In vitro pharmacokinetics of antimicrobial cationic peptides alone and in combination with antibiotics against methicillin resistant Staphylococcus aureus biofilms

Antibiotic therapy for methicillin-resistant Staphylococcus aureus (MRSA) infections is becoming more difficult in hospitals and communities because of strong biofilm-forming properties and multidrug resistance. Biofilm-associated MRSA is not affected by therapeutically achievable concentrations of antibiotics. Therefore, we investigated the in vitro pharmacokinetic activities of antimicrobial cationic peptides (AMPs; indolicidin, cecropin [1–7]-melittin A [2–9] amide [CAMA], and nisin), either alone or in combination with antibiotics (daptomycin, linezolid, teicoplanin, ciprofloxacin, and azithromycin), against standard and 2 clinically obtained MRSA biofilms. The minimum inhibitory concentrations (MIC) and minimum biofilm-eradication concentrations (MBEC) were determined by microbroth dilution technique. The time-kill curve (TKC) method was used to determine the bactericidal activities of the AMPs alone and in combination with the antibiotics against standard and clinically obtained MRSA biofilms. The MIC values of the AMPs and antibiotics ranged between 2 to 16 and 0.25 to 512 mg/L, and their MBEC values were 640 and 512 to 5120 mg/L, respectively. The TKC studies demonstrated that synergistic interactions occurred most frequently when using nisin + daptomycin/ciprofloxacin, indolicidin + teicoplanin, and CAMA + ciprofloxacin combinations. No antagonism was observed with any combination. AMPs appear to be good candidates for the treatment of MRSA biofilms, as they act as both enhancers of anti-biofilm activities and help to prevent or delay the emergence of resistance when used either alone or in combination with antibiotics.     

The human antimicrobial peptide LL-37 and its fragments possess both antimicrobial and antibiofilm activities against multidrug-resistant Acinetobacter baumannii

Acinetobacter baumannii infections are difficult to treat due to multidrug resistance. Biofilm formation by A. baumannii is an additional factor in its ability to resist antimicrobial therapy. The antibacterial and antibiofilm activities of the human antimicrobial peptide LL-37 and its fragments KS-30, KR-20 and KR-12 against clinical isolates of multidrug-resistant (MDR) A. baumannii were evaluated. The minimal inhibitory concentration (MIC) of LL-37 against MDR A. baumannii isolates ranged from 16 to 32 μg/mL. The MIC of KS-30 fragment varied from 8.0 to16 μg/mL and the KR-20 fragment MIC ranged from 16 to 64 μg/mL. LL-37 and KS-30 fragment exhibited 100% bactericidal activity against five A. baumannii strains, including four MDR clinical isolates, within 30 min at concentrations of 0.25–1 μg/mL. By 0.5 h, the fragments KR-20 and KR-12 eliminated all tested strains at 8 and 64 μg/mL respectively. LL-37 and its fragments displayed anti-adherence activities between 32-128 μg/mL. A minimum biofilm eradication concentration (MBEC) biofilm assay demonstrated that LL-37 inhibited and dispersed A. baumannii biofilms at 32 μg/mL respectively. Truncated fragments of LL-37 inhibited biofilms at concentrations of 64–128 μg/mL. KS-30, the truncated variant of LL-37, effectively dispersed biofilms at 64 μg/mL. At 24 h, no detectable toxicity was observed at the efficacious doses when cytotoxicity assays were performed. Thus, LL-37, KS-30 and KR-20 exhibit significant antimicrobial activity against MDR A. baumannii. The prevention of biofilm formation in vitro by LL-37, KS-30 and KR-20 adds significance to their efficacy. These peptides can be potential therapeutics against MDR A. baumannii infections.     

Antimicrobial and anti-biofilm effect of Bac8c on major bacteria associated with dental caries and Streptococcus mutans biofilms

