Control of Bacillus cereus in foods by Rhodomyrtus tomentosa (Ait.) Hassk. Leaf extract and its purified compound

The Diverse Activities and Mechanisms of the Acylphloroglucinol Antibiotic Rhodomyrtone: Antibacterial Activity and Beyond

Background/Objectives: The rose myrtle Rhodomyrtus tomentosa is a medicinal plant used in traditional Asian medicine. The active compound in R. tomentosa leaf extracts is rhodomyrtone, a chiral acylphloroglucinol. Rhodomyrtone exhibits an impressive breadth of activities, including antibacterial, antiviral, antiplasmodial, immunomodulatory, and anticancer properties. Its antibacterial properties have been extensively studied. Methods: We performed a comprehensive literature review on rhodomyrtone and summarized the current knowledge about this promising acylphloroglucinol antibiotic and its diverse functions in this review. Results: Rhodomyrtone shows nano to micromolar activities against a broad range of Gram-positive pathogens, including multidrug-resistant clinical isolates, and possesses a unique mechanism of action. It increases membrane fluidity and creates hyperfluid domains that attract membrane proteins prior to forming large membrane vesicles, effectively acting as a membrane protein trap. This mechanism affects a multitude of cellular processes, including cell division and cell wall synthesis. Additionally, rhodomyrtone reduces the expression of inflammatory cytokines, such as TNF-α, IL-17A, IL1β, and IL8. Generally showing low toxicity against mammalian cells, rhodomyrtone does inhibit the proliferation of cancer cell lines, such as epidermal carcinoma cells. The primary mechanism behind this activity appears to be the downregulation of adhesion kinases and growth factors. Furthermore, rhodomyrtone has shown antioxidant activity and displays cognitive effects, such as decreasing depressive symptoms in mice. Conclusions: Rhodomyrtone shows great promise as therapeutic agent, mostly for antibacterial but also for diverse other applications. Yet, bottlenecks such as resistance development and a better understanding of mammalian cell toxictiy demand careful assessment.

Potential applications of Rhodomyrtus tomentosa leaf extract as natural anti-staphylococcal additive in food systems: Efficacy and in vivo safety evaluation

This work aimed to explore the potential use of Rhodomyrtus tomentosa ethanol leaf extract (RTEL) as an alternative food preservative agent for controlling the growth of Staphylococcus aureus. Antibacterial activities against food-isolated S. aureus were performed using disc diffusion and broth microdilution assays, followed by evaluating in vivo subacute oral toxicity of the extract. Salad dressing was used as a food model to study bactericidal properties and consumer acceptability. RTEL remarkably inhibited S. aureus with minimum inhibitory concentrations (MICs) ranging from 7.81-62.5 µg/mL. Repeated oral doses (5, 50, and 300 mg/kg RTEL) for 28 days did not affect any of the measured toxicity parameters. The no-observed-adverse-effect-level (NOAEL) of RTEL was noted as more than 300 mg/kg body weight/day. The utilization of RTEL (12.5 mg/mL) in the vinaigrette salad dressing did not affect the consumer acceptability of the product, remarkably killed the pathogen within 3-9 h of exposure. The results indicated that RTEL is safe and effective as a natural anti-staphylococcal controlling agent that could be utilized in food systems. Further work is required on the effects of enterotoxin production, an important virulence factor of S. aureus responsible for food-borne disease.

Transferosomes stabilized hydrogel incorporated rhodomyrtone-rich extract from Rhodomyrtus tomentosa leaf fortified with phosphatidylcholine for the management of skin and soft-tissue infections

