The objective of this study was to further elucidate the role of membrane potential in the mechanism of action of daptomycin, a novel lipopeptide antibiotic. Membrane depolarization was measured by both fluorimetric and flow cytometric assays. Adding daptomycin (5 g/ml) to Staphylococcus aureus gradually dissipated membrane potential. In both assays, cell viability was reduced by >99% and membrane potential was reduced by >90% within 30 min of adding daptomycin. Cell viability decreased in parallel with changes in membrane potential, demonstrating a temporal correlation between bactericidal activity and membrane depolarization. Decreases in viability and potential also showed a dose-dependent correlation. Depolarization is indicative of ion movement across the cytoplasmic membrane. Fluorescent probes were used to demonstrate Ca 2؉ -dependent, daptomycin-triggered potassium release from S. aureus. Potassium release was also correlated with bactericidal activity. This study demonstrates a clear correlation between dissipation of membrane potential and the bactericidal activity of daptomycin. A multistep model for daptomycin's mechanism of action is proposed.Daptomycin is a novel lipopeptide antibiotic in late-stage clinical development for the treatment of serious gram-positive infections. Daptomycin exhibits rapid in vitro bactericidal activity against clinically significant strains of gram-positive pathogens including hemolytic streptococci, methicillin-resistant Staphylococcus aureus, and vancomycin-resistant enterococci (4,10,12,14,19,22,23). Daptomycin acts at the cytoplasmic membrane of susceptible bacteria (8), as demonstrated by binding and fractionation studies. Additionally, the activity of daptomycin is dependent on the presence of physiologic levels of free calcium ions (50 mg/liter).Debate over daptomycin's mechanism of action has continued for more than a decade. One hypothesis suggests that daptomycin bactericidal activity is mediated by inhibition of lipoteichoic acid (LTA) biosynthesis (7,8). However, a recent investigation has failed to find evidence of a role for LTA in the mechanism of action of daptomycin in S. aureus or Enterococcus faecalis, suggesting that the in vitro bactericidal activity of daptomycin is independent of LTA biosynthesis (V. Laganas, J. Alder, and J. A. Silverman, submitted for publication). A second proposed mechanism of action for daptomycin is that the antibiotic causes dissipation of bacterial membrane potential, resulting in disruption of multiple aspects of cellular function (1, 2). Bactericidal activity via disruption of membrane potential is the proposed mechanism of action for a variety of antimicrobial peptides, including the pore-forming antibiotic nisin (18,20). We wished to further investigate the role of bacterial membrane potential in the mechanism of action of daptomycin. In this study, we demonstrate a significant correlation between membrane depolarization and bactericidal activity. Furthermore, we demonstrate that one possible mechanism of membrane depol...

For 70 years antibiotics have saved countless lives and enabled the development of modern medicine, but it is becoming clear that the success of antibiotics may have only been temporary and we now anticipate a long-term, generational and perhaps never-ending challenge to find new therapies to combat antibiotic-resistant bacteria. As the search for new conventional antibiotics has become less productive and there are no clear strategies to improve success, a broader approach to address bacterial infection is needed. This review of potential alternatives to antibiotics (A2As) was commissioned by the Wellcome Trust, jointly funded by the Department of Health, and involved scientists and physicians from academia and industry. For the purpose of this review, A2As were defined as non-compound approaches (that is, products other than classical antibacterial agents) that target bacteria or approaches that target the host. In addition, the review was limited to agents that had potential to be administered orally, by inhalation or by injection for treatment of systemic/invasive infection. Within these criteria, the review has identified 19 A2A approaches now being actively progressed. The feasibility and potential clinical impact of each approach was considered. The most advanced approaches (and the only ones likely to deliver new treatments by 2025) are antibodies, probiotics, and vaccines now in Phase II and Phase III trials. These new agents will target infections caused by P. aeruginosa, C. difficile and S. aureus. However, other than probiotics for C. difficile, this first wave will likely best serve as adjunctive or preventive therapies. This suggests that conventional antibiotics will still be needed. The economics of pathogen-specific therapies must improve to encourage innovation, and greater investment into A2As with broad-spectrum activity (e.g. antimicrobial-, host defense-and, anti-biofilm peptides) is needed. Increased funding, estimated at >£1.5 bn over 10 years is required to validate and then develop these A2As. Investment needs to be partnered with translational expertise and targeted to support the validation of these approaches at Clinical Phase II proof of concept. Such an approach could transform our understanding of A2As as effective new therapies and should provide the catalyst required for both active engagement and investment by the pharma/biotech industry. Only a sustained, concerted and coordinated international effort will provide the solutions needed for the next decade.

Daptomycin is a lipopeptide antibiotic with potent activity against gram-positive bacteria. Complete-genome comparisons of laboratory-derived Staphylococcus aureus with decreased susceptibility to daptomycin and their susceptible parent were used to identify genes that contribute to reduced susceptibility to daptomycin. Selective pressure of growth in sublethal concentrations of daptomycin resulted in the accumulation of mutations over time correlating with incremental decreases in susceptibility. Single point mutations resulting in amino acid substitutions occurred in three distinct proteins: MprF, a lysylphosphatidylglycerol synthetase; YycG, a histidine kinase; and RpoB and RpoC, the ␤ and ␤ subunits of RNA polymerase. Sequence analysis of mprF, yycF, yycG, rpoB, and rpoC in clinical isolates that showed treatment-emergent increases in daptomycin MICs revealed point mutations in mprF and a nucleotide insertion in yycG, suggesting a role for these genes in decreased susceptibility to daptomycin in the hospital setting.

The lipopeptide daptomycin has been approved for use in skin and skin-structure infections but has failed to meet statistical noninferiority criteria in a clinical trial for severe community-acquired pneumonia. Daptomycin exhibited an unusual pattern of activity in pulmonary animal models: efficacy in Staphylococcus aureus hematogenous pneumonia and inhalation anthrax but no activity against Streptococcus pneumoniae in simple bronchial-alveolar pneumonia. Daptomycin was shown to interact in vitro with pulmonary surfactant, resulting in inhibition of antibacterial activity. This effect was specific to daptomycin and consistent with its known mechanism of action. This represents the first example of organ-specific inhibition of an antibiotic.

bDaptomycin is a lipopeptide antibiotic used clinically for the treatment of certain types of Gram-positive infections, including those caused by methicillin-resistant Staphylococcus aureus (MRSA). Details of the mechanism of action of daptomycin continue to be elucidated, particularly the question of whether daptomycin acts on the cell membrane, the cell wall, or both. Here, we use fluorescence microscopy to directly visualize the interaction of daptomycin with the model Gram-positive bacterium Bacillus subtilis. We show that the first observable cellular effects are the formation of membrane distortions (patches of membrane) that precede cell death by more than 30 min. Membrane patches are able to recruit the essential cell division protein DivIVA. Recruitment of DivIVA correlates with membrane defects and changes in cell morphology, suggesting a localized alteration in the activity of enzymes involved in cell wall synthesis that could account for previously described effects of daptomycin on cell wall morphology and septation. Membrane defects colocalize with fluorescently labeled daptomycin, DivIVA, and fluorescent reporters of peptidoglycan biogenesis (Bocillin FL and BODIPY FL-vancomycin), suggesting that daptomycin plays a direct role in these events. Our results support a mechanism for daptomycin with a primary effect on cell membranes that in turn redirects the localization of proteins involved in cell division and cell wall synthesis, causing dramatic cell wall and membrane defects, which may ultimately lead to a breach in the cell membrane and cell death. These results help resolve the longstanding questions regarding the mechanism of action of this important class of antibiotics.