Dental caries is a common oral bacterial infectious disease. Its prevention and treatment requires control of the causative pathogens within dental plaque, especially Streptococcus mutans (S. mutans). Antimicrobial peptides (AMPs), one of the promising substitutes for conventional antibiotics, have been widely tested and used for controlling bacterial infections. The present study focuses on evaluating the potential of the novel AMPs cyclic bactenecin and its derivatives against bacteria associated with dental caries. The results indicate that Bac8c displayed highest activity against the bacteria tested, whereas both cyclic and linear bactenecin had weak antimicrobial activity. The cytotoxicity assay showed that Bac8c did not cause detectable toxicity at concentrations of 32–128 μg/ml for 5 min or 32–64 μg/ml for 60 min. S. mutans and Lactobacillus fermenti treated with Bac8c showed variable effects on bacterial structure via scanning electron microscopy and transmission electron microscopy. There appeared to be a large amount of extracellular debris and obvious holes on the cell surface, as well as loss of cell wall and nucleoid condensation. The BioFlux system was employed to generate S. mutans biofilms under a controlled flow, which more closely resemble the formation process of natural biofilms. Bac8c remarkably reduced the viability of cells in biofilms formed in the BioFlux system. This phenomenon was further analyzed and verified by real-time PCR results of a significant suppression of the genes involved in S. mutans biofilm formation. Taken together, this study suggests that Bac8c has a potential clinical application in preventing and treating dental caries.     

γ-Alkylidene-γ-lactones and isobutylpyrrol-2(5H)-ones analogues to rubrolides as inhibitors of biofilm formation by Gram-positive and Gram-negative bacteria

Several molecules have been discovered that interfere with formation of bacterial biofilms, opening a new strategy for the development of more efficient treatments in case of antibiotic resistant bacteria. Amongst the most active compounds are some natural brominated furanones from marine algae Delisea pulchra that have proven to be able to control pathogenic biofilms. We have recently reported that some rubrolide analogues are able to inhibit biofilm formation of Enterococcus faecalis. In the present Letter we describe results of the biological evaluation of a small library of 28 compounds including brominated furanones and the corresponding lactams against biofilm formation of Staphylococcus aureus, Pseudomonas aeruginosa, Staphylococcus epidermidis and Streptococcus mutans. Our results showed that in general these compounds were more active against biofilms of S. epidermidis and P. aeruginosa, with little or no inhibition of planktonic bacterial growth. In some cases they were able to prevent biofilm formation of P. aeruginosa at concentrations as low as 0.6 μg/mL (1.3 μM, compound 3d) and 0.7 μg/mL (1.3 μM, 3f). Results also indicate that, in general, lactams are more active against biofilms than their precursors, thus designating this class of molecules as good candidates for the development of a new generation of antimicrobial drugs targeted to biofilm inhibition.     

An in vitro urinary tract catheter system to investigate biofilm development in catheter-associated urinary tract infections

Biofilm development in urinary tract catheters is an often underestimated problem. However, this form of infection leads to high mortality rates and causes significant costs in health care. Therefore, it is important to analyze these biofilms and establish avoiding strategies. In this study a continuous flow-through system for the cultivation of biofilms under catheter-associated urinary tract infection conditions was established and validated. The in vitro urinary tract catheter system implies the composition of urine (artificial urine medium), the mean volume of urine of adults (1 mL min^–1), the frequently used silicone catheter (foley silicon catheter) as well as the infection with uropathogenic microorganisms like Pseudomonas aeruginosa. Three clinical isolates from urine of catheterized patients were chosen due to their ability to form biofilms, their mobility and their cell surface hydrophobicity. As reference strain P. aeruginosa PA14 has been used. Characteristic parameters as biofilm thickness, specific biofilm growth rate and substrate consumption were observed. Biofilm thicknesses varied from 105 ± 16 μm up to 246 ± 67 μm for the different isolates. The specific biofilm growth rate could be determined with a non invasive optical biomass sensor. This sensor allows online monitoring of the biofilm growth in the progress of the cultivation.     

Efficacy of Indolicidin,Cecropin A (1-7)-Melittin (CAMA) and Their Combination Against Biofilm-Forming Multidrug-Resistant Enteroaggregative Escherichia coli