Rhodomyrtus tomentosa leaf (RT)-incorporated transferosomes were developed with lecithin and cholesterol blends with edge activators at different ratios. RT-transferosomes were characterized and employed in transferosomal gel formulations for the management of skin and soft-tissue infections. The optimized formulation entrapped up to 81.90 ± 0.31% of RT with spherical vesicles (405.3 ± 2.0 nm), polydispersity index value of 0.16 ± 0.08, and zeta potential of - 61.62 ± 0.86 mV. Total phenolic and flavonoid contents of RT-transferosomes were 15.65 ± 0.04 μg GAE/g extract and 43.13 ± 0.91 μg QE/g extract, respectively. RT-transferosomes demonstrated minimum inhibitory and minimum bactericidal concentrations at 8-256 and 64-1024 μg/mL, respectively. Free radical scavenging assay showed RT-transferosomes with high scavenging activity against DPPH and ABTS radicals. Moreover, RT-transferosomes demonstrated moderate activity against mushroom tyrosinase, with IC50 values of 245.32 ± 1.32 μg/mL. The biocompatibility results against L929 fibroblast and Vero cells demonstrated IC50 at 7.05 ± 0.17 and 4.73 ± 0.13 μg/mL, respectively. In addition, nitric oxide production significantly decreased by 6.78-88.25% following the treatment with 31.2-500 ng/mL RT-transferosomes (p < 0.001). Furthermore, the freeze-thaw stability study displayed no significant change in stability in the sedimentation and pH of gel fortified with RT-transferosomes. The results suggested that RT-transferosome formulation can be effectively employed as natural biomedicines for scar prevention and the management of skin soft-tissue infections.

Effects of cinnamaldehyde against planktonic bacteria and biofilm formation of Shigella flexneri

Cinnamaldehyde (CA) has demonstrated anti-inflammatory, anti-tumor and anti-cancer activities; Its antimicrobial and antibiofilm actions against Shigella flexneri, on the other hand, have not been investigated. Sh. flexneri is a gram-negative foodborne pathogen that can be widely found in nature and some industrial production environments. In this current research, our aim was to examine the influences of CA on planktonic bacteria and biofilm formation. The minimum inhibitory concentration (MIC) of CA against Sh. flexneri strain was 100 μg/mL, while bacteria treated with CA showed a longer lag phase compared with the untreated control. CA effectively inactivated the Sh. flexneri in LB broth and fresh lettuce juice. CA treatment resulted in cell membrane permeability changes and dysfunction, as proven by cell membrane depolarization, decreased intracellular ATP concentration. In addition, CA was also discovered to increase the level of reactive oxygen species (ROS) in cells, and induce morphological changes in cells. Crystal violet staining showed that the biomass of biofilm was decreased significantly with CA in 24 h. Light microscopy and field emission scanning electron microscopy (FESEM) observations demonstrated decreased biofilm adhesion and destruction of biofilm architecture after treatment with CA. These findings indicated that CA acts as a natural bacteriostatic agent to control Sh. flexneri in food processing and production.

Antimicrobial Activity and Action Mechanism of Thymoquinone against Bacillus cereus and Its Spores

In this study, thymoquinone (TQ), a natural active substance, was investigated for its antibacterial activity against Bacillus cereus, and its inhibitory effect on B. cereus in reconstituted infant formula (RIF) was evaluated. In addition, the inhibitory effect of TQ on B. cereus spore germination was explored. The minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) of TQ against eight B. cereus strains ranged from 4.0 to 8.0 μg/mL, whereas B. cereus treated with TQ displayed a longer lag phase than the untreated control. TQ exerted a good bactericidal effect on B. cereus in Luria–Bertani broth. In addition, TQ obviously reduced the intracellular ATP concentration of B. cereus, which caused depolarization of the cell membrane, increased the intracellular reactive oxygen species level, impaired the cell morphology, and destroyed proteins or inhibited proteins synthesis. This provides a mechanism for its bacteriostatic effect. TQ also inactivated B. cereus growth in RIF. Moreover, reverse transcription–quantitative polymerase chain reaction illustrated that TQ downregulated the transcription of genes related to hemolysin, non-hemolytic enterotoxin, enterotoxin, and cytotoxin K. Meanwhile, TQ displayed the ability to inhibit the germination of B. cereus spores. These findings indicate that TQ, as an effective natural antimicrobial preservative, has potential applications in controlling food contamination and foodborne diseases caused by B. cereus.

