Synthesis and anti-biofilm activity of thiazole Schiff bases

Highly Fluorescent π-Conjugated Azomethines and Divalent Metal Complexes as Antibacterial and Antibiofilm Nominees

π-Conjugated azomethine ligands differing in the naphthalene or phenylmethane-centered core structure and their divalent cobalt, nickel, copper, and zinc metal complexes were prepared and well-characterized by spectral analyses in solid state. Magnetic natures of the complexes were determined by magnetic susceptibility measurements in solid-state. Their remarkable photophysical characteristics were recorded by Uv-vis and Fluorescence spectroscopic techniques. At their excitation wavelenght of 265 nm, all molecules exhibited triple fluorescence emission bands with promising intensities above 673 nm in near infra-red region. Antibacterial and antibiofilm activities of the π-conjugated azomethines are promising for potential applications in medical and healthcare settings. Hence, the antibacterial/antibiofilm activity of the π-conjugated azomethine ligands and their metal complexes against some clinically important bacteria namely Staphylococcus aureus (MSSA), Methicillin-resistant Staphylococcus aureus (MRSA), Staphylococcus epidermidis, Escherichia coli, Pseudomonas aeruginosa and Proteus mirabilis was investigated, and the obtained results have shown that the ligands and complexes had a remarkable antibacterial effect, especially on Proteus mirabilis. Metal complexes have been found to have a significant inhibitory effect on biofilm formation by MRSA, MSSA, and P. mirabilis compared to ligands. The copper (II) complex of ligand-2 showed the highest inhibition percentage, significantly reducing biofilm formation for MRSA and MSSA. Furthermore, cobalt (II) complexes of the ligands selectively inhibited the growth of the opportunistic pathogen P. mirabilis biofilms, indicating that metal complexes might be a good choice for future antibiofilm studies.

Design, synthesis and biological evaluation of thiazolyl-halogenated pyrroles or pyrazoles as novel antibacterial and antibiofilm agents

The formation of biofilm is one of the important factors for bacteria to develop drug-resistant. A series of halogenated-pyrroles or pyrazoles containing thiazole groups as antibacterial agents were designed and synthesized to target biofilms. Among them, compound 8c showed antibacterial activity against various Gram-positive bacteria, particularly against vancomycin-resistant Enterococcus faecalis (MIC ≤0.125 μg/mL). Additionally, this compound significantly inhibited biofilm formation of Staphylococcus aureus and Pseudomonas aeruginosa at sub-MIC doses. Furthermore, compound 8c exhibited significantly lower mammalian cell toxicity compared to pyrrolomycin C and its hepatic microsomal metabolic stability in various species was also evaluated. Further experiment on the infection model of Galleria mellonella proved that the compound was effective in vivo.

Role of small molecules and nanoparticles in effective inhibition of microbial biofilms: A ray of hope in combating microbial resistance

Microbial biofilms pose a severe threat to global health, as they are associated with deadly chronic infections and antibiotic resistance. To date, very few drugs are in clinical practice that specifically target microbial biofilms. Therefore, there is an urgent need for the development of novel therapeutic options targeting biofilm-related infections. In this review, we discuss nearly seventy-five different molecular scaffolds published over the last decade (2010-2023) which have exhibited their biofilm inhibition potential. For convenience, we have classified these into five different sub-groups based on their origin and design (excluding peptides as they are placed in between small molecules and biologics), namely, heterocycles; inorganic small molecules & metal complexes; small molecules decorated nanoparticles; small molecules derived from natural products (both plant and marine sources); and small molecules designed by in-silico approach. These antibiofilm agents are capable of disrupting microbial biofilms and can offer a promising avenue for future developments in human medicine. A hitherto review of this kind will lay a platform for the researchers to find new molecular entities to curb the serious menace of antimicrobial resistance especially caused by biofilms.

Small Schiff Base Molecules-A Possible Strategy to Combat Biofilm-Related Infections

Microorganisms participating in the development of biofilms exhibit heightened resistance to antibiotic treatment, therefore infections involving biofilms have become a problem in recent years as they are more difficult to treat. Consequently, research efforts are directed towards identifying novel molecules that not only possess antimicrobial properties but also demonstrate efficacy against biofilms. While numerous investigations have focused on antimicrobial capabilities of Schiff bases, their potential as antibiofilm agents remains largely unexplored. Thus, the objective of this article is to present a comprehensive overview of the existing scientific literature pertaining to small molecules categorized as Schiff bases with antibiofilm properties. The survey involved querying four databases (Web of Science, ScienceDirect, Scopus and Reaxys). Relevant articles published in the last 10 years were selected and categorized based on the molecular structure into two groups: classical Schiff bases and oximes and hydrazones. Despite the majority of studies indicating a moderate antibiofilm potential of Schiff bases, certain compounds exhibited a noteworthy effect, underscoring the significance of considering this type of molecular modeling when seeking to develop new molecules with antibiofilm effects.

