Promysalin is a salicylate-containing antimicrobial with a cell-membrane-disrupting mechanism of action on Gram-positive bacteria

Enzymatic reconstitution of salicylate formation in promysalin biosynthesis

Promysalin is an amphipathic antibiotic isolated from Pseudomonas promysalinigenes (previously Pseudomonas putida RW10S1) which shows potent antibacterial activities against Gram-negative pathogens by inactivating succinate dehydrogenase. Based on the in-vivo studies, promysalin is hypothesized to be assembled from three building blocks: salicylic acid, proline, and myristic acid via a proposed but uncharacterized hybrid NRPS-PKS biosynthetic pathway. So far, no in-vitro biosynthetic studies have been reported for this promising antibiotic. Here, we report the first in-vitro reconstitution and biochemical characterization of two early enzymes on the pathway: PpgH, an isochorismate synthase (IS), and PpgG, an isochorismate pyruvate lyase (IPL) which are involved in the biosynthesis of salicylic acid, the polar fragment of promysalin. We also report a secondary chorismate mutase (CM) activity for PpgG. Based on our biochemical experiments, preliminary mechanistic proposals have been postulated for PpgH and PpgG. We believe this study will lay a strong foundation for elucidating the functions and mechanisms of other intriguing enzymes of the promysalin biosynthesis pathway, which may potentially unravel interesting enzyme chemistries and promote pathway engineering in the future.

A lipophilic chitosan-modified self-nanoemulsifying system influencing cellular membrane metabolism enhances antibacterial and anti-biofilm efficacy for multi-drug resistant Pseudomonas aeruginosa wound infection

Wound infections, especially infections with multidrug-resistant bacteria, are a serious public health issue worldwide. In addition, the accumulation microbial biofilm of multidrug-resistant Pseudomonas aeruginosa increases the risk and physically obstruct its healing activity at the wound site. Therefore, the development of an eminent agent to control wound infection is urgently needed. Here, we report a novel chitosan (a natural biological macromolecule)-modified self-nanoemulsifying system (CSN) with lipophilic chlorhexidine acetate (CAA, a poorly water-soluble agent) that was designed and prepared using low-energy emulsification methods. We found that CSN displays better antibacterial efficacy, which occurs more quickly than its aqueous solution, in destroying the structure of the bacterial cell membrane and promoting the leakage of nucleic acids, proteins, K⁺, and Mg²⁺ from Pseudomonas aeruginosa cells. Importantly, CSN also accelerates skin wound healing after Pseudomonas aeruginosa infection by inhibiting biofilm formation and eradicating mature biofilms. Moreover, the proteomic results suggested that CSN altered membrane permeability and cellular membrane metabolism, allowing more drug molecules to enter the cytosol. Based on these results, this lipophilic self-nanoemulsifying system may be applied in the treatment of skin wounds caused by multidrug-resistant bacteria, especially Pseudomonas aeruginosa.

A Food-Grade Resin with LDH–Salicylate to Extend Mozzarella Cheese Shelf Life

Mozzarella cheese can be considered by far the world’s most popular Italian dairy product. Extending the shelf life of mozzarella cheese is an important issue in the dairy industry due to the high risk of contamination by several bacteria species, including spoilage pseudomonads. In this work, active packaging was prepared by coating traditional polyethylene terephthalate (PET) containers of “ovoline” mozzarella cheese with a food-grade resin mixed with a layered double hydroxide (LDH) in which salicylate anion was intercalatedby ionic exchange.. This antimicrobial molecule is listed in EC-Directive 10/2011/EC of 14 January 2011. Morphological arrangement of the molecule into the LDH layers was evaluated by X-ray diffraction (XRD) and controlled release followed by UV spectroscopy. Then, active trays were used to pack the mozzarella cheeses stored for 20 days at 4 °C and under thermal abuse (15 °C). Samples from both conditions showed coliform reduction (by ca. 2 log CFU/g) throughout the storage period. Depending on temperature, total mesophilic aerobic bacteria, Pseudomonas spp., yeasts, and mold loads were reduced in the first 3 days; at 4 °C. Slower acidification and lower proteolysis were also found in treated samples in comparison to control ones. The fitting of the Gompertz function to coliforms and spoilage pseudomonads highlighted an increase in the shelf life of mozzarella cheese of ca. 2 days at 4 °C. These results suggest that salicylate–LDH-coated PET may be applied to extend the shelf-life of mozzarella cheese and also counteract its spoilage if accidental interruptions to refrigeration occur.

