiroo Itohiya, Yuji Matsushima, Satoshi Shirakawa, Sohtaro Kajiyama, Akihiro Yashima, Takatoshi Nagano, Kazuhiro Gomi, Amitava Mukherjee.
Rapid progress has been made in terms of metal nanoparticles studied in numerous fields. Metal nanoparticles have also been used in medical research, and antibacterial properties and anticancer effects have been reported. However, the underlying mechanism responsible for these effects has not been fully elucidated. Therefore, the present study focused on platinum nanoparticles (PtNPs) and examined their antibacterial properties and functional potential for decomposing organic matter, considering potential applications in the dental field. PtNPs were allowed to react with dental-related bacteria (Streptococcus mutans; Enterococcus faecalis, caries; Porphyromonas gingivalis, and endodontic and periodontal lesions). Antibacterial properties were evaluated by measuring colony formation. In addition, PtNPs were allowed to react with albumin and lipopolysaccharides (LPSs), and the functional potential to decompose organic matter was evaluated. All evaluations were performed in vitro. Colony formation in all bacterial species was completely suppressed by PtNPs at concentrations of >5 ppm. The addition of PtNPs at concentrations of >10 ppm significantly increased fragmentation and decomposition. The addition of PtNPs at concentrations of >125 pico/mL to 1 EU/mL LPS resulted in significant amounts of decomposition and elimination. The results revealed that PtNPs had antibacterial effects against dental-related bacteria and proteolytic potential to decompose proteins and LPS, an inflammatory factor associated with periodontal disease. Therefore, the use and application of PtNPs in periodontal and endodontic treatment is considered promising.
Metal nanoparticles, which are obtained by converting metals into fine particles (< 100 nm in diameter), have a large surface area and exhibit properties that differ from those of bulk metals because of the quantum size effect . Metal nanoparticles have been studied in numerous fields, such as chemistry, biology, materials science, and medicine, and rapid advances have been reported in recent years [2–6]. The antibacterial characteristics of metal nanoparticles (silver, zinc oxide, copper oxide, etc.) have previously been reported [7–12]. In particular, silver nanoparticles (AgNPs) have attracted much interest in many fields due to their excellent broad-spectrum antibacterial activity [9,13]. In contrast, gold nanoparticles (AuNPs) are commonly evaluated for use in biosensing or drug delivery applications, given that these particles are inert and highly stable . Antibacterial AuNPs are produced by coating AuNPs with organic molecules that have antibacterial properties . In this experiment, 100 ppm PtNPs (particle size: 2–19 nm) solution, which was used as a stock solution, were obtained from the manufacturer (Ceramics Craft Co. Ltd. Shizuoka, Japan). This PtNPs were made directly from platinum by irradiation with infrared pulsed laser in liquid. The storage of PtNPs was kept in the dark at room temperature avoiding direct sunlight as the stock solution. Moreover, when using for experiment, the stock solution was diluted in sterile purified water and used each time. The PtNPs were examined by transmission electron microscope (JEM-1200EX II, JEOL, Tokyo, Japan) Fig 1 shows a transmission electron micrograph of PtNPs used in this experiment. Aqueous solutions of nano-sized metal particles reportedly exhibit more potent antibacterial activity as particle size enters the nanometer range . In particular, AgNPs are known to have excellent antibacterial effects, but they are also known to possess strong cytotoxicity . Recently, metal NPs have attracted much attention not only due to their antibacterial properties but also because of their application in other areas of medicine, such as cancer treatment . This study focused on platinum, a stable metal that does not tend to induce allergic reactions, and investigated the effects of platinum nanoparticles. In this study, we used the PtNPs, which were made directly from platinum by irradiation with infrared pulsed laser in liquid according to the Liquid phase laser ablation method . When PtNPs solutions were added to S. mutans, E. faecalis, and P. gingivalis adjusted to 1 × 104 cells/mL, clear antibacterial properties were observed at concentrations of > 5 ppm. In a similar experiment that was previously reported, nanoplatinum particles (2–19 nm) were allowed to react for 15 minutes. These nanoparticles exhibited the same levels of antibacterial activity as that reported in our study, even though the bacterial strains were different [31,32]. The reaction time of the solution used and platinum particle size were almost the same, and the results obtained were similar to those reported in our study. Kanieczny et al. conducted a study to compare the antibacterial properties against gram-positive and gram-negative bacteria using PtNPs. The results showed a stronger antibacterial effect against gram-negative bacteria  because of the thin cell wall of gram-negative bacteria. In addition, Rosenberg et al. have also reported on antibacterial activity against E. coli, a gram-negative bacterium . In this study, PtNPs demonstrated strong antibacterial activity not only against P. gingivalis, a gram-negative bacterium, but also against S. mutans and E. faecalis, which are gram-positive bacteria. While, it is inferred that the antibacterial effect of NPs on anaerobic bacteria of oral origin is lower than that on aerobic bacteria. It is considered that this is because the antibacterial property is controlled by the availability of oxygen and the particle size . These interactions may have shown similar antibacterial activity to P. gingivalis, S. mutans and E. faecalis.
PtNPs have been shown to mediate antibacterial effects related to caries, endodontic lesions, and periodontal diseases. PtNPs also exhibit functional potential to decompose proteins and strong effectiveness against LPS, the cell wall constituent of gram-negative bacteria. However, this study was limited to only in vitro findings, and there is a necessity to further investigate whether similar results can be obtained in the clinic.