Antibacterial and antibiofilm activity of permanently ionized quaternary ammonium fluoroquinolones

A series of quaternary ammonium fluoroquinolones was obtained by exhaustive methylation of the amine groups present at the 7-position of fluoroquinolones, including ciprofloxacin, enoxacin, gatifloxacin, lomefloxacin, and norfloxacin. The synthesized molecules were tested for their antibacterial and antibiofilm activities against Gram-positive and Gram-negative human pathogens, i.e. Staphylococcus aureus and Pseudomonas aeruginosa. The study showed that the synthesized compounds are potent antibacterial agents (MIC values at the lowest 6.25 μM) with low cytotoxicity in vitro as assessed on the BALB 3T3 mouse embryo cell line. Further experiments proved that the tested derivatives are able to bind to the DNA gyrase and topoisomerase IV active sites in a fluoroquinolone-characteristic manner. The most active quaternary ammonium fluoroquinolones, in contrast to ciprofloxacin, reduce the total biomass of P. aeruginosa ATCC 15442 biofilm in post-exposure experiments. The latter effect may be due to the dual mechanism of action of the quaternary fluoroquinolones, which also involves disruption of bacterial cell membranes. IAM-HPLC chromatographic experiments with immobilized artificial membranes (phospholipids) showed that the most active compounds were those with moderate lipophilicity and containing a cyclopropyl group at the N1 nitrogen atom in the fluoroquinolone core.

Repurposing the Sphingosine-1-Phosphate Receptor Modulator Etrasimod as an Antibacterial Agent Against Gram-Positive Bacteria

New classes of antibiotics are urgently needed in the fight against multidrug-resistant bacteria. Drug repurposing has emerged as an alternative approach to accelerate antimicrobial research and development. In this study, we screened a library of sphingosine-1-phosphate receptor (S1PR) modulators against Staphylococcus aureus and identified five active compounds. Among them, etrasimod (APD334), an investigational drug for the treatment of ulcerative colitis, displayed the best inhibitory activity against S. aureus when growing as free-floating planktonic cells and within biofilms. In follow-up studies, etrasimod showed bactericidal activity and drastic reduction of viable bacteria within 1 h of exposure. It also displayed a potent activity against other Gram-positive bacteria, including penicillin- and methicillin-resistant S. aureus strains, S. epidermidis, and Enterococcus faecalis, with a minimum inhibitory concentration (MIC) ranging from 5 to 10 μM (2.3–4.6 μg/mL). However, no inhibition of viability was observed against Gram-negative bacteria Acinetobacter baumannii, Escherichia coli, and Pseudomonas aeruginosa, showing that etrasimod preferably acts against Gram-positive bacteria. On the other hand, etrasimod was shown to inhibit quorum sensing (QS) signaling in Chromobacterium violaceum, suggesting that it may block the biofilm formation by targeting QS in certain Gram-negative bacteria. Furthermore, etrasimod displayed a synergistic effect with gentamicin against S. aureus, thus showing potential to be used in antibiotic combination therapy. Finally, no in vitro toxicity toward mammalian cells was observed. In conclusion, our study reports for the first time the potential of etrasimod as a repurposed antibacterial compound against Gram-positive bacteria.

Screening of natural compounds identifies ferutinin as an antibacterial and anti-biofilm compound

Antibacterial screenings are most commonly targeted at planktonic bacteria but less effort is dedicated to the exploration of agents acting on biofilms. Here, a natural compounds library was screened against Staphylococcus aureus using a 384-well plate platform to identify compounds preventing biofilm formation. Five structurally diverse hits were selected for follow-up studies: honokiol, tschimganidin, ferutinin, oridonin and deoxyshikonin. The compounds were evaluated against different bacterial species for their capacity to prevent and disrupt biofilms. The development of resistance and cytotoxicity were also investigated. Ferutinin displayed the best antibacterial activity, with a minimum inhibitory, bactericidal and biofilm preventive concentration of 25 µM against S. aureus. It efficiently disrupted pre-formed biofilms (over 5-log reduction of viable cells) and reduced biofilm formation on a catheter in the presence of neutrophils. This work provides new information on the antibacterial activity of five natural compounds and identified ferutinin as a promising candidate against S. aureus biofilms.