Salicylic Acid Biosynthesis and Metabolism: A Divergent Pathway for Plants and Bacteria

Salicylic acid (SA) is an active secondary metabolite that occurs in bacteria, fungi, and plants. SA and its derivatives (collectively called salicylates) are synthesized from chorismate (derived from shikimate pathway). SA is considered an important phytohormone that regulates various aspects of plant growth, environmental stress, and defense responses against pathogens. Besides plants, a large number of bacterial species, such as Pseudomonas, Bacillus, Azospirillum, Salmonella, Achromobacter, Vibrio, Yersinia, and Mycobacteria, have been reported to synthesize salicylates through the NRPS/PKS biosynthetic gene clusters. This bacterial salicylate production is often linked to the biosynthesis of small ferric-ion-chelating molecules, salicyl-derived siderophores (known as catecholate) under iron-limited conditions. Although bacteria possess entirely different biosynthetic pathways from plants, they share one common biosynthetic enzyme, isochorismate synthase, which converts chorismate to isochorismate, a common precursor for synthesizing SA. Additionally, SA in plants and bacteria can undergo several modifications to carry out their specific functions. In this review, we will systematically focus on the plant and bacterial salicylate biosynthesis and its metabolism.

Target-Based Design of Promysalin Analogues Identifies a New Putative Binding Cleft in Succinate Dehydrogenase

Promysalin is a small-molecule natural product that specifically inhibits growth of the Gram-negative pathogen Pseudomonas aeruginosa (PA). This activity holds promise in the treatment of multidrug resistant infections found in immunocompromised patients with chronic illnesses, such as cystic fibrosis. In 2015, our lab completed the first total synthesis; subsequent analogue design and SAR investigation enabled identification of succinate dehydrogenase (Sdh) as the biological target in PA. Herein, we report the target-guided design of new promysalin analogues with varying alkyl chains, one of which is on par with our most potent analogue to date. Computational docking revealed that some analogues have a different orientation in the Sdh binding pocket, placing the terminal carbon proximal to a tryptophan residue. This inspired the design of an extended side chain analogue bearing a terminal phenyl moiety, providing a basis for the design of future analogues.

Antimicrobial activity of resveratrol-derived monomers and dimers against foodborne pathogens

Plant polyphenolic compounds are considered a promising source for new antibacterial agents. In this study, we evaluated the antimicrobial activity of a collection of resveratrol-derived monomers and dimers screened as single molecules against a panel of nine foodborne pathogens. The results demonstrated that two monomers (i.e., pterostilbene 2 and (E)-3-hydroxy-4′,5-dimethoxystilbene 9) and three dimers (i.e., δ-viniferin 10, viniferifuran 14 and dehydro-δ-viniferin 15) were endowed with significant antibacterial activity against gram-positive bacteria. The exposure of gram-positive foodborne pathogens to 100 µg/mL of 2, 9 and 15 induced severe cell membrane damage, resulting in the disruption of the phospholipid bilayer. The most promising dimeric compound, dehydro-δ-viniferin 15, was tested against Listeria monocytogenes, resulting in a loss of cultivability, viability and cell membrane potential. TEM analysis revealed grave morphological modifications on the cell membrane and leakage of intracellular content, confirming that the cell membrane was the principal biological target of the tested derivative.

Transcriptomic Profiling Suggests That Promysalin Alters the Metabolic Flux, Motility, and Iron Regulation in Pseudomonas putida KT2440

Promysalin, a secondary metabolite produced by P. putida RW10S1, is a narrow-spectrum antibiotic that targets P. aeruginosa over other Pseudomonas spp. P. putida KT2440, a nonproducing strain, displays increased swarming motility and decreased pyoverdine production in the presence of exogenous promysalin. Herein, proteomic and transcriptomic experiments were used to provide insight about how promysalin elicits responses in PPKT2440 and rationalize its species selectivity. RNA-sequencing results suggest that promysalin affects PPKT2440 by (1) increasing swarming in a flagella-independent manner; (2) causing cells to behave as if they were experiencing an iron-deficient environment, and (3) shifting metabolism away from glucose conversion to pyruvate via the Entner-Doudoroff pathway. These findings highlight nature's ability to develop small molecules with specific targets, resulting in exquisite selectivity.

Chemical and Biological Aspects of Nutritional Immunity-Perspectives for New Anti-Infectives that Target Iron Uptake Systems

Upon bacterial infection, one of the defense mechanisms of the host is the withdrawal of essential metal ions, in particular iron, which leads to "nutritional immunity". However, bacteria have evolved strategies to overcome iron starvation, for example, by stealing iron from the host or other bacteria through specific iron chelators with high binding affinity. Fortunately, these complex interactions between the host and pathogen that lead to metal homeostasis provide several opportunities for interception and, thus, allow the development of novel antibacterial compounds. This Review focuses on iron, discusses recent highlights, and gives some future perspectives which are relevant in the fight against antibiotic resistance.