High-level iron mitigates fusaricidin-induced membrane damage and reduces membrane fluidity leading to enhanced drug resistance inBacillus subtilis

Rapid Detection of Escherichia coli Antibiotic Susceptibility Using Live/Dead Spectrometry for Lytic Agents

Antibiotic resistance is a serious threat to public health. The empiric use of the wrong antibiotic occurs due to urgency in treatment combined with slow, culture-based diagnostic techniques. Inappropriate antibiotic choice can promote the development of antibiotic resistance. We investigated live/dead spectrometry using a fluorimeter (Optrode) as a rapid alternative to culture-based techniques through application of the LIVE/DEAD® BacLightTM Bacterial Viability Kit. Killing was detected by the Optrode in near real-time when Escherichia coli was treated with lytic antibiotics-ampicillin and polymyxin B-and stained with SYTO 9 and/or propidium iodide. Antibiotic concentration, bacterial growth phase, and treatment time used affected the efficacy of this detection method. Quantification methods of the lethal action and inhibitory action of the non-lytic antibiotics, ciprofloxacin and chloramphenicol, respectively, remain to be elucidated.

TonB-Dependent Receptors Affect the Spontaneous Oxytetracycline Resistance Evolution in Aeromonas hydrophila

It is well known that most microbial populations develop their intrinsic antibiotics resistance at low concentrations in antibiotics environments, but the factors influencing spontaneous resistance are still largely unknown. In this study, Aeromonas hydrophila strains with different resistance levels to oxytetracycline (OXY) were induced by sublethal antibiotic selection pressure, and differential expression of proteins was compared among them using iTRAQ-based quantitative proteomics. Our following bioinformatic analysis showed that energy metabolism-related proteins were downregulated, while several iron-related proteins were upregulated in high OXY-resistant strains. To further investigate the role of spontaneous OXY resistance evolution, four TonB-dependent receptor-coded genes were deleted, and their OXY susceptibility capabilities and antibiotic evolutionary assays were performed, respectively. Our results showed that the deletion of these genes did not affect the susceptibility to OXY, but showed different evolution rates in the spontaneous OXY evolution compared with wild-type strain, especially for AHA_0971 and AHA_4251. Therefore, this study indicates the important role of TonB-dependent receptor proteins during the bacterial antibiotics resistance evolution and may provide a new prophylactic strategy against the development of antibiotic resistance.

Role of Lipid Composition, Physicochemical Interactions, and Membrane Mechanics in the Molecular Actions of Microbial Cyclic Lipopeptides

Several experimental and theoretical studies have extensively investigated the effects of a large diversity of antimicrobial peptides (AMPs) on model lipid bilayers and living cells. Many of these peptides disturb cells by forming pores in the plasma membrane that eventually lead to the cell death. The complexity of these peptide-lipid interactions is mainly related to electrostatic, hydrophobic and topological issues of these counterparts. Diverse studies have shed some light on how AMPs act on lipid bilayers composed by different phospholipids, and how mechanical properties of membranes could affect the antimicrobial effects of such compounds. On the other hand, cyclic lipopeptides (cLPs), an important class of microbial secondary metabolites, have received comparatively less attention. Due to their amphipathic structures, cLPs exhibit interesting biological activities including interactions with biofilms, anti-bacterial, anti-fungal, antiviral, and anti-tumoral properties, which deserve more investigation. Understanding how physicochemical properties of lipid bilayers contribute and determining the antagonistic activity of these secondary metabolites over a broad spectrum of microbial pathogens could establish a framework to design and select effective strategies of biological control. This implies unravelling-at the biophysical level-the complex interactions established between cLPs and lipid bilayers. This review presents, in a systematic manner, the diversity of lipidated antibiotics produced by different microorganisms, with a critical analysis of the perturbing actions that have been reported in the literature for this specific set of membrane-active lipopeptides during their interactions with model membranes and in vivo. With an overview on the mechanical properties of lipid bilayers that can be experimentally determined, we also discuss which parameters are relevant in the understanding of those perturbation effects. Finally, we expose in brief, how this knowledge can help to design novel strategies to use these biosurfactants in the agronomic and pharmaceutical industries.

Antibacterial and Antibiofilm Activities of a Novel Synthetic Cyclic Lipopeptide against Cariogenic Streptococcus mutans UA159

Despite continuous efforts to control cariogenic dental biofilms, very few effective antimicrobial treatments exist. In this study, we characterized the activity of the novel synthetic cyclic lipopeptide 4 (CLP-4), derived from fusaricidin, against the cariogenic pathogen Streptococcus mutans UA159. We determined CLP-4's MIC, minimum bactericidal concentration (MBC), and spontaneous resistance frequency, and we performed time-kill assays. Additionally, we assessed CLP-4's potential to inhibit biofilm formation and eradicate preformed biofilms. Our results demonstrate that CLP-4 has strong antibacterial activity in vitro and is a potent bactericidal agent with low spontaneous resistance frequency. At a low concentration of 5 μg/ml, CLP-4 completely inhibited S. mutans UA159 biofilm formation, and at 50 μg/ml, it reduced the viability of established biofilms by >99.99%. We also assessed CLP-4's cytotoxicity and stability against proteolytic digestion. CLP-4 withstood trypsin or chymotrypsin digestion even after treatment for 24 h, and our toxicity studies showed that CLP-4 effective concentrations had negligible effects on hemolysis and the viability of human oral fibroblasts. In summary, our findings showed that CLP-4 is a potent antibacterial and antibiofilm agent with remarkable stability and low nonspecific cytotoxicity. Hence, CLP-4 is a promising novel antimicrobial peptide with potential for clinical application in the prevention and treatment of dental caries.

Transcriptional Profiling Analysis of Bacillus subtilis in Response to High Levels of Fe(3.)

Iron is essential to microorganisms for its important biological function but could be highly toxic in excess. We have used genome-wide transcriptional analysis in Fe(3+)-treated (4 mM) Bacillus subtilis to reveal the effect of excess Fe(3+) on B. subtilis and characterized the potential pathways involved in Fe(3+) stress tolerance. A total of 366 and 400 genes were identified as significantly up-regulated and down-regulated, respectively. We found excess Fe(3+) had four major influences on B. subtilis: Fe(3+) resulted in oxidative stress and induced genes involved in oxidative stress resistance including the SigB-regulated genes, but the PerR regulon was not inducible in Fe(3+)-mediated oxidative stress except zosA; Fe(3+) significantly disturbed homeostasis of Mn(2+) and Zn(2+), and the mechanism was proposed in this article; the acidity of Fe(3+)-induced genes involved in acid consuming and production of bases and shifted B. subtilis to carbon starvation state; Fe(3+)-induced genes related to membrane remodeling (bkd operon), which prevents Fe(3+)'s incorporation to membrane lipids. Moreover, Fe(3+) repressed the stringent control response, consistent with the induction of stringent control in iron limitation, demonstrating that iron might be a signal in stringent control of B. subtilis. This study was the first to provide a comprehensive overview of the genetic response of B. subtilis to ecxess Fe(3+).