The present study examined the anti-biofilm efficacy of two short-chain antimicrobial peptides (AMPs), namely, indolicidin and cecropin A (1-7)-melittin (CAMA) against biofilm-forming multidrug-resistant enteroaggregative Escherichia coli (MDR-EAEC) isolates. The typical EAEC isolates re-validated by PCR and confirmed using HEp-2 cell adherence assay was subjected to antibiotic susceptibility testing to confirm its MDR status. The biofilm-forming ability of MDR-EAEC isolates was assessed by Congo red binding, microtitre plate assays and hydrophobicity index; broth microdilution technique was employed to determine minimum inhibitory concentrations (MICs) and minimum biofilm eradication concentrations (MBECs). The obtained MIC and MBEC values for both AMPs were evaluated alone and in combination against MDR-EAEC biofilms using crystal violet (CV) staining and confocal microscopy-based live/dead cell quantification methods. All the three MDR-EAEC strains revealed weak to strong biofilm-forming ability and were found to be electron-donating and weakly electron-accepting (hydrophobicity index). Also, highly significant (P < 0.001) time-dependent hydrodynamic growth of the three MDR-EAEC strains was observed at 48 h of incubation in Dulbecco’s modified Eagle’s medium (DMEM) containing 0.45% D-glucose. AMPs and their combination were able to inhibit the initial biofilm formation at 24 h and 48 h as evidenced by CV staining and confocal quantification. Further, the application of AMPs (individually and combination) against the preformed MDR-EAEC biofilms resulted in highly significant eradication (P < 0.001) at 24 h post treatment. However, significant differences were not observed between AMP treatments (individually or in combination). The AMPs seem to be an effective candidates for further investigations such as safety, stability and appropriate biofilm-forming MDR-EAEC animal models.

     

Antimicrobial and anti-biofilm properties of new taxodione derivative from hairy roots of Salvia austriaca

The aim of the present report was to evaluate antimicrobial/anti-biofilm activity of 7-(2-oxohexyl)-taxodione, a novel taxodione derivative isolated from n-hexane extract of Salvia austriaca hairy roots. Antimicrobial assays showed that 7-(2-oxohexyl)-taxodione was at least 4 times more active than taxodione against methicillin-susceptible as well against methicillin-resistant staphylococci with MIC of 1.25–2.5 μg ml⁻¹. This compound was less active against vancomycin-resistant enterococci (VRE), on the same level as taxodione (MIC ranged 10.0–20.0 μg ml⁻¹). The presence of 7-(2-oxohexyl)-taxodione in the culture medium (at MIC, ½ MIC or ¼ MIC) decreased adhesion of staphylococci to abiotic surfaces, which in turn caused a reduction in biofilm formation during 24 h, by approximately 25–30%. Also, the extent of established biofilm eradication was found to be significant, although it required an increased concentration of the compound. This is the first report on the antimicrobial activity of this, up to now not known compound, isolated from transformed roots of S. austriaca.     

In-vitro effect of human cathelicidin antimicrobial peptide LL-37 on dengue virus type 2

Human Cathelicidin antimicrobial peptide LL-37 is known to have antiviral activity against many viruses. In the present study, we investigated the in-vitro effect of LL-37 on dengue virus type 2 (DENV-2) infection and replication in Vero E6 cells. To study the effect of pretreatment of virus or cells with LL-37, the virus was pretreated with different concentrations of LL-37 (2.5 μM–15 μM) or scrambled (Scr) LL-37(5 μM–15 μM) and used for infection or the cells were first treated with LL-37 and infected. To study the effect of LL-37 post infection (PI), the cells were infected first followed by addition of LL-37 to the culture medium 24 h after infection. In all conditions, after the incubation, the culture supernatant was assessed for viral RNA copy number by real time RT-PCR, infectious virus particles by focus forming unit assay (FFU) and non structural protein 1 (NS1) antigen levels by ELISA. Percentage of infection was assessed using immunoflourescence assay (IFA). The results revealed that pretreatment of virus with 10–15 μM LL-37 significantly reduced its infectivity as compared to virus control (P < 0.0001). Moreover, pretreatment of virus with 10–15 μM LL-37 significantly reduced the levels of viral genomic RNA and NS1 antigen (P < 0.0001). Treatment of virus with 10–15 μM LL-37 resulted in two to three log reduction of mean log₁₀ FFU/ml as compared to virus control (P < 0.0001). Treatment of the virus with scrambled LL-37 had no effect on percentage of infection and viral load as compared to virus control cultures (P > 0.05). Pretreatment of cells before infection or addition of LL-37 to the culture 24 h PI had no effect on viral load. Molecular docking studies revealed possible binding of LL-37 to both the units of DENV envelope (E) protein dimer. Together, the in-vitro experiments and in-silico analyses suggest that LL-37 inhibits DENV-2 at the stage of entry into the cells by binding to the E protein. The results might have implications for prophylaxis against DENV infections and need further in-vivo studies.     

