Repurposing Mitomycin C in Combination with Pentamidine or Gentamicin to Treat Infections with Multi-Drug-Resistant (MDR) Pseudomonas aeruginosa

The aims of this study were (i) to determine if the combination of mitomycin C with pentamidine or existing antibiotics resulted in enhanced efficacy versus infections with MDR P. aeruginosa in vivo; and (ii) to determine if the doses of mitomycin C and pentamidine in combination can be reduced to levels that are non-toxic in humans but still retain antibacterial activity. Resistant clinical isolates of P. aeruginosa, a mutant strain over-expressing the MexAB-OprM resistance nodulation division (RND) efflux pump and a strain with three RND pumps deleted, were used. MIC assays indicated that all strains were sensitive to mitomycin C, but deletion of three RND pumps resulted in hypersensitivity and over-expression of MexAB-OprM caused some resistance. These results imply that mitomycin C is a substrate of the RND efflux pumps. Mitomycin C monotherapy successfully treated infected Galleria mellonella larvae, albeit at doses too high for human administration. Checkerboard and time-kill assays showed that the combination of mitomycin C with pentamidine, or the antibiotic gentamicin, resulted in synergistic inhibition of most P. aeruginosa strains in vitro. In vivo, administration of a combination therapy of mitomycin C with pentamidine, or gentamicin, to G. mellonella larvae infected with P. aeruginosa resulted in enhanced efficacy compared with monotherapies for the majority of MDR clinical isolates. Notably, the therapeutic benefit conferred by the combination therapy occurred with doses of mitomycin C close to those used in human medicine. Thus, repurposing mitomycin C in combination therapies to target MDR P. aeruginosa infections merits further investigation.

Droplet Tn-Seq identifies the primary secretion mechanism for yersiniabactin in Yersinia pestis

Nutritional immunity includes sequestration of transition metals from invading pathogens. Yersinia pestis overcomes nutritional immunity by secreting yersiniabactin to acquire iron and zinc during infection. While the mechanisms for yersiniabactin synthesis and import are well-defined, those responsible for yersiniabactin secretion are unknown. Identification of this mechanism has been difficult because conventional mutagenesis approaches are unable to inhibit trans-complementation by secreted factors between mutants. To overcome this obstacle, we utilized a technique called droplet Tn-seq (dTn-seq), which uses microfluidics to isolate individual transposon mutants in oil droplets, eliminating trans-complementation between bacteria. Using this approach, we first demonstrated the applicability of dTn-seq to identify genes with secreted functions. We then applied dTn-seq to identify an AcrAB efflux system as required for growth in metal-limited conditions. Finally, we showed this efflux system is the primary yersiniabactin secretion mechanism and required for virulence during bubonic and pneumonic plague. Together, these studies have revealed the yersiniabactin secretion mechanism that has eluded researchers for over 30 years and identified a potential therapeutic target for bacteria that use yersiniabactin for metal acquisition.

Combination Therapy with Ciprofloxacin and Pentamidine against Multidrug-Resistant Pseudomonas aeruginosa: Assessment of In Vitro and In Vivo Efficacy and the Role of Resistance-Nodulation-Division (RND) Efflux Pumps

The aim of this work was to (i) evaluate the efficacy of a combination treatment of pentamidine with ciprofloxacin against Galleria mellonella larvae infected with an MDR strain of P. aeruginosa and (ii) determine if pentamidine acts as an efflux-pump inhibitor. Resistant clinical isolates, mutant strains overexpressing one of three RND efflux pumps (MexAB-OprM, MexCD-OprJ, and MexEF-OprN), and a strain with the same three pumps deleted were used. MIC assays confirmed that the clinical isolates and the mutants overexpressing efflux pumps were resistant to ciprofloxacin and pentamidine. The deletion of the three efflux pumps induced sensitivity to both compounds. Exposure to pentamidine and ciprofloxacin in combination resulted in the synergistic inhibition of all resistant strains in vitro, but no synergy was observed versus the efflux-pump deletion strain. The treatment of infected G. mellonella larvae with the combination of pentamidine and ciprofloxacin resulted in enhanced efficacy compared with the monotherapies and significantly reduced the number of proliferating bacteria. Our measurement of efflux activity from cells revealed that pentamidine had a specific inhibitory effect on the MexCD-OprJ and MexEF-OprN efflux pumps. However, the efflux activity and membrane permeability assays revealed that pentamidine also disrupted the membrane of all cells. In conclusion, pentamidine does possess some efflux-pump inhibitory activity, in addition to a more general disruptive effect on membrane integrity that accounts for its ability to potentiate ciprofloxacin activity. Notably, the enhanced efficacy of combination therapy with pentamidine and ciprofloxacin versus MDR P. aeruginosa strains in vivo merits further investigation into its potential to treat infections via this pathogen in patients.