Facile deposition of biogenic silver nanoparticles on porous alumina discs, an efficient antimicrobial, antibiofilm, and antifouling strategy for functional contact surfaces

Surface modification is an emerging strategy for the design of contact materials. Fabricated alumina discs were functionalized by deposition of biogenic silver nanoparticles. The surfaces were characterized for physico-chemical, antibacterial and antibiofilm properties against microbial pathogens. The surface demonstrated improved hydrophobicity and a surface silver nanoparticle content of 6.4 w%. A reduction of more than 99.9% in CFU mL^-i was observed against the Gram-positive and Gram-negative bacteria tested, with >90% reduction of the fungal isolate. After 4 h, microbial adhesion was reduced by >99.9 and 90% for Escherichia coli and Staphylococcus aureus, respectively. Scanning electron micrographs further revealed a biofilm reduction. Cell viability tests indicated a bioincompatibility higher than 80% with Caco-2 and HaCaT cell lines after 48 h contact. The results suggest that deposition of biogenic silver nanoparticles on the surface of contact materials could be employed as a strategy to prevent biofilm formation.

Antimicrobial Activity of Methanolic and Aqueous Extract of Rhodomyrtus tomentosa Leaves against Staphylococcus aureus and Escherichia coli

Food poisoning is one of the riskiest illnesses caused by an organism like bacteria, viruses and parasites, which can contaminate food at any stage of food production. Most reports are associated with bacterial contamination through the food preparation process. Researcher nowadays attempts to discover a safer agent to use in food preparation as a food preservative agent. Thus, the potential of Rhodomyrtus tomentosa leaves as an antimicrobial agent was explored in this study. It was tested against two bacteria which are Staphylococcus aureus (gram-positive) and Eschericia coli (gram-negative). Antimicrobial activity of methanol and aqueous R. tomentosa leaves extract was compared using gram staining, bacterial identification, antimicrobial sensitivity testing (AST), minimal inhibitory concentration (MIC), minimal bactericidal concentration (MBC) and statistical analysis. The disc diffusion technique was used for AST where the mean area of inhibition methanol and aqueous extract against S. aureus was 18 mm and 16 mm respectively, whereas there was no area of inhibition for E. coli. MIC was done to determine the minimum concentration of the extract needed to inhibit the bacteria. After that, MBC was performed to confirm the previous MIC result. Statistical analysis was done by conduct it through SPSS software program. Then, the result for S. aureus is obtained from finding its p-value. Statistically, a significant difference between the two bacteria was analyzed by using independent t-test. A result of p-value less than 0.05 was considered significantly different as one another. Statistical analysis showed no significant difference between methanol and aqueous extract on antimicrobial activity. In conclusion, both aqueous and methanol leaves R. tomentosa yielded similar antibacterial activity which susceptible to S. aureus and can be used safely toward foodborne pathogen.

Molecular Basis of Rhodomyrtone Resistance in Staphylococcus aureus

Rhodomyrtone (Rom) is a plant-derived broad-spectrum antibiotic active against many Gram-positive pathogens. A single point mutation in the regulatory farR gene (farR*) confers resistance to Rom in Staphylococcus aureus (RomR). The mutation in farR* alters the activity of the regulator, FarR*, in such a way that not only its own gene, farR*, but also the divergently transcribed farE gene and genes controlled by the global regulator, agr, are highly upregulated. Here, we show that mainly the upregulation of the fatty acid efflux pump FarE causes the RomR phenotype, as farE deletion in either the parent or the RomR strain (RomR ΔfarE) yielded hypersensitivity to Rom. Comparative lipidome analysis of the supernatant (exolipidomics) and the pellet fraction revealed that the RomR strain excreted about 10 times more phospholipids (PGs) than the parent strain or the ΔfarE mutants. Since the PG content in the supernatant (2,244 ng/optical density [OD]) was more than 100-fold higher than that of fatty acids (FA), we assumed that PG interacts with Rom, thereby abrogating its antimicrobial activity. Indeed, by static and dynamic light scattering (SLS and DLS) and isothermal titration calorimetry (ITC) analyses, we could demonstrate that both PG and Rom were vesicular and reacted with each other in milliseconds to form a 1:1.49 [Rom-PG(32:0), where PG(32:0) is PG with C32:0 lipids] complex. The binding is entropically driven and hence hydrophobic and of low specificity in nature. Our results indicate that the cytoplasmic membrane is the actual target of Rom, which is also in agreement with Rom's induced rapid collapse of the membrane potential and decreased membrane integrity. IMPORTANCE Antibiotic resistance is a growing public health problem, and alternative antibiotics are urgently needed. Rhodomyrtone (Rom), an antimicrobial compound originally isolated from Rhodomyrtus tomentosa, is active against multidrug-resistant Gram-positive pathogens. However, Rom-resistant (RomR) mutants occur with low frequency. In this study, we unraveled the underlying resistance mechanism, which is based on a point mutation in the farR regulator gene, causing overexpression of FarE, which most likely acts as a phospholipid (PG) efflux pump, as large amounts of PG were found in the supernatant and the pellet fraction. We show that PG can bind to Rom, thereby abrogating its antimicrobial activity. The direct interaction of Rom with PG suggests that Rom's actual target is the cytoplasmic membrane. Antibiotics that interact with PG are rare. Since Rom can be chemically synthesized, it serves as a lead compound for synthesis of improved variants.