Anti-quorum sensing and anti-biofilm activity of Amomum tsaoko (Amommum tsao-ko Crevost et Lemarie) on foodborne pathogens

Cell-to-cell communication or quorum sensing (QS) leads to biofilm formation and causing other virulence factors which are extreme problems for food safety, biofilm related infectious diseases etc. This study evaluated the anti-QS activity of the Amomum tsaoko extract (0.5–4 mg/ml) by using Chromobacterium violaceum a biosensor strain and biofilm formation by crystal violate assay. Experimental results demonstrated that the overall yield of Amomum tsao-ko extract was 11.33 ± 0.3% (w/w). MIC for Staphylococcus aureus (Gram positive), Salmonella Typhimurium and Pseudomonas aeruginosa (Gram negative) was 1, 2 and 2 mg/ml, respectively. A concentration of 4 mg/ml extract showed highest biofilm inhibition 51.96% on S. Typhimurium when 47.06%, 45.28% were shown by S. aureus, P. aeruginosa respectively. The damage of biofilm architecture was observed by Confocal Laser Scanning Microscopy (CLSM). A level of 44.59% inhibition of violacein production was demonstrated when the dose was 4 mg/ml. Swarming motility inhibition was observed in a dose dependent manner. Taken together, the treatment of A. tsaoko extract can deliver value to food product and medicine by controlling pathogenesis.     

Design,expression and characterization of the hybrid antimicrobial peptide LHP7, connected by a flexible linker,against Staphylococcus and Streptococcus

The emergence of bacterial resistance to common antibiotics poses a threat to human health and has rekindled an interest in antimicrobial peptides (AMPs). LHP7, a novel hybrid AMP containing 83 amino acid residues was designed on the basis of the LH28 and plectasin. LHP7 was expressed in Pichia pastoris, the total concentration of secreted protein reached 0.906 g/L after 108 h of methanol induction. Its antimicrobial activity was higher than that of the parent AMPs; the minimal inhibitory concentrations (MICs) of LHP7 against Staphylococcus aureus, Streptococcus pneumoniae and Streptococcus suis were 0.091, 0.023 and 0.18 μM, respectively. The antibacterial activity of LHP7 against clinical MRSA isolates (MICs = 0.73–2.91 μM) was enhanced over that of plectasin. The fractional inhibitory concentration (FIC) indicated a synergistic effect between LHP7 and ampicillin against MRSA (FIC = 0.375), and combinations of LHP7 with gentamicin, rifampin or tetracycline provided evidence of additive effects (FIC = 0.625–1.0). LHP7 exhibited a broad range of pH stability and thermostability, and a hemolytic activity of less than 5% below a concentration of 500 μg/mL. It was resistant to pepsin and papain digestion, but sensitive to trypsin digestion. These results suggest that LHP7 might have potential as a broadly applied and clinically useful antimicrobial agent.     

Structure–activity studies of echinomycin antibiotics against drug-resistant and biofilm-forming Staphylococcus aureus and Enterococcus faecalis

Four echinomycin antibiotics were isolated from the culture broth of a marine streptomycete, and their structures were determined by a combination of chemical and spectroscopic analyses. Antibiotic activities were measured against drug-resistant and biofilm-forming strains of Staphylococcus aureus and Enterococcus faecalis. Minimum inhibitory concentrations ranging from 0.01 μM to greater than 14 μM clearly defined structure–activity relationships for antibiotic potency. Echinomycin was the most active compound with a MIC of 0.03 μM against methicillin-resistant S. aureus and 0.01 μM against biofilm-forming E. faecalis.     

Identification and characterization of an anti-pseudomonal dichlorocarbazol derivative displaying anti-biofilm activity

Pseudomonas aeruginosa strains resistant towards all currently available antibiotics are increasingly encountered, raising the need for new anti-pseudomonal drugs. We therefore conducted a medium-throughput screen of a small-molecule collection resulting in the identification of the N-alkylated 3,6-dihalogenocarbazol 1-(sec-butylamino)-3-(3,6-dichloro-9H-carbazol-9-yl)propan-2-ol (MIC = 18.5 μg mL⁻¹). This compound, compound 1, is bacteriostatic towards a broad spectrum of Gram-positive and Gram-negative pathogens, including P. aeruginosa. Importantly, 1 also eradicates mature biofilms of P. aeruginosa. 1 displays no cytotoxicity against various human cell types, pointing to its potential for further development as a novel antibacterial drug.     