Phenylalanine-arginine β-naphthylamide could enhance neomycin-sensitivity on Riemerella anatipestifer in vitro and in vivo

Riemerella anatipestifer is an important duck pathogen responsible for septicemia and infectious serositis, which has caused great economic losses to the duck industry. Phenylalanine-arginine β-naphthylamide (PAβN) is an efflux pump inhibitor, which mainly reduces the efflux effect by competing with antibiotics for efflux pump channels. Here, we found that R. anatipestifer strain GD2019 showed resistances to gentamicin, amikacin, kanamycin, and neomycin. Notably, PAβN could significantly reduce the Minimal inhibitory concentrations (MICs) of neomycin on the GD2019 strain. Moreover, PAβN combined with neomycin significantly decreased bacterial loads, relieved pathological injury and increase survival rate (p < 0.05) for the ducks lethally challenged by the GD2019 strain. Therefore, our results suggested, in vitro and in vivo, PAβN could reduce neomycin-resistant of R. anatipestifer. Importantly, finding of this study provide a new approach for treating antibiotic-resistant R. anatipestifer infection.

The culmination of multidrug-resistant efflux pumps vs. meager antibiotic arsenal era: Urgent need for an improved new generation of EPIs

Efflux pumps function as an advanced defense system against antimicrobials by reducing the concentration of drugs inside the bacteria and extruding the substances outside. Various extraneous substances, including antimicrobials, toxic heavy metals, dyes, and detergents, have been removed by this protective barrier composed of diverse transporter proteins found in between the cell membrane and the periplasm within the bacterial cell. In this review, multiple efflux pump families have been analytically and widely outlined, and their potential applications have been discussed in detail. Additionally, this review also discusses a variety of biological functions of efflux pumps, including their role in the formation of biofilms, quorum sensing, their survivability, and the virulence in bacteria, and the genes/proteins associated with efflux pumps have also been explored for their potential relevance to antimicrobial resistance and antibiotic residue detection. A final discussion centers around efflux pump inhibitors, particularly those derived from plants.

Assessment of synergistic activity of rhamnolipid and linezolid against methicillin-resistant Staphylococcus aureus in-vitro and in-vivo with Galleria mellonella larvae model

Antibiotic resistance, one of the most crucial public health problems, has increased the interest in synergy studies of antibiotics with existing antibiotics and natural compounds to make current treatment more effective in addition to new drug development. In this study, the effectiveness of rhamnolipid and linezolid on the Galleria mellonella larvae model in-vitro and in-vivo against Methicillin-resistant Staphylococcus aureus isolates, which are problematic in treatment, were investigated. Four S.aureus (One ATCC 29213 strain and three methicillin-resistant strains) were used in the study. Two MRSA isolates were resistant to linezolid, and one was susceptible. Partial synergy was observed in one resistant strain, and although no synergy was observed in the other resistant strain, the minimum inhibitory concentration of the resistant strain decreased from 16 to 4 μg/mL with a four-fold decrease and reached the susceptibility limit. No change was observed in the MIC of linezolid-susceptible strains. The G.mellonella larval model demonstrated that combined therapy was more effective than monotherapy by survival function tests and CFU determination. RML/LNZ combination improved survival compared to monotherapy and decreased the bacterial burden from 10⁸ to 10³.