(-)-Tetrahydroberberrubine∙acetate accelerates antioxidant potential and inhibits food associated Bacillus cereus in rice

A protoberberine alkaloid, (-)-tetrahydroberberrubine∙acetate (THBA) was assessed for its antioxidant potential and ability to inhibit the growth of a food hazard bacterium Bacillus cereus in vitro and in situ. THBA displayed significant and dose-dependent cellular antioxidant potential against hydrogen peroxide-induced oxidative stress in NIH 3T3 fibroblast cells and decreased the ROS levels as well as increased the expression levels of SOD1 and SOD2 enzymes. The inhibitory spectrum of THBA confirmed its mechanistic role in the disruption of the membrane integrity of B. cereus as evidenced by the results of time-inactivation, cell membrane integrity, NPN membrane uptake, membrane potential, and electron microscopy analyses. Moreover, THBA inhibited biofilm formation by B. cereus and disrupted pre-established biofilms on a glass surface. Furthermore, THBA was also able to inhibit B. cereus in raw rice with a significant amount of reduction in CFU counts, suggesting its potential role as a natural antioxidant and antimicrobial agent.

Inactivation of farR Causes High Rhodomyrtone Resistance and Increased Pathogenicity in Staphylococcus aureus

Rhodomyrtone (Rom) is an acylphloroglucinol antibiotic originally isolated from leaves of Rhodomyrtus tomentosa. Rom targets the bacterial membrane and is active against a wide range of Gram-positive bacteria but the exact mode of action remains obscure. Here we isolated and characterized a spontaneous Rom-resistant mutant from the model strain Staphylococcus aureus HG001 (Rom^R) to learn more about the resistance mechanism. We showed that Rom-resistance is based on a single point mutation in the coding region of farR [regulator of fatty acid (FA) resistance] that causes an amino acid change from Cys to Arg at position 116 in FarR, that affects FarR activity. Comparative transcriptome analysis revealed that mutated farR affects transcription of many genes in distinct pathways. FarR represses for example the expression of its own gene (farR), its flanking gene farE (effector of FA resistance), and other global regulators such as agr and sarA. All these genes were consequently upregulated in the Rom^R clone. Particularly the upregulation of agr and sarA leads to increased expression of virulence genes rendering the Rom^R clone more cytotoxic and more pathogenic in a mouse infection model. The Rom-resistance is largely due to the de-repression of farE. FarE is described as an efflux pump for linoleic and arachidonic acids. We observed an increased release of lipids in the Rom^R clone compared to its parental strain HG001. If farE is deleted in the Rom^R clone, or, if native farR is expressed in the Rom^R strain, the corresponding strains become hypersensitive to Rom. Overall, we show here that the high Rom-resistance is mediated by overexpression of farE in the Rom^R clone, that FarR is an important regulator, and that the point mutation in farR (Rom^R clone) makes the clone hyper-virulent.