Synthetic non-toxic anti-biofilm agents as a strategy in combating bacterial resistance

The tremendous increase in the bacterial resistance to the available antibiotics is a serious problem for the treatment of various infections. Biofilm formation in bacteria significantly contributes to the bacterial survival in host cells, and is considered as an crucial factor, responsible for bacterial resistance. The response of the bacterial cells in the biofilm to antibiotics is completely different from that of the free floating planktonic cells of the same strain. The anti-biofilm agents that could inhibit the biofilm production without affecting the bacterial growth, apply less selective pressure over the bacterial strains than the traditional antibiotics; thus the development of bacterial resistance would be of low incidence. Many attempts have been performed to discover novel agents capable of interfering with the bacterial biofilm life cycle, and several compounds have shown promising activities in suppressing the biofilm production or in dispersing mature existing biofilms. This review describes the different chemical classes that have anti-biofilm effects against different Gram-positive and Gram-negative bacteria without affecting the bacterial growth.

Quorum sensing inhibitors as Therapeutics: Bacterial biofilm inhibition

The overuse and inappropriate use of antibiotics to treat bacterial infections has led to the development of multiple drug resistant strains. Biofilm is a complex microorganism aggregation defined by the presence of a dynamic, sticky, and protective extracellular matrix made of polysaccharides, proteins, and nucleic acids. The infectious diseases are caused by bacteria that flourish within quorum sensing (QS) mediated biofilms. Efforts to disrupt biofilms have enabled the identification of bioactive molecules produced by prokaryotes and eukaryotes. The QS system is quenched predominantly by these molecules. The phenomenon is also termed as quorum sensing (QS). Both synthetic and natural substances have been discovered to be useful in QS. This review describes natural and synthetic quorum sensing inhibitors (QSIs) with the potential to treat bacterial infections. It includes the discussion on quorum sensing, mechanism of quorum sensing, effect of substituents on the activity. These discoveries could result in effective therapies using far lower dosages of medications, particularly antibiotics, are currently needed.

Analysis of the Antimicrobial and Anti-Biofilm Activity of Natural Compounds and Their Analogues against Staphylococcus aureus Isolates

(1) Background: Staphylococcus aureus (S. aureus) is one of the most frequent causes of biofilm-associated infections. With the emergence of antibiotic-resistant, especially methicillin-resistant S. aureus (MRSA), there is an urgent need to discover novel inhibitory compounds against this clinically important pathogen. In this study, we evaluated the antimicrobial and anti-biofilm activity of 11 compounds, including phenyl propenes and phenolic aldehydes, eugenol, ferulic acid, sinapic acid, salicylaldehyde, vanillin, cinnamoyl acid, and aldehydes, against drug-resistant S. aureus isolates. (2) Methods: Thirty-two clinical S. aureus isolates were obtained from Alkhidmat Diagnostic Center and Blood Bank, Karachi, Pakistan, and screened for biofilm-forming potential, and susceptibility/resistance against ciprofloxacin, chloramphenicol, ampicillin, amikacin, cephalothin, clindamycin, streptomycin, and gentamicin using the Kirby-Bauer disk diffusion method. Subsequently, 5 representative clinical isolates were selected and used to test the antimicrobial and anti-biofilm potential of 11 compounds using both qualitative and quantitative assays, followed by qPCR analysis to examine the differences in the expression levels of biofilm-forming genes (ica-A, fnb-B, clf-A and cna) in treated (with natural compounds and their derivatives) and untreated isolates. (3) Results: All isolates were found to be multi-drug resistant and dominant biofilm formers. The individual Minimum Inhibitory Concentration (MIC) of natural compounds and their analogues ranged from 0.75−160 mg/mL. Furthermore, the compounds, Salicylaldehyde (SALI), Vanillin (VAN), α-methyl-trans-cinnamaldehyde (A-MT), and trans-4-nitrocinnamic acid (T4N) exhibited significant (15−92%) biofilm inhibition/reduction percentage capacity at the concentration of 1−10 mg/mL. Gene expression analysis showed that salicylaldehyde, α-methyl-trans-cinnamaldehyde, and α-bromo-trans-cinnamaldehyde resulted in a significant (p < 0.05) downregulation of the expression of ica-A, clf-A, and fnb-A genes compared to the untreated resistant isolate. (4) Conclusions: The natural compounds and their analogues used in this study exhibited significant antimicrobial and anti-biofilm activity against S. aureus. Biofilms persist as the main concern in clinical settings. These compounds may serve as potential candidate drug molecules against biofilm forming S. aureus.