Role of Efflux Pumps on Antimicrobial Resistance in Pseudomonas aeruginosa

Antimicrobial resistance is an old and silent pandemic. Resistant organisms emerge in parallel with new antibiotics, leading to a major global public health crisis over time. Antibiotic resistance may be due to different mechanisms and against different classes of drugs. These mechanisms are usually found in the same organism, giving rise to multidrug-resistant (MDR) and extensively drug-resistant (XDR) bacteria. One resistance mechanism that is closely associated with the emergence of MDR and XDR bacteria is the efflux of drugs since the same pump can transport different classes of drugs. In Gram-negative bacteria, efflux pumps are present in two configurations: a transmembrane protein anchored in the inner membrane and a complex formed by three proteins. The tripartite complex has a transmembrane protein present in the inner membrane, a periplasmic protein, and a porin associated with the outer membrane. In Pseudomonas aeruginosa, one of the main pathogens associated with respiratory tract infections, four main sets of efflux pumps have been associated with antibiotic resistance: MexAB-OprM, MexXY, MexCD-OprJ, and MexEF-OprN. In this review, the function, structure, and regulation of these efflux pumps in P. aeruginosa and their actions as resistance mechanisms are discussed. Finally, a brief discussion on the potential of efflux pumps in P. aeruginosa as a target for new drugs is presented.

Carbapenem-Only Combination Therapy against Multi-Drug Resistant Pseudomonas aeruginosa: Assessment of In Vitro and In Vivo Efficacy and Mode of Action

The aim of the study was to determine the efficacy of carbapenem-only combination treatments derived from four approved drugs (meropenem, doripenem, ertapenem and imipenem) against a MDR strain of P. aeruginosa in a Galleria mellonella larvae infection model. G. mellonella larvae were infected with P. aeruginosa NCTC 13437 (carrying the VIM 10 carbapenamase) and the efficacy of the six possible dual, four triple, and one quadruple carbapenem combination(s) were compared to their constituent monotherapies. Four of these combinations showed significantly enhanced survival compared to monotherapies and reduced the bacterial burden inside infected larvae but without complete elimination. Bacteria that survived combination therapy were slower growing, less virulent but with unchanged carbapenem MICs-observations that are consistent with a persister phenotype. In vitro time-kill assays confirmed that the combinations were bactericidal and confirmed that a low number of bacteria survived exposure. Mass spectrometry was used to quantify changes in the concentration of carbapenems in the presence of carbapenemase-carrying P. aeruginosa. The rate of degradation of individual carbapenems was altered, and often significantly reduced, when the drugs were in combinations compared with the drugs alone. These differences may account for the enhanced inhibitory effects of the combinations against carbapenem-resistant P. aeruginosa and are consistent with a 'shielding' hypothesis. In conclusion, carbapenem combinations show promise in combating MDR P. aeruginosa and are worthy of additional study and development.

Role of the multi-drug efflux systems on the baseline susceptibility to ceftazidime/avibactam and ceftolozane/tazobactam in clinical isolates of non-carbapenemase-producing carbapenem-resistant Pseudomonas aeruginosa

Carbapenem-resistant Pseudomonas aeruginosa (CRPA) is one of the pathogens that urgently needs new drugs and new alternatives for its control. The primary strategy to combat this bacterium is combining treatments of beta-lactam with a beta-lactamase inhibitor. The most used combinations against P. aeruginosa are ceftazidime/avibactam (CZA) and ceftolozane/tazobactam (C/T). Although mechanisms leading to CZA and C/T resistance have already been described, among which are the resistance-nodulation-division (RND) efflux pumps, the role that these extrusion systems may play in CZA, and C/T baseline susceptibility of clinical isolates remains unknown. For this purpose, 161 isolates of non-carbapenemase-producing (Non-CP) CRPA were selected, and susceptibility tests to CZA and C/T were performed in the presence and absence of the RND efflux pumps inhibitor, Phenylalanine-arginine β-naphthylamide (PAβN). In the absence of PAβN, C/T showed markedly higher activity against Non-CP-CRPA isolates than observed for CZA. These results were even more evident in isolates classified as extremely-drug resistant (XDR) or with difficult-to-treat resistance (DTR), where CZA decreased its activity up to 55.2% and 20.0%, respectively, whereas C/T did it up to 82.8% (XDR), and 73.3% (DTR). The presence of PAβN showed an increase in both CZA (37.6%) and C/T (44.6%) activity, and 25.5% of Non-CP-CRPA isolates increased their susceptibility to these two combined antibiotics. However, statistical analysis showed that only the C/T susceptibility of Non-CP-CRPA isolates was significantly increased. Although the contribution of RND activity to CZA and C/T baseline susceptibility was generally low (two-fold decrease of minimal inhibitory concentrations [MIC]), a more evident contribution was observed in a non-minor proportion of the Non-CP-CRPA isolates affected by PAβN [CZA: 25.4% (15/59); C/T: 30% (21/70)]. These isolates presented significantly higher MIC values for C/T. Therefore, we conclude that RND efflux pumps are participating in the phenomenon of baseline susceptibility to CZA and, even more, to C/T. However, the genomic diversity of clinical isolates is so great that deeper analyzes are necessary to determine which elements are directly involved in this phenomenon.