Structures and Bioactive Properties of Myrtucommulones and Related Acylphloroglucinols from Myrtaceae

Myrtaceae are a group of plants that include a number of renowned species used in ethnomedicine in many areas worldwide. Their valuable therapeutic properties have stimulated a fruitful research activity addressed to the identification of the bioactive components of their extracts yielding a great diversity of terpenes; polyphenols; and other exclusive products. Among the latter, starting with the discovery of myrtucommulone A from myrtle (Myrtus communis), a series of structurally-related acylphloroglucinol compounds have been characterized from several species that represent the basic active principles to be considered in view of possible drug development. Aspects concerning chemical and biological properties of these products are reviewed in the present paper.

Apoptosis and antimigration induction in human skin cancer cells by rhodomyrtone

Rhodomyrtone is a bioactive compound extracted from Rhodomyrtus tomentosa leaves. It has been used as a traditional herb medicine for many years. Rhodomyrtone exhibits antibacterial activity, anti-inflammatory and antioxidant activities. However, the anticancer activity of rhodomyrtone has not been previously reported. The present study investigated the anticancer effect of rhomyrtone on human epidermoid carcinoma A431 cells. The cytotoxic and antiproliferative effects of rhodomyrtone on A431 cells were investigated by an MTT assay. Cell morphological alterations and apoptotic cells were observed with Hoechst 33342 staining following rhodomyrtone treatment. Flow cytometry and western blotting were performed to detect cell cycle and apoptosis induction. The results demonstrated that rhodomyrtone inhibited proliferation of A431 cells in a dose-dependent manner with IC⁵⁰ value of 8.04±0.11 µg/ml. The results also indicated that rhodomyrtone increased chromatin condensation, nuclear fragmentation and apoptotic bodies in treated A431 cells in a time-dependent manner. Apoptosis was also induced through the activation of caspase-7 and poly (ADP-Ribose) polymerase cleavage. Flow cytometry analysis revealed that rhodomyrtone induced cell cycle arrest at the G1 phase. Notably, the non-toxic concentration of rhodomyrtone markedly inhibited A431 cell migration in a dose- and time-dependent manner. These finding suggested that rhodomyrtone may be used as an anticancer agent for human skin cancer.

Rhodomyrtone (Rom) is a membrane-active compound

Particularly in Asia medicinal plants with antimicrobial activity are used for therapeutic purpose. One such plant-derived antibiotic is rhodomyrtone (Rom) isolated from Rhodomyrtus tomentosa leaves. Rom shows high antibacterial activity against a wide range of Gram-positive bacteria, however, its mode of action is still unclear. Reporter gene assays and proteomic profiling experiments in Bacillus subtilis indicate that Rom does not address classical antibiotic targets like translation, transcription or DNA replication, but acts at the cytoplasmic membrane. In Staphylococcus aureus, Rom decreases the membrane potential within seconds and at low doses, causes release of ATP and even the excretion of cytoplasmic proteins (ECP), but does not induce pore-formation as for example nisin. Lipid staining revealed that Rom induces local membrane damage. Rom's antimicrobial activity can be antagonized in the presence of a very narrow spectrum of saturated fatty acids (C15:0, C16:0, or C18:0) that most likely contribute to counteract the membrane damage. Gram-negative bacteria are resistant to Rom, presumably due to reduced penetration through the outer membrane and its neutralization by LPS. Rom is cytotoxic for many eukaryotic cells and studies with human erythrocytes showed that Rom induces eryptosis accompanied by erythrocyte shrinkage, cell membrane blebbing, and membrane scrambling with phosphatidylserine translocation to the erythrocyte surface. Rom's distinctive interaction with the cytoplasmic membrane reminds on the amphipathic, alpha-helical peptides, the phenol-soluble modulins (PSMs), and renders Rom an important tool for the investigation of membrane physiology.

Tomentodione E, a new sec-pentyl syncarpic acid-based meroterpenoid from the leaves of Rhodomyrtus tomentosa

A new meroterpenoid, tomentodione E (1), along with four known ones (2-5) were isolated from the leaves of Rhodomyrtus tomentosa. Their structures were elucidated based on extensive spectroscopic data as well as computational methods. Compound 1 represents the first example of meroterpenoid possessing a sec-pentyl syncarpic acid motif coupled with a caryophyllene. Compounds 1-4 were evaluated for their in vitro antiviral activity against respiratory syncytial virus (RSV) with cytopathic effect (CPE) reduction assay, and 2 showed potent in vitro anti-RSV effect.

