Rhodomyrtone Accumulates in Bacterial Cell Wall and Cell Membrane and Inhibits the Synthesis of Multiple Cellular Macromolecules in Epidemic Methicillin-Resistant Staphylococcus aureus.

As the burden of antibacterial resistance worsens and treatment options become narrower, rhodomyrtone-a novel natural antibiotic agent with a new antibacterial mechanism-could replace existing antibiotics for the treatment of infections caused by multi-drug resistant Gram-positive bacteria. In this study, rhodomyrtone was detected within the cell by means of an easy an inexpensive method. The antibacterial effects of rhodomyrtone were investigated on epidemic methicillin-resistant Staphylococcus aureus. Thin-layer chromatography demonstrated the entrapment and accumulation of rhodomyrtone within the bacterial cell wall and cell membrane. The incorporation of radiolabelled precursors revealed that rhodomyrtone inhibited the synthesis of macromolecules including DNA, RNA, proteins, the cell wall, and lipids. Following the treatment with rhodomyrtone at MIC (0.5-1 µg/mL), the synthesis of all macromolecules was significantly inhibited (p ≤ 0.05) after 4 h. Inhibition of macromolecule synthesis was demonstrated after 30 min at a higher concentration of rhodomyrtone (4× MIC), comparable to standard inhibitor compounds. In contrast, rhodomyrtone did not affect lipase activity in staphylococci-both epidemic methicillin-resistant S. aureus and S. aureus ATCC 29213. Interfering with the synthesis of multiple macromolecules is thought to be one of the antibacterial mechanisms of rhodomyrtone.

In vivo safety assessment of rhodomyrtone, a potent compound, from Rhodomyrtus tomentosa leaf extract.

BACKGROUND: Rhodomyrtus tomentosa (Aiton) Hassk. has been traditionally used to relieve various diseases. Rhodomyrtone, a bioactive acylphloroglucinol compound isolated from the leaves of Rhodomyrtus tomentosa, has been scientifically evidenced as a potential antibacterial agent. This study aimed to assess safety of rhodomyrtone in both invertebrate and vertebrate models. MATERIAL AND METHODS: Safety of rhodomyrtone was determined in an invertebrate model, Galleria mellonella as well as vertebrate models including zebrafish (Danio rerio) and murine. In addition, toxicity to human erythrocytes was also measured. RESULTS: Treatment of Galleria mellonella with rhodomyrtone at 100 mg/kg body weight up to four days showed no visible toxic effects (100 % survival). In zebrafish embryo model, at least 80 % survival of embryos was demonstrated when treated with rhodomyrtone at 0.5 µg/mL for three days. Prior to clinical trial, it is a prerequisite that rhodomyrtone has to be evaluated for its biocompatibility with human blood components. The results displayed that rhodomyrtone at 256 µg/mL did not cause any observable human erythrocyte haemolysis. Furthermore, preclinical assessment of rhodomyrtone formulation justified potential applications of rhodomyrtone in humans. Oral toxicity testing in a mouse model indicated the absence of systemic toxicity when the animals received up to 5000 mg/kg body weight of rhodomyrtone formulation for a period of fourteen days. CONCLUSIONS: As the minimal inhibitory concentration of rhodomyrtone against most Gram-positive pathogens is 0.5-1 µg/mL, the results suggest that it should produce no toxic effects at concentrations used in human, thus support further development in pharmaceutical industries and public health applications.

Metallic nanoparticles augmented the antibacterial potency of Rhodomyrtus tomentosa acetone extract against Escherichia coli.

The present study was focused on the preparation of gold, silver, and gold-silver-alloy nanoparticles using Rhodomyrtus tomentosa acetone extract (RAE). The synthesized nanoparticles showed the surface plasmon resonance absorption peak corresponding to gold and silver nanoparticle. However, Au-Ag-Alloy nanoparticles showed the single peak between the peaks of AuNPs and AgNPs. TEM observation ascertained the shape and size of nanoparticles. FTIR results indicated the involvement of RAE for the synthesis and capping of nanoparticles. Study on antibacterial activity demonstrated the enhanced activity of RAE capped on silver and Au-Ag-Alloy nanoparticles against Escherichia coli.

Antibacterial mechanisms of rhodomyrtone against important hospital-acquired antibiotic-resistant pathogenic bacteria.

The antibacterial mechanisms of rhodomyrtone, a member of the acylphloroglucinols isolated from Rhodomyrtus tomentosa leaves, against important hospital-acquired antibiotic-resistant pathogenic bacteria were assessed. The results indicated that rhodomyrtone exhibited pronounced antibacterial activity against key antibiotic-resistant pathogens including epidemic meticillin-resistant Staphylococcus aureus (EMRSA), vancomycin-intermediate S. aureus and vancomycin-resistant enterococcal strains. The strains EMRSA-16, Enterococcus faecalis ATCC 29212 and VRE-3 demonstrated a significant decrease in survival ability after treatment with rhodomyrtone at 1× (0.5 µg ml(-1)), 2×, 4× and 8× MIC for 24 h. Moreover, the compound was observed in the cytoplasmic fraction of rhodomyrtone-treated S. aureus, and only a very fine band of the compound was seen following separation of the cell-wall and cell-membrane fractions of the treated cells. In addition, exposure of S. aureus to rhodomyrtone at 4×, 2× and 1× MIC for 24 h produced no significant effect on the bacterial cell membrane and cell lysis, suggesting that neither of these is the main target of rhodomyrtone action in these organisms. Stepwise isolation of the bacterial cells with increasing resistance to rhodomyrtone was not induced in either S. aureus or EMRSA-16 after 45 passages on Luria-Bertani agar supplemented with rhodomyrtone. In addition, in vitro toxicity of rhodomyrtone at 128× MIC on human erythrocytes was not observed. These results provide evidence to support therapeutic challenges of rhodomyrtone against Gram-positive pathogens.

Inhibition of staphyloxanthin biosynthesis in Staphylococcus aureus by rhodomyrtone, a novel antibiotic candidate.

Staphyloxanthin is the eponymous feature of the human pathogen Staphylococcus aureus, and the pigment promotes resistance to reactive oxygen species and host neutrophil-based killing. To probe the possible use of rhodomyrtone isolated from Rhodomyrtus tomentosa (Aiton) Hassk. leaves to inhibit pigment production in S. aureus, experiments were carried out to compare pigment production and the susceptibility of rhodomyrtone-treated S. aureus and untreated cells to oxidants in vitro. In addition, we observed the innate immune clearance of S. aureus after incubation with rhodomyrtone using an ex vivo assay system - human whole-blood survival. The results indicated that rhodomyrtone-treated S. aureus exhibited reduced pigmentation, and that rhodomyrtone treatment led to a dose-dependent increase in the susceptibility of the pathogen to H(2)O(2) and singlet oxygen killing. Consequently, the survival ability of the treated organisms decreased in freshly isolated human whole blood due to less carotenoid pigment to act as an antioxidant scavenger. Rhodomyrtone may be acting via effects on DnaK and/or σ(B), resulting in many additional effects on bacterial virulence.