Crystalline Derivatives of Dipyrazolo-1,5-diazocine and Dipyrazolopyrimidine: A Case of Unexpected Synthesis and Isostructural Polymorphism

Pyrazole-phenylmethanimines (Shiff bases), Py–N=CH–Ph, form molecular crystals whose supramolecular and self-assembly properties can be tuned according to the substitution made on the aromatic and pyrazole rings. In pursuit of the first pyrazole-pyridinemethanimine member, Py–N=CH–Pyr, by following the well-known synthetic scheme for these Shiff bases, two hitherto unknown crystalline derivatives of dipyrazolo-1,5-diazocine and dipyrazolopyrimidine were obtained instead, this depending on the use or not of acetic acid as the catalyst. 1,5-diazocine crystallizes in a single P-1 triclinic packing system (Z = 2, Z′ = 1), while dipyrazolopyrimidine exhibits isostructural dimorphic behavior by adopting two (pale pink and yellow) alike P21/c monoclinic systems (both Z = 4, Z′ = 1) as a function of the solvent used. Crystal structures were resolved by means of X-ray diffraction technique and their intramolecular, intermolecular, and supramolecular assemblies analyzed with the assistance of decorated Hirshfeld surfaces and the topology study of electron density using the quantum-theory of atoms in molecules (QTAIM). Although both dipyrazolopyrimidine polymorphs are stabilized by the same type of noncovalent motifs, the pale pink crystal has a slightly more compact structure, with more efficient inter- and intramolecular interactions.

From the Physicochemical Characteristic of Novel Hesperetin Hydrazone to Its In Vitro Antimicrobial Aspects

Microorganisms are able to give rise to biofilm formation on food matrixes and along food industry infrastructures or medical equipment. This growth may be reduced by the application of molecules preventing bacterial adhesion on these surfaces. A new Schiff base ligand, derivative of hesperetin, HABH (2-amino-N'-(2,3-dihydro-5,7-dihydroxy-2-(3-hydroxy-4-methoxyphenyl)chromen-4-ylidene)benzohydrazide), and its copper complex, CuHABH [CuLH₂(OAc)], were designed, synthesized and analyzed in terms of their structure and physicochemical properties, and tested as antibacterial agents. Their structures both in a solid state and in solution were established using several methods: FT-IR, ¹H NMR, ¹³C NMR, UV-Vis, FAB MS, EPR, ESI-MS and potentiometry. Coordination binding of the copper(II) complex dominating at the physiological pH region in the solution was found to be the same as that detected in the solid state. Furthermore, the interaction between the HABH and CuHABH with calf-thymus DNA (CT-DNA) were investigated. These interactions were tracked by UV-Vis, CD (circular dichroism) and spectrofluorimetry. The results indicate a stronger interaction of the CuHABH with the CT-DNA than the HABH. It can be assumed that the nature of the interactions is of the intercalating type, but in the high concentration range, the complex can bind to the DNA externally to phosphate residues or to a minor/major groove. The prepared compounds possess antibacterial and antibiofilm activities against Gram-positive and Gram-negative bacteria. Their antagonistic activity depends on the factor-strain test system. The glass was selected as a model surface for the experiments on antibiofilm activity. The adhesion of bacterial cells to the glass surface in the presence of the compounds was traced by luminometry and the best antiadhesive action against both bacterial strains was detected for the CuHABH complex. This molecule may play a crucial role in disrupting exopolymers (DNA/proteins) in biofilm formation and can be used to prevent bacterial adhesion especially on glass equipment.

Thiazole Analogues of the Marine Alkaloid Nortopsentin as Inhibitors of Bacterial Biofilm Formation

Anti-virulence strategy is currently considered a promising approach to overcome the global threat of the antibiotic resistance. Among different bacterial virulence factors, the biofilm formation is recognized as one of the most relevant. Considering the high and growing percentage of multi-drug resistant infections that are biofilm-mediated, new therapeutic agents capable of counteracting the formation of biofilms are urgently required. In this scenario, a new series of 18 thiazole derivatives was efficiently synthesized and evaluated for its ability to inhibit biofilm formation against the Gram-positive bacterial reference strains Staphylococcus aureus ATCC 25923 and S. aureus ATCC 6538 and the Gram-negative strain Pseudomonas aeruginosa ATCC 15442. Most of the new compounds showed a marked selectivity against the Gram-positive strains. Remarkably, five compounds exhibited BIC₅₀ values against S. aureus ATCC 25923 ranging from 1.0 to 9.1 µM. The new compounds, affecting the biofilm formation without any interference on microbial growth, can be considered promising lead compounds for the development of a new class of anti-virulence agents.