Use of Rhodomyrtus tomentosa ethanolic leaf extract for the bio-control of Listeria monocytogenes post-cooking contamination in cooked chicken meat

Controlling foodborne pathogen in ready-to-eat food is important in food safety. The present study accessed the potential use of Rhodomyrtus tomentosa ethanolic leaf extract as a bio-control agent against Listeria monocytogenes in cooked chicken meat model system. The antilisterial activity of the plant extract was better under microwave condition and enhanced as storage temperature increased from 4 to 37 °C. The extract could reduce L. monocytogenes numbers at low (10⁴ CFU/g) and high (10⁶ CFU/g) inoculum levels in cooked chicken by both rinse and injection application methods. A 5 min rinse in 8% w/v R. tomentosa extract reduced the bacterial number by ≥2-log before storage and ≥3-log after storage at 4 °C for 5 days. Injection with 0.4% w/w R. tomentosa extract resulted in approximately 2-log reduction in the cell numbers both before and after storage at 4 °C for 5 days. Five minutes rinse in the extract bath demonstrated better sensory preferences which were not significantly different from the control. Addition of black pepper powder to the extract rinsed samples improved odour but not appearance, colour, and texture preferences. Rhodomyrtus tomentosa extract was significantly effective for the bio-control of L. monocytogenes contaminations in cooked chicken meat model. The extract was observed as a potent bio-additive agent to control contaminations from L. monocytogenes and ensure safety in ready-to-eat meat.

Effects of Rhodomyrtus tomentosa Extract on Killing Activity of Human Neutrophils and Membrane Integrity of Enterohaemorrhagic Escherichia coli O157:H7

Enterohaemorrhagic Escherichia coli (E. coli) O157:H7 is one of the most virulent causative agents of foodborne disease. Use of antibiotics for the treatment against E. coli O157:H7 infection leads to hemolytic uremic syndrome. The present study evaluated the potential of ethanolic leaf extract of a medicinal plant, Rhodomyrtus tomentosa in enhancing the killing activity of human neutrophils against E. coli O157:H7. In addition, the effects of the extract on membrane permeability of the organisms were studied. In the killing assay, percentage survival of the bacterial cells after being exposed to human neutrophils in the presence of various concentrations of the extract were determined. At 45 min, percentage survival of E. coli O157:H7 and E. coli ATCC 25922 after treated with neutrophils in the presence of the extract at 125-250 µg/mL was 58.48%-50.28% and 69.13%-35.35%, respectively. Furthermore, upon treatment with R. tomentosa at 250 µg/mL uptake of crystal violet by E. coli O157:H7 and E. coli ATCC 25922 was increased to 40.07% and 36.16%, respectively. Therefore, it is suggested that the extract exhibited dual effects as immunostimulant and membrane permeabilizing agent perhaps resulted in enhancing the killing activity of neutrophils against the organisms.

Effects of Rhodomyrtus tomentosa Leaf Extract on Staphylococcal Adhesion and Invasion in Bovine Udder Epidermal Tissue Model

Bovine mastitis is one of the most important infectious diseases in dairy herds, and staphylococci are the most important etiologic agents of this disease. Antibiotics and chemical agents used in livestock for prevention and cure of the disease can accumulate in milk and give rise to food safety concerns. Rhodomyrtus tomentosa leaf extract was studied as an alternative approach to reduce the bacterial infections. The ethanolic extract of this plant demonstrated antibacterial activity with minimum inhibitory concentration (MIC) values as low as 16–64 μg/mL against staphylococcal isolates. In addition, the extract had an effect on the bacterial cell surface properties by increasing its hydrophobicity in a concentration dependent manner. To further extend the antibacterial efficacy, silver nanoparticles synthesized with the extract, a pure rhodomyrtone, and liposomal encapsulated rhodomyrtone were applied and their inhibitory effects on bacterial adhesion and invasion were determined by ex vivo study in a bovine udder epidermal tissue model. These agents exerted remarkable antibacterial activity against staphylococci and decreased the adhesion of the bacterial cells to the tissues. These results supported that R. tomentosa ethanolic extract could be applied as an alternative agent for bovine udder care in dairy farms.