Antibiofilm activity and post antifungal effect of lemongrass oil on clinical Candida dubliniensis isolate

Candidal infections are often difficult to eradicate due to the resistance of biofilms to antifungal agents. This study aimed at determining the effects of lemongrass (Cymbopogon citratus DC) oil against Candida dubliniensis in both planktonic and biofilms form. The results from broth microdilution method revealed that the minimum inhibitory and minimum fungicidal concentration of lemongrass oil on C. dubliniensis were 0.43 and 0.86 mg/ml, respectively. Employing a formazan salt (XTT tetrazolium) reduction assay for biofilm study, the results showed that the average percentage (mean ± SD) inhibition of lemongrass oil (0.43 mg/ml) on biofilm formation was 91.57 ± 1.31%, while it exhibited more than 80% killing activity against C. dubliniensis in biofilm at concentrations of 1.7 mg/ml. In addition, a significant reduction (P = 0.03) of candidal adhesion to acrylic occurred after a 15 min exposure to 1.7 mg/ml of lemongrass oil. Moreover, limited exposure of yeasts to lemongrass oil at subcidal concentration can suppress growth for more than 24 h. Altogether, the results obtained indicate that lemongrass oil possessed antifungal and antibiofilm activities and could modulate candidal colonization. Therefore, the efficacy of lemongrass oil merits further development of this agent for the therapy of oral candidiasis.     

Identification of aryl 2-aminoimidazoles as biofilm inhibitors in Gram-negative bacteria

The synthesis and biofilm inhibitory activity of a 30-member aryl amide 2-aminoimidazole library against the three biofilm forming Gram-negative bacteria Escherichia coli, Psuedomonas aeruginosa, and Acinetobacter baumannii is presented. The most active compound identified inhibits the formation of E. coli biofilms with an IC₅₀ of 5.2 μM and was observed to be non-toxic to planktonic growth, demonstrating that analogues based on an aryl framework are viable options as biofilm inhibitors within the 2-aminoimidazole family.     

Synthesis and antimicrobial activity of polyhalo isophthalonitrile derivatives

A series of polyhalo isophthalonitrile derivatives (3 and 4) that incorporate a variety of substituents at the 2-, 4-, 5- and/or 6-positions of the isophthalonitrile moieties have been designed and synthesized. These derivatives were evaluated for their antimicrobial activity against Staphylococcus aureus, Bacillus cereus (Gram-positive bacteria), Escherichia coli, Pseudomonas aeruginosa (Gram-negative bacteria); and Candida albicans (Fungi). Compounds 3 and 4 showed stronger inhibition of Gram-positive bacteria and fungi growth, and the antimicrobial ability of compound 3j (a 4-(benzylamino)-5-chloro-2,6-difluoro analog, MIC[SA] = 0.5 μg/mL; MIC[BC] = 0.4 μg/mL; MIC[CA] = 0.5 μg/mL) were close to nofloxacin and fluconazole and identified as the most potent antimicrobial agents in the series. The preliminary analysis of structure–activity relationships is also discussed.     

Synthesis and antimicrobial profile of N-substituted imidazolium oximes and their monoquaternary salts against multidrug resistant bacteria

Two different series of N-substituted imidazolium oximes and their monoquaternary salts were synthesized and biologically tested with respect to their ability to inhibit growth a diverse panel of antibiotic susceptible Gram-positive and antibiotic resistant Gram-negative bacteria as well fungal strains. The newly synthesized compounds were analyzed by spectral studies to confirm their structure. The preliminary results showed that all compounds tested possess promising antimicrobial potential against both susceptible Gram-positive and antibiotic resistant Gram-negative isolates, exhibiting a wide range of MIC values from 0.14 to 100.0 μg/mL. The structure–activity relationship demonstrates that the p-methylphenyl and p-fluorophenyl groups in monoquaternary salts 6 and 7 attached directly to the imidazolium ring could be essential for observed remarkable inhibitory profiles against clinically important pathogens Pseudomonas aeruginosa (MIC = 0.14 μg/mL) and Klebsiella pneumoniae (MIC = 1.56 μg/mL). Furthermore, the broth microdilution assay was then used to investigate the antiresistance efficacy of compound 7 against fourteen extended-spectrum β-lactamase (ESBL)-producing strains in comparison to eight clinically relevant antibiotics. Compound 7 exhibited a remarkable antiresistance profiles ranging between 0.39 and 12.50 μg/mL against all of ESBL-producing strains, which leads to the suggestion that may be interesting candidate for development of new antimicrobials to combat multidrug resistant Gram-negative bacteria.     