Synthesis and Biological Evaluation of Fingolimod Derivatives as Antibacterial Agents

We recently identified fingolimod as a potent antibiofilm compound by screening FDA-approved drugs. To study if the antibacterial activity of fingolimod could be further improved and to explore in-depth structure-activity relationships, we synthesized 28 novel fingolimod derivatives and evaluated their efficacy against Staphylococcus aureus grown in planktonic/single cell and biofilms. The most effective derivatives were tested on preformed S. aureus biofilms and against Gram-negative bacteria Acinetobacter baumannii and Pseudomonas aeruginosa, using fingolimod as the reference compound. Seven derivatives were more effective against S. aureus, while five other derivatives showed improved activity against P. aeruginosa and/or A. baumannii, with no apparent change in cytotoxicity on human cells. The most interesting derivatives, compounds 43 and 55, displayed a broader spectrum of antibacterial activity, possibly exerted by the change of the para-hydrocarbon chain to a meta position for 43 and by an additional hydroxyl group for 55.

Surfaceome and Exoproteome Dynamics in Dual-Species Pseudomonas aeruginosa and Staphylococcus aureus Biofilms

Bacterial biofilms are an important underlying cause for chronic infections. By switching into the biofilm state, bacteria can evade host defenses and withstand antibiotic chemotherapy. Despite the fact that biofilms at clinical and environmental settings are mostly composed of multiple microbial species, biofilm research has largely been focused on single-species biofilms. In this study, we investigated the interaction between two clinically relevant bacterial pathogens (Staphylococcus aureus and Pseudomonas aeruginosa) by label-free quantitative proteomics focusing on proteins associated with the bacterial cell surfaces (surfaceome) and proteins exported/released to the extracellular space (exoproteome). The changes observed in the surfaceome and exoproteome of P. aeruginosa pointed toward higher motility and lower pigment production when co-cultured with S. aureus. In S. aureus, lower abundances of proteins related to cell wall biosynthesis and cell division, suggesting increased persistence, were observed in the dual-species biofilm. Complementary phenotypic analyses confirmed the higher motility and the lower pigment production in P. aeruginosa when co-cultured with S. aureus. Higher antimicrobial tolerance associated with the co-culture setting was additionally observed in both species. To the best of our knowledge, this study is among the first systematic explorations providing insights into the dynamics of both the surfaceome and exoproteome of S. aureus and P. aeruginosa dual-species biofilms.

Screening of FDA-Approved Drugs Using a 384-Well Plate-Based Biofilm Platform: The Case of Fingolimod

In an effort to find new repurposed antibacterial compounds, we performed the screening of an FDA-approved compounds library against Staphylococcus aureus American Type Culture Collection (ATCC) 25923. Compounds were evaluated for their capacity to prevent both planktonic growth and biofilm formation as well as to disrupt pre-formed biofilms. One of the identified initial hits was fingolimod (FTY720), an immunomodulator approved for the treatment of multiple sclerosis, which was then selected for follow-up studies. Fingolimod displayed a potent activity against S. aureus and S. epidermidis with a minimum inhibitory concentration (MIC) within the range of 12–15 µM at which concentration killing of all the bacteria was confirmed. A time–kill kinetic study revealed that fingolimod started to drastically reduce the viable bacterial count within two hours and we showed that no resistance developed against this compound for up to 20 days. Fingolimod also displayed a high activity against Acinetobacter baumannii (MIC 25 µM) as well as a modest activity against Escherichia coli and Pseudomonas aeruginosa. In addition, fingolimod inhibited quorum sensing in Chromobacterium violaceum and might therefore target this signaling pathway in certain Gram-negative bacteria. In conclusion, we present the identification of fingolimod from a compound library and its evaluation as a potential repurposed antibacterial compound.

Optimization of a High-Throughput 384-Well Plate-Based Screening Platform with Staphylococcus aureus ATCC 25923 and Pseudomonas aeruginosa ATCC 15442 Biofilms

In recent years, bacterial infections have become a main concern following the spread of antimicrobial resistance. In addition, bacterial biofilms are known for their high tolerance to antimicrobials and they are regarded as a main cause of recalcitrant infections in humans. Many efforts have been deployed in order to find new antibacterial therapeutic options and the high-throughput screening (HTS) of large libraries of compounds is one of the utilized strategies. However, HTS efforts for anti-biofilm discovery remain uncommon. Here, we miniaturized a 96-well plate (96WP) screening platform, into a 384-well plate (384WP) format, based on a sequential viability and biomass measurements for the assessment of anti-biofilm activity. During the assay optimization process, different parameters were evaluated while using Staphylococcus aureus and Pseudomonas aeruginosa as the bacterial models. We compared the performance of the optimized 384WP platform to our previously established 96WP-based platform by carrying out a pilot screening of 100 compounds, followed by the screening of a library of 2000 compounds to identify new repurposed anti-biofilm agents. Our results show that the optimized 384WP platform is well-suited for screening purposes, allowing for the rapid screening of a higher number of compounds in a run in a reliable manner.