Thiazoles, Their Benzofused Systems, and Thiazolidinone Derivatives: Versatile and Promising Tools to Combat Antibiotic Resistance

Thiazoles, their benzofused systems, and thiazolidinone derivatives are widely recognized as nuclei of great value for obtaining molecules with various biological activities, including analgesic, anti-inflammatory, anti-HIV, antidiabetic, antitumor, and antimicrobial. In particular, in the past decade, many compounds bearing these heterocycles have been studied for their promising antibacterial properties due to their action on different microbial targets. Here we assess the recent development of this class of compounds to address mechanisms underlying antibiotic resistance at both bacterial-cell and community levels (biofilms). We also explore the SAR and the prospective clinical application of thiazole and its benzofused derivatives, which act as inhibitors of mechanisms underlying antibiotic resistance in the treatment of severe drug-resistant infections. In addition, we examined all bacterial targets involved in their antimicrobial activity reporting, when described, their spontaneous frequencies of resistance.

Synthesis, biological evaluations and computational studies of N-(3-(-2-(7-Chloroquinolin-2-yl)vinyl) benzylidene)anilines as fungal biofilm inhibitors

In the present investigation, new chloroquinoline derivatives bearing vinyl benzylidene aniline substituents at 2nd position were synthesized and screed for biofilm inhibitory, antifungal and antibacterial activity. The result of biofilm inhibition of C. albicans suggested that compounds 5j (IC₅₀ value = 51.2 μM) and 5a (IC₅₀ value = 66.2 μM) possess promising antibiofilm inhibition when compared with the standard antifungal drug fluconazole (IC₅₀ = 40.0 μM). Two compounds 5a (MIC = 94.2 μg/mL) and 5f (MIC = 98.8 μg/mL) also exhibited good antifungal activity comparable to standard drug fluconazole (MIC = 50.0 μg/mL). The antibacterial screening against four strains of bacteria viz. E. coli, P. aeruginosa, B. subtilis, and S. aureus suggested their potential antibacterial activity and especially all the compounds except 5g were found more active than the standard drug ciprofloxacin against B. subtilis. To further gain insights into the possible mechanism of these compounds in biofilm inhibition through the agglutinin like protein (Als), molecular docking and molecular dynamics simulation studies were carried out. Molecular modeling studies suggested the clear role in inhibition of this protein and the resulting biofilm inhibitory activity.

Synthesis and Biological Evaluation of Pyrimidine-oxazolidin-2-arylimino Hybrid Molecules as Antibacterial Agents

Pyrimidine-1,3-oxazolidin-2-arylimino hybrids have been synthesized as a new class of antibacterial agents. The synthetic approach exploits a Cu(II)-catalyzed intramolecular halkoxyhalogenation of alkynyl ureas, followed by a Suzuki coupling reaction with 2,4-dimethoxypyrimidin-5-boronic acid. Biological screenings revealed that most of the compounds showed moderate to good activity against two Gram-positive (B. subtilis, S. aureus) and three Gram-negative (P. aeruginosa, S. typhi, K. pneumonia) pathogenic strains. A molecular docking study, performed in the crystal structure of 50S ribosomal unit of Haloarcula marismortui, indicated that pyrimidine-oxazolidin-2-arylimino hybrids 8c and 8h exhibited a high binding affinity (−9.65 and −10.74 kcal/mol), which was in agreement with their good antibacterial activity. The obtained results suggest that the combination of pyrimidine and oxazolidone moieties can be considered as a valid basis to develop new further modifications towards more efficacious antibacterial compounds.

A thiadiazole reduces the virulence of Xanthomonas oryzae pv. oryzae by inhibiting the histidine utilization pathway and quorum sensing

Thiazole, isothiazole, thiadiazole and their derivatives are widely thought to induce host defences against plant pathogens. In this article, we report that bismerthiazol, a thiadiazole molecule, reduces disease by inhibiting the histidine utilization (Hut) pathway and quorum sensing (QS). Bismerthiazol provides excellent control of bacterial rice leaf blight (BLB) caused by Xanthomonas oryzae pv. oryzae (Xoo), but does not greatly inhibit Xoo growth in vitro. According to RNA-sequencing analysis, the transcription of the Hut pathway genes of Xoo ZJ173 was inhibited after 4.5 and 9.0 h of bismerthiazol treatment. Functional studies of hutG and hutU indicated that the Hut pathway had little effect on the growth and bismerthiazol sensitivity of Xoo in vitro, but significantly reduced the aggregation of Xoo cells. Deletion mutants of hutG or hutU were more motile, produced less biofilm and were less virulent than the wild-type, indicating that the Hut pathway is involved in QS and contributes to virulence. The overexpression of the hutG-U operons in ZJ173 reduced Xoo control by bismerthiazol. Bismerthiazol did not inhibit the transcription of Hut pathway genes, QS or virulence of the bismerthiazol-resistant strain 2-1-1. The results indicate that bismerthiazol reduces Xoo virulence by inhibiting the Hut pathway and QS.

