bDue to their abilities to form strong biofilms, Staphylococcus aureus and Staphylococcus epidermidis are the most frequently isolated pathogens in persistent and chronic implant-associated infections. As biofilm-embedded bacteria are more resistant to antibiotics and the immune system, they are extremely difficult to treat. Therefore, biofilm-active antibiotics are a major challenge. Here we investigated the effect of the lantibiotic gallidermin on two representative biofilm-forming staphylococcal species. Gallidermin inhibits not only the growth of staphylococci in a dose-dependent manner but also efficiently prevents biofilm formation by both species. The effect on biofilm might be due to repression of biofilm-related targets, such as ica (intercellular adhesin) and atl (major autolysin). However, gallidermin's killing activity on 24-h and 5-day-old biofilms was significantly decreased. A subpopulation of 0.1 to 1.0% of cells survived, comprising "persister" cells of an unknown genetic and physiological state. Like many other antibiotics, gallidermin showed only limited activity on cells within mature biofilms.

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The anti-staphylococcal activity of an ethanol extract of Rhodomyrtus tomentosa and its pure compound, rhodomyrtone, as well as their effects on staphylococcal biofilm formation and biofilm-grown cells were assessed. MIC and minimal bactericidal concentration values of the ethanol extract and rhodomyrtone against planktonic cultures and biofilms of five clinical strains each of Staphylococcus aureus and Staphylococcus epidermidis, and American Type Culture Collection (ATCC) strains of both species, were 32-512 and 0.25-2 mg ml "1 , respectively.Results from time-kill studies indicated that rhodomyrtone at a concentration of 4¾ MIC could reduce the number of Staphylococcus aureus ATCC 25923 and Staphylococcus epidermidis ATCC 35984 cells by 99.9 % within 3 and 13 h, respectively. The ability of rhodomyrtone and the ethanol extract to prevent biofilm formation and kill mature biofilms was assessed: both demonstrated better activity than vancomycin at inhibiting staphylococcal biofilm formation. In addition, the viability of 24 h and 5-day staphylococcal biofilm-grown cells decreased after treatment with the ethanol extract and rhodomyrtone. The ability to reduce biofilm formation and kill mature biofilms occurred in a dose-dependent manner. Scanning electron microscopy clearly confirmed that treatment with rhodomyrtone at 16¾ MIC could reduce 24 h biofilm formation and the numbers of staphylococci, whilst at 64¾ MIC this compound destroyed the organisms in the 5-day established biofilm. These results suggest that rhodomyrtone has the potential for further drug development for the treatment of biofilm-forming staphylococcal infections.

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.