Synthesis and antimicrobial activity of novel amphiphilic aromatic amino alcohols

We report in this work the preparation and in vitro antimicrobial evaluation of novel amphiphilic aromatic amino alcohols synthesized by reductive amination of 4-alkyloxybenzaldehyde with 2-amino-2-hydroxymethyl-propane-1,3-diol. The antibacterial activity was determined against four standard strains (Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli, Pseudomonas aeruginosa) and 21 clinical isolates of methicillin-resistant Staphylococcus aureus. The antifungal activity was evaluated against four yeast (Candida albicans, Candida tropicalis, Candida glabrata and Candida parapsilosis). The results obtained showed a strong positive correlation between the lipophilicity and the antibiotic activity of the tested compounds. The best activities were obtained against the Gram-positive bacteria (MIC = 2–16 μg ml^?1) for the five compounds bearing longer alkyl chains (4c–g; 8–14 carbons), which were also the most active against Candida (MIC = 2–64 μg ml^?1). Compound 4e exhibited the highest levels of inhibitory activity (MIC = 2–16 μg ml^?1) against clinical isolates of MRSA. A concentration of twice the MIC resulted in bactericidal activity of 4d against 19 of the 21 clinical isolates.     

Promethazine improves antibiotic efficacy and disrupts biofilms of Burkholderia pseudomallei

Efflux pumps are important defense mechanisms against antimicrobial drugs and maintenance of Burkholderia pseudomallei biofilms. This study evaluated the effect of the efflux pump inhibitor promethazine on the structure and antimicrobial susceptibility of B. pseudomallei biofilms. Susceptibility of planktonic cells and biofilms to promethazine alone and combined with antimicrobials was assessed by the broth microdilution test and biofilm metabolic activity was determined with resazurin. The effect of promethazine on 48 h-grown biofilms was also evaluated through confocal and electronic microscopy. The minimum inhibitory concentration (MIC) of promethazine was 780 mg l^?1, while the minimum biofilm elimination concentration (MBEC) was 780–3,120 mg l^?1. Promethazine reduced the MIC values for erythromycin, trimethoprim/sulfamethoxazole, gentamicin and ciprofloxacin and reduced the MBEC values for all tested drugs (p<0.05). Microscopic analyses demonstrated that promethazine altered the biofilm structure of B. pseudomallei, even at subinhibitory concentrations, possibly facilitating antibiotic penetration. Promethazine improves antibiotics efficacy against B. pseudomallei biofilms, by disrupting biofilm structure.     

Synthesis and antibacterial studies of binaphthyl-based tripeptoids. Part 1

An efficient synthesis of 29 new binaphthyl-based neutral, and mono- and di-cationic, peptoids is described. Some of these compounds had antibacterial activities with MIC values of 1.9–3.9 μg/mL against Staphylococcus aureus. One peptoid had a MIC value of 6 μg/mL against a methicillin-resistant strain of S. aureus (MRSA) and a MIC value of 2 μg/mL against vancomycin-resistant strains of enterococci (VRE).     

Derivatives (halogen,nitro and amino) of 8-hydroxyquinoline with highly potent antimicrobial and antioxidant activities

8-Hydroxyquinoline (8HQ) compounds have been reported to possess diverse bioactivities. In recent years, drug repositioning has gained considerable attention in drug discovery and development. Herein, 8HQ (1) and its derivatives (2–9) bearing various substituents (amino, nitro, cyano and halogen) were investigated for their antimicrobial against 27 microorganisms (agar dilution method) and antioxidant (DPPH method) activities. The parent 8HQ (1) exerted a highly potent antimicrobial activity against Gram-positive bacteria including diploid fungi and yeast with MIC values in the range of 3.44–13.78 μM. Moreover, the halogenated 8HQ, especially 7-bromo-8HQ (4) and clioquinol (6), displayed a high antigrowth activity against Gram-negative bacteria compared with the parent compound (1). Apparently, the derivatives with a relatively high safely index, e.g., nitroxoline (2), exhibited strong antibacterial activity against Aeromonas hydrophila (MIC=5.26 μM) and selectively inhibited the growth of P. aeruginosa with the MIC value of 84.14 μM; cloxyquin (3) showed a strong activity against Listseria monocytogenes and Plesiomonas shigelloides with MIC values of 5.57 and 11.14 μM, respectively. Most compounds displayed an antioxidant activity. Specifically, 5-amino-8HQ (8) was shown to be the most potent antioxidant (IC₅₀=8.70 μM) compared with the positive control (α-tocopherol) with IC₅₀ of 13.47 μM. The findings reveal that 8HQ derivatives are potential candidates to be further developed as antimicrobial and antioxidant agents.