Potential Bio-Control Agent from Rhodomyrtus tomentosa against Listeria monocytogenes

Listeria monocytogenes is an important foodborne pathogen implicated in many outbreaks of listeriosis. This study aimed at screening for the potential use of Rhodomyrtus tomentosa ethanolic leaf extract as a bio-control agent against L. monocytogenes. Twenty-two L. monocytogenes isolates were checked with 16 commercial antibiotics and isolates displayed resistance to 10 antibiotics. All the tested isolates were sensitive to the extract with inhibition zones ranging from 14 to 16 mm. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values ranged from 16 to 32 µg/mL and 128 to 512 µg/mL, respectively. Time-kill assay showed that the extract had remarkable bactericidal effects on L. monocytogenes. The extract at a concentration of 16 µg/mL reduced tolerance to 10% NaCl in L. monocytogenes in 4 h. Stationary phase L. monocytogenes cells were rapidly inactivated by greater than 3-log units within 30 min of contact time with R. tomentosa extract at 128 µg/mL. Electron microscopy revealed fragmentary bacteria with changes in the physical and morphological properties. Our study demonstrates the potential of the extract for further development into a bio-control agent in food to prevent the incidence of L. monocytogenes contamination.

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.

Acetone Extract from Rhodomyrtus tomentosa: A Potent Natural Antioxidant

Rhodomyrtus tomentosa (Myrtaceae) has been employed in traditional Thai medicine to treat colic diarrhoea, dysentery, abscesses, haemorrhage, and gynaecopathy. In addition, it has been used to formulate skin-whitening, anti-aging and skin beautifying agents. Ethnomedical activities of this plant may be due its antioxidant property. Hence, the aim of this study was to evaluate both in vitro and in vivo antioxidant activities of R. tomentosa leaf extract. In vitro antioxidant activity of the extract was assessed by lipid peroxidation inhibition capacity, ferric reducing antioxidant power, and metal chelating activity. R. tomentosa extract demonstrated its free radical scavenging effects in concentration dependent manner. In vivo antioxidant activity of the extract was conducted in Swiss Albino mice. Levels of thio-barbituric acid reactive substances (TBARS), glutathione (GSH), and the activities of antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) in blood, liver, and kidney were analyzed using microtitre plate photometer. Administration of CCl₄ caused significant increase in TBARS and decrease in GSH, SOD, CAT and GPx levels. In contrast, R. tomentosa extract (0.8 g/kg) effectively prevented these alterations and maintained the antioxidant status. The results suggest that R. tomentosa extract can serve as a potent antioxidant.

Anti Propionibacterium acnes activity of rhodomyrtone, an effective compound from Rhodomyrtus tomentosa (Aiton) Hassk. leaves

Propionibacterium acnes have been recognized as one of the main causative agents in pathogenesis of acne. Twenty one isolates of P. acnes isolated from acne lesions were screened for lipase and protease activity which are reported to be associated in acne and inflammation. Interestingly, all P. acnes isolates demonstrated lipase activity. Similarly, 90% of test P. acnes produced protease enzyme. Antibacterial activity of the ethanol extract of Rhodomyrtus tomentosa (Aiton) Hassk. leaves and rhodomyrtone, its principle compound were tested against P. acnes using broth macrodilution method. The MIC(90) values of the ethanol extract and rhodomyrtone were 32 and 0.5 μg/mL, respectively. The numbers of the bacterial cells were reduced at least 99% after treatment with the ethanol extract and rhodomyrtone within 72 and 24 h, respectively. Cytotoxicity test of the extract and rhodomyrtone was performed on human normal fibroblast. The IC(50) values of the ethanol extract and rhodomyrtone were 476 and more than 200 μg/mL, approximately 15 and 400 folds higher than the MIC(90) values indicating that both substances were very low cytotoxic which could be applied as topical therapeutic anti-acne agents.