Novel amalgamation of phthalazine-quinolines as biofilm inhibitors: One-pot synthesis, biological evaluation and in silico ADME prediction with favorable metabolic fate

A facile and highly efficient one-pot synthesis of phthalazine-quinoline derivatives is reported via four component reaction of phthalic anhydride, hydrazine hydrate, 5,5-dimethyl 1,3 cyclohexanedione and various quinoline aldehydes using PrxCoFe2-xO4 (x=0.1) nanoparticles as a catalyst. The synthesized compounds have been evaluated for anti-biofilm activity against Pseudomonas aeruginosa and Candida albicans. The compounds 12a (IC50=30.0μM) and 12f (IC50=34.5μM) had shown promising anti-biofilm activity against P. aeruginosa and C. albicans, respectively, when compared with standards without affecting the growth of cells (and thus behave as anti-quorum sensing agents). Compounds 12a (MIC=45.0μg/mL) and 12f (MIC=57.5μg/mL) showed significant potent antimicrobial activity against P. aeruginosa and C. albicans, respectively. Thus, the active derivatives were not only potent biofilm inhibitors but also efficient antimicrobial agents. In silico ADME and metabolic site prediction studies were also held out to set an effective lead candidate for the future antimicrobial drug discovery initiatives.

Two thiadiazole compounds promote rice defence against Xanthomonas oryzae pv. oryzae by suppressing the bacterium's production of extracellular polysaccharides

Thiazole, isothiazole, thiadiazole, and their derivatives are used to control various human, animal and plant diseases. In addition to having direct anti-microbial and anti-fungal properties, these compounds are thought to induce host defences, but the mechanism of defence induction remains poorly understood. This article reports that the thiadiazoles of zinc thiazole and bismerthiazol induce H2 O2 accumulation, up-regulation of defence-related genes, callose deposition and hypersensitive response-like cell death in rice leaves infected with Xanthomonas oryaze pv. oryzae (Xoo) strain ZJ173, but not in non-infected leaves. These defence responses in Xoo-infected leaves were suppressed by the exogenous application of catalase, which reduces H2 O2 accumulation. The application of extracellular polysaccharides (EPSs) extracted from strain ZJ173 significantly compromised rice defence against ZJ173 with or without thiadiazole treatment. The EPS-deficient Xoo mutant ∆gumH triggered a stronger defence than its parent strain ZJ173. The thiadiazole treatments reduced EPS production by strain ZJ173, but not by the thiadiazole-resistant strain 2-1-1, which is thiadiazole resistant in vivo, but not in vitro; moreover, enhanced defence was not detected in thiadiazole-treated rice inoculated with 2-1-1. Based on these data, we infer that zinc thiazole and bismerthiazol promote rice defence against Xoo by inhibiting the production of bacterial EPS.

Anti-biofilm activity and synergism of novel thiazole compounds with glycopeptide antibiotics against multidrug-resistant staphylococci

Methicillin-resistant Staphylococcus aureus (MRSA) infections are a leading cause of death among all fatalities caused by antibiotic-resistant bacteria. With the rise of increasing resistance to current antibiotics, new antimicrobials and treatment strategies are urgently needed. Thiazole compounds have been shown to possess potent antimicrobial activity. A lead thiazole 1 and a potent derivative 2 were synthesized and their activity in combination with glycopeptide antibiotics was determined against an array of MRSA and vancomycin-resistant S. aureus (VRSA) clinical isolates. In addition, the anti-biofilm activity of the novel thiazoles was investigated against S. epidermidis. Compound 2 behaved synergistically with vancomycin against MRSA and was able to resensitize VRSA to vancomycin, reducing its MIC by 512-fold in two strains. In addition, both thiazole compounds were superior to vancomycin in significantly reducing S. epidermidis biofilm mass. Collectively, the results obtained demonstrate that compounds 1 and 2 possess potent antimicrobial activity alone or in combination with vancomycin against multidrug-resistant staphylococci and show potential for use in disrupting staphylococcal biofilm.