Role for the shelterin protein TRF2 in human herpesvirus 6A/B chromosomal integration

Human herpesviruses 6A and 6B (HHV-6A/B) are unique among human herpesviruses in their ability to integrate their genome into host chromosomes. Viral integration occurs at the ends of chromosomes within the host telomeres. The ends of the HHV-6A/B genomes contain telomeric repeats that facilitate the integration process. Here, we report that productive infections are associated with a massive increase in telomeric sequences of viral origin. The majority of the viral telomeric signals can be detected within viral replication compartments (VRC) that contain the viral DNA processivity factor p41 and the viral immediate-early 2 (IE2) protein. Components of the shelterin protein complex present at telomeres, including TRF1 and TRF2 are also recruited to VRC during infection. Biochemical, immunofluorescence coupled with in situ hybridization and chromatin immunoprecipitation demonstrated the binding of TRF2 to the HHV-6A/B telomeric repeats. In addition, approximately 60% of the viral IE2 protein localize at cellular telomeres during infection. Transient knockdown of TRF2 resulted in greatly reduced (13%) localization of IE2 at cellular telomeres (p<0.0001). Lastly, TRF2 knockdown reduced HHV-6A/B integration frequency (p<0.05), while no effect was observed on the infection efficiency. Overall, our study identified that HHV-6A/B IE2 localizes to telomeres during infection and highlight the role of TRF2 in HHV-6A/B infection and chromosomal integration.

Synthesis and biological evaluation of hybrid quinolone-based quaternary ammonium antibacterial agents

A series of novel fluoroquinolone-Safirinium dye hybrids was synthesized by means of tandem Mannich-electrophilic amination reactions from profluorophoric isoxazolones and antibiotics bearing a secondary amino group at position 7 of the quinoline ring. The obtained fluorescent spiro fused conjugates incorporating quaternary nitrogen atoms were characterized by ¹H NMR, IR, MS, and elemental analysis. All the synthetic analogues (3a-h and 4a-h) were evaluated for their in vitro antimicrobial, bactericidal, and antibiofilm activities against a panel of Gram positive and Gram-negative pathogenic bacteria. The most active Safirinium Q derivatives of lomefloxacin (4d) and ciprofloxacin (4e) exhibited molar-based antibacterial activities comparable to the unmodified drugs and displayed considerable inhibitory potencies in E. coli DNA gyrase supercoiling assays with IC₅₀ values in the low micromolar range. Zwiterionic hybrids were noticeably less lipophilic than the parent quinolones in micellar electrokinetic chromatography (MECK) experiments. The tests performed in the presence of phenylalanine-arginine β-naphthylamide (PAβN) or carbonyl cyanide m-chlorophenylhydrazone (CCCP) revealed that the conjugates are to some extent subject to bacterial efflux and cellular accumulation, respectively. Moreover, the hybrids did not exhibit notable cytotoxicity towards the HEK 293 control cell line and demonstrated low propensity for resistance development, as exemplified for compounds 3g and 4b. Finally, molecular docking experiments revealed that the synthesized compounds were able to bind in the fluoroquinolone-binding mode at S. aureus DNA gyrase and S. pneumoniae topoisomerase IV active sites.

Characterization of human herpesvirus 6A/B U94 as ATPase, helicase, exonuclease and DNA-binding proteins

Human herpesvirus-6A (HHV-6A) and HHV-6B integrate their genomes into the telomeres of human chromosomes, however, the mechanisms leading to integration remain unknown. HHV-6A/B encode a protein that has been proposed to be involved in integration termed U94, an ortholog of adeno-associated virus type 2 (AAV-2) Rep68 integrase. In this report, we addressed whether purified recombinant maltose-binding protein (MBP)-U94 fusion proteins of HHV-6A/B possess biological functions compatible with viral integration. We could demonstrate that MBP-U94 efficiently binds both dsDNA and ssDNA containing telomeric repeats using gel shift assay and surface plasmon resonance. MBP-U94 is also able to hydrolyze adenosine triphosphate (ATP) to ADP, providing the energy for further catalytic activities. In addition, U94 displays a 3′ to 5′ exonuclease activity on dsDNA with a preference for 3′-recessed ends. Once the DNA strand reaches 8–10 nt in length, the enzyme dissociates it from the complementary strand. Lastly, MBP-U94 compromises the integrity of a synthetic telomeric D-loop through exonuclease attack at the 3′ end of the invading strand. The preferential DNA binding of MBP-U94 to telomeric sequences, its ability to hydrolyze ATP and its exonuclease/helicase activities suggest that U94 possesses all functions required for HHV-6A/B chromosomal integration.