Options and Limitations in Clinical Investigation of Bacterial Biofilms.

Bacteria can form single- and multispecies biofilms exhibiting diverse features based upon the microbial composition of their community and microenvironment. The study of bacterial biofilm development has received great interest in the past 20 years and is motivated by the elegant complexity characteristic of these multicellular communities and their role in infectious diseases. Biofilms can thrive on virtually any surface and can be beneficial or detrimental based upon the community's interplay and the surface. Advances in the understanding of structural and functional variations and the roles that biofilms play in disease and host-pathogen interactions have been addressed through comprehensive literature searches. In this review article, a synopsis of the methodological landscape of biofilm analysis is provided, including an evaluation of the current trends in methodological research. We deem this worthwhile because a keyword-oriented bibliographical search reveals that less than 5% of the biofilm literature is devoted to methodology. In this report, we (i) summarize current methodologies for biofilm characterization, monitoring, and quantification; (ii) discuss advances in the discovery of effective imaging and sensing tools and modalities; (iii) provide an overview of tailored animal models that assess features of biofilm infections; and (iv) make recommendations defining the most appropriate methodological tools for clinical settings.

Attaching NorA efflux pump inhibitors to methylene blue enhances antimicrobial photodynamic inactivation of Escherichia coli and Acinetobacter baumannii in vitro and in vivo.

Resistance of bacteria to antibiotics is a public health concern worldwide due to the increasing failure of standard antibiotic therapies. Antimicrobial photodynamic inactivation (aPDI) is a promising non-antibiotic alternative for treating localized bacterial infections that uses non-toxic photosensitizers and harmless visible light to produce reactive oxygen species and kill microbes. Phenothiazinium photosensitizers like methylene blue (MB) and toluidine blue O are hydrophobic cations that are naturally expelled from bacterial cells by multidrug efflux pumps, which reduces their effectiveness. We recently reported the discovery of a NorA efflux pump inhibitor-methylene blue (EPI-MB) hybrid compound INF55-(Ac)en-MB that shows enhanced photodynamic inactivation of the Gram-positive bacterium methicillin-resistant Staphylococcus aureus (MRSA) relative to MB, both in vitro and in vivo. Here, we report the surprising observation that INF55-(Ac)en-MB and two related hybrids bearing the NorA efflux pump inhibitors INF55 and INF271 also show enhanced aPDI activity in vitro (relative to MB) against the Gram-negative bacteria Escherichia coli and Acinetobacter baumannii, despite neither species expressing the NorA pump. Two of the hybrids showed superior effects to MB in murine aPDI infection models. The findings motivate wider exploration of aPDI with EPI-MB hybrids against Gram-negative pathogens and more detailed studies into the molecular mechanisms underpinning their activity.

Light-based technologies for management of COVID-19 pandemic crisis.

The global dissemination of the novel coronavirus disease (COVID-19) has accelerated the need for the implementation of effective antimicrobial strategies to target the causative agent SARS-CoV-2. Light-based technologies have a demonstrable broad range of activity over standard chemotherapeutic antimicrobials and conventional disinfectants, negligible emergence of resistance, and the capability to modulate the host immune response. This perspective article identifies the benefits, challenges, and pitfalls of repurposing light-based strategies to combat the emergence of COVID-19 pandemic.

Structure-activity relationships of 2-aryl-1H-indole inhibitors of the NorA efflux pump in Staphylococcus aureus.

The synthesis of 22 2-aryl-1H-indoles, including 12 new compounds, has been achieved via Pd- or Rh-mediated methodologies, or selective electrophilic substitution. All three methods were based on elaborations from simple indole precursors. SAR studies on these indoles and 2-phenyl-1H-indole in Staphylococcus aureus as NorA efflux pump inhibitors indicated 5-nitro-2-(3-methoxycarbonyl)phenyl-1H-indole was a slightly more potent inhibitor than the lead INF55. A promising new antibacterial lead compound against S. aureus (2-phenyl-1H-indol-5-yl)-methanol, was also found.

Attaching the NorA Efflux Pump Inhibitor INF55 to Methylene Blue Enhances Antimicrobial Photodynamic Inactivation of Methicillin-Resistant Staphylococcus aureus in Vitro and in Vivo.

Antimicrobial photodynamic inactivation (aPDI) uses photosensitizers (PSs) and harmless visible light to generate reactive oxygen species (ROS) and kill microbes. Multidrug efflux systems can moderate the phototoxic effects of PSs by expelling the compounds from cells. We hypothesized that increasing intracellular concentrations of PSs by inhibiting efflux with a covalently attached efflux pump inhibitor (EPI) would enhance bacterial cell phototoxicity and reduce exposure of neighboring host cells to damaging ROS. In this study, we tested the hypothesis by linking NorA EPIs to methylene blue (MB) and examining the photoantimicrobial activity of the EPI-MB hybrids against the human pathogen methicillin-resistant Staphylococcus aureus (MRSA). Photochemical/photophysical and in vitro microbiological evaluation of 16 hybrids carrying four different NorA EPIs attached to MB via four linker types identified INF55-(Ac)en-MB 12 as a lead. Compound 12 showed increased uptake into S. aureus cells and enhanced aPDI activity and wound healing effects (relative to MB) in a murine model of an abrasion wound infected by MRSA. The study supports a new approach for treating localized multidrug-resistant MRSA infections and paves the way for wider exploration of the EPI-PS hybrid strategy in aPDI.

