Gramicidin D enhances the antibacterial activity of fluoride

Fluoride releasing photothermal responsive TiO2 matrices for antibiosis, biosealing and bone regeneration

Peri-implantitis induced by infection leads to gingival recession, alveolar resorption and eventual dental implant failure. So, antibiosis and biosealing of abutments as well as osseointegration of roots need to be projected seriously during the whole service lifespan of dental implants. In this work, a multipurpose photothermal therapy strategy based on Si/P/F doped TiO2 matrix is proposed to address the above issues. This TiO2 matrix not only has outstanding photothermal response, but also triggers the release of F ions under near-infrared (NIR) light irradiation. Local hyperthermia assisted with the released F ions reduces adenosine triphosphate (ATP) synthesis of staphylococcus aureus (S. aureus), increases bacterial membrane permeability, and induces abundant of reactive oxygen species, resulting in the oxidation of cellular components and eventual death of bacteria. Furthermore, the synergic action of mild photothermal stimulation and Si/P/F ions of TiO2 matrix up-regulates gingival epithelial cells behavior (e.g., hemidesmosome formation) and osteoblasts response in vitro. In an infected model, this TiO2 matrix obviously eliminates bacteria, reduces inflammatory response, improves epithelial sealing and osseointegration, and reduces alveolar resorption by regulating NIR irradiation.

Na+/K+ carrier ionophore antibiotics valinomycin and monensin enhance the antibacterial activity of fluoride

Fluoride is routinely used as a highly effective antibacterial agent that interferes with bacterial metabolism through fundamentally different mechanisms. One of the major bacterial evasion mechanisms against fluoride is the impermeability of cell envelope to the anion that limits its cellular uptake. Therefore, translating such compounds to clinical settings requires novel mechanisms to facilitate the uptake of membrane-impermeant molecules. Published data have indicated antibiotic synergy between fluoride and membrane destabilizing agents that induce strong fluoride toxicity in bacteria via enhancing the permeability of bacterial membranes to fluoride. Here, we report a similar mechanism of antibiotic synergy between fluoride and potassium ion carriers, valinomycin and monensin against Gram-positive bacteria, B. subtilis and S. aureus. Molecular dynamics simulations were performed to understand the effect of potassium on the binding affinity of fluoride to monensin and valinomycin. The trajectory results strongly indicated that the monensin molecules transport fluoride ions across the cell membrane via formation of ion-pair between the monensin-K⁺ complex and a fluoride. This study provides new insights to design novel compounds to enhance the uptake of small toxic anions via synergistic interactions and thus exert strong antibacterial activity against a wide variety of pathogens.

Membrane Exporters of Fluoride Ion

Microorganisms contend with numerous and unusual chemical threats and have evolved a catalog of resistance mechanisms in response. One particularly ancient, pernicious threat is posed by fluoride ion (F^-), a common xenobiotic in natural environments that causes broad-spectrum harm to metabolic pathways. This review focuses on advances in the last ten years toward understanding the microbial response to cytoplasmic accumulation of F^-, with a special emphasis on the structure and mechanisms of the proteins that microbes use to export fluoride: the CLC^F family of F^-/H⁺ antiporters and the Fluc/FEX family of F^- channels.

Metalloriboswitches: RNA-based inorganic ion sensors that regulate genes

Divalent ions fulfill essential cellular roles and are required for virulence by certain bacteria. Free intracellular Mg²⁺ can approach 5 mm, but at this level Mn²⁺, Ni²⁺, or Co²⁺ can be growth-inhibitory, and magnesium fluoride is toxic. To maintain ion homeostasis, many bacteria have evolved ion sensors embedded in the 5'-leader sequences of mRNAs encoding ion uptake or efflux channels. Here, we review current insights into these "metalloriboswitches," emphasizing ion-specific binding by structured RNA aptamers and associated conformational changes in downstream signal sequences. This riboswitch-effector interplay produces a layer of gene regulatory feedback that has elicited interest as an antibacterial target.

Small molecule fluoride toxicity agonists

Fluoride is a ubiquitous anion that inhibits a wide variety of metabolic processes. Here, we report the identification of a series of compounds that enhance fluoride toxicity in Escherichia coli and Streptococcus mutans. These molecules were isolated by using a high-throughput screen (HTS) for compounds that increase intracellular fluoride levels as determined via a fluoride riboswitch reporter fusion construct. A series of derivatives were synthesized to examine structure-activity relationships, leading to the identification of compounds with improved activity. Thus, we demonstrate that small molecule fluoride toxicity agonists can be identified by HTS from existing chemical libraries by exploiting a natural fluoride riboswitch. In addition, our findings suggest that some molecules might be further optimized to function as binary antibacterial agents when combined with fluoride.

Bacterial fluoride resistance, Fluc channels, and the weak acid accumulation effect

Fluoride ion (F(-)) is a ubiquitous environmental threat to microorganisms, which have evolved a family of highly selective "Fluc" F(-) channels that export this inhibitory anion from their cytoplasm. It is unclear, however, how a thermodynamically passive mechanism like an ion channel can protect against high concentrations of external F(-). We monitored external F(-) concentrations in Escherichia coli suspensions and showed that, in bacteria lacking Fluc, F(-) accumulates when the external medium is acidified, as a predicted function of the transmembrane pH gradient. This weak acid accumulation effect, which results from the high pKa (3.4) and membrane permeability of HF, is abolished by Fluc channels. We also found that, although bacterial growth is inhibited by high concentrations of F(-), bacteria can withstand cytoplasmic F(-) at levels a hundred times higher than those that inhibit proliferation, resuming growth when the F(-) challenge is removed.