What Approaches to Thwart Bacterial Efflux Pumps-Mediated Resistance?

An effective response that combines prevention and treatment is still the most anticipated solution to the increasing incidence of antimicrobial resistance (AMR). As the phenomenon continues to evolve, AMR is driving an escalation of hard-to-treat infections and mortality rates. Over the years, bacteria have devised a variety of survival tactics to outwit the antibiotic’s effects, yet given their great adaptability, unexpected mechanisms are still to be discovered. Over-expression of efflux pumps (EPs) constitutes the leading strategy of bacterial resistance, and it is also a primary driver in the establishment of multidrug resistance (MDR). Extensive efforts are being made to develop antibiotic resistance breakers (ARBs) with the ultimate goal of re-sensitizing bacteria to medications to which they have become unresponsive. EP inhibitors (EPIs) appear to be the principal group of ARBs used to impair the efflux system machinery. Due to the high toxicity of synthetic EPIs, there is a growing interest in natural, safe, and innocuous ones, whereby plant extracts emerge to be excellent candidates. Besides EPIs, further alternatives are being explored including the development of nanoparticle carriers, biologics, and phage therapy, among others. What roles do EPs play in the occurrence of MDR? What weapons do we have to thwart EP-mediated resistance? What are the obstacles to their development? These are some of the core questions addressed in the present review.

Demonstration of the efficacy of curcumin on carbapenem-resistant Pseudomonas aeruginosa with Galleria mellonella larvae model

Due to increasing antimicrobial resistance, studies where new treatment options are investigated along with the synergistic effects of natural products with antibiotics have arisen. Pseudomonas aeruginosa (P. aeruginosa) is an opportunistic pathogen and infection with multi-drug resistant (MDR) P. aeruginosa poses a critical problem during treatment. Curcumin (CUR) is listed in the literature as one of the promising natural ingredients with its strong antimicrobial activity. In our study, our aim was to investigate the in vitro synergistic effect of CUR with imipenem (IMP) and Colistin (CST) in MDR P. aeruginosa isolates and in vivo activity on Galleria mellonella (G. mellonella) larvae. Three clinical isolates of MDR P. aeruginosa, which were determined to be phenotypically resistant to carbapenems, were used, and KPC and OXA48 resistance genes were determined by PCR method. The synergistic effect of CUR with antibiotics were investigated by the checkerboard method. Larval survival and bacterial load were compared with the in vivo study. In this study, IMP MIC values were significantly reduced (two to eight-fold decrease) in the presence of CUR, and partial synergy was observed. For CST, this value decreased two-fold. Bacterial load was evaluated to investigate the effect of antimicrobials during infection. While the CFUs increased over time in non-treated larvae as compared to the initial inoculum, bacterial load was significantly decreased for the groups treated with CUR, IMP and CST compared to the untreated group (p < 0.05). It was concluded CUR-antibiotic combinations can provide an alternative approach in the treatment of infections with MDR bacteria.

Bacterial Multidrug Efflux Pumps at the Frontline of Antimicrobial Resistance: An Overview

Multidrug efflux pumps function at the frontline to protect bacteria against antimicrobials by decreasing the intracellular concentration of drugs. This protective barrier consists of a series of transporter proteins, which are located in the bacterial cell membrane and periplasm and remove diverse extraneous substrates, including antimicrobials, organic solvents, toxic heavy metals, etc., from bacterial cells. This review systematically and comprehensively summarizes the functions of multiple efflux pumps families and discusses their potential applications. The biological functions of efflux pumps including their promotion of multidrug resistance, biofilm formation, quorum sensing, and survival and pathogenicity of bacteria are elucidated. The potential applications of efflux pump-related genes/proteins for the detection of antibiotic residues and antimicrobial resistance are