Microbial efflux systems and inhibitors: approaches to drug discovery and the challenge of clinical implementation.

Conventional antimicrobials are increasingly ineffective due to the emergence of multidrug-resistance among pathogenic microorganisms. The need to overcome these deficiencies has triggered exploration for novel and unconventional approaches to controlling microbial infections. Multidrug efflux systems (MES) have been a profound obstacle in the successful deployment of antimicrobials. The discovery of small molecule efflux system blockers has been an active and rapidly expanding research discipline. A major theme in this platform involves efflux pump inhibitors (EPIs) from natural sources. The discovery methodologies and the available number of natural EPI-chemotypes are increasing. Advances in our understanding of microbial physiology have shed light on a series of pathways and phenotypes where the role of efflux systems is pivotal. Complementing existing antimicrobial discovery platforms such as photodynamic therapy (PDT) with efflux inhibition is a subject under investigation. This core information is a stepping stone in the challenge of highlighting an effective drug development path for EPIs since the puzzle of clinical implementation remains unsolved. This review summarizes advances in the path of EPI discovery, discusses potential avenues of EPI implementation and development, and underlines the need for highly informative and comprehensive translational approaches.

Strategies to potentiate antimicrobial photoinactivation by overcoming resistant phenotypes.

Conventional antimicrobial strategies have become increasingly ineffective due to the emergence of multidrug resistance among pathogenic microorganisms. The need to overcome these deficiencies has triggered the exploration of alternative treatments and unconventional approaches towards controlling microbial infections. Photodynamic therapy (PDT) was originally established as an anticancer modality and is currently used in the treatment of age-related macular degeneration. The concept of photodynamic inactivation requires cell exposure to light energy, typically wavelengths in the visible region that causes the excitation of photosensitizer molecules either exogenous or endogenous, which results in the production of reactive oxygen species (ROS). ROS produce cell inactivation and death through modification of intracellular components. The versatile characteristics of PDT prompted its investigation as an anti-infective discovery platform. Advances in understanding of microbial physiology have shed light on a series of pathways, and phenotypes that serve as putative targets for antimicrobial drug discovery. Investigations of these phenotypic elements in concert with PDT have been reported focused on multidrug efflux systems, biofilms, virulence and pathogenesis determinants. In many instances the results are promising but only preliminary and require further investigation. This review discusses the different antimicrobial PDT strategies and highlights the need for highly informative and comprehensive discovery approaches.

Identification of novel antimicrobials using a live-animal infection model.

The alarming increase of antibiotic-resistant bacterial pathogens points to the need for novel therapeutic approaches to combat infection. To discover novel antimicrobials, we devised a screen to identify compounds that promoted the survival of the model laboratory nematode Caenorhabditis elegans infected with the human opportunistic pathogen Enterococcus faecalis. E. faecalis colonizes the nematode intestinal tract, forming a persistent lethal infection. Infected nematodes were rescued by antibiotic treatment in a dose-dependent manner, and antibiotic treatment markedly reduced the number of bacteria colonizing the nematode intestine. To facilitate high throughput screening of compound libraries, we adapted a previously developed agar-based C. elegans-E. faecalis infection assay so that it could be carried out in liquid medium in standard 96-well microtiter plates. We used this simple infection system to screen 6,000 synthetic compounds and 1,136 natural product extracts. We identified 16 compounds and 9 extracts that promoted nematode survival. Some of the compounds and extracts inhibited E. faecalis growth in vitro, but, in contrast to traditional antibiotics, the in vivo effective dose of many of these compounds was significantly lower than the minimum inhibitory concentration needed to prevent the growth of E. faecalis in vitro. Moreover, many of the compounds and extracts had little or no affect on in vitro bacterial growth. Our findings indicate that the whole-animal C. elegans screen identifies not only traditional antibiotics, but also compounds that target bacterial virulence or stimulate host defense.

Conjugating berberine to a multidrug efflux pump inhibitor creates an effective antimicrobial.

In bacteria, multidrug-resistance pumps (MDRs) confer resistance to chemically unrelated amphipathic toxins. A major challenge in developing efficacious antibiotics is identifying antimicrobial compounds that are not rapidly pumped out of bacterial cells. The plant antimicrobial berberine, the active component of the medicinal plants echinacea and golden seal, is a cation that is readily extruded by bacterial MDRs, thereby rendering it relatively ineffective as a therapeutic agent. However, inhibition of MDR efflux causes a substantial increase in berberine antimicrobial activity, suggesting that berberine and potentially many other compounds could be more efficacious if an effective MDR pump inhibitor could be identified. Here we show that covalently linking berberine to INF 55 , an inhibitor of Major Facilitator MDRs, results in a highly effective antimicrobial that readily accumulates in bacteria. The hybrid molecule showed good efficacy in a Caenorhabditis elegans model of enterococcal infection, curing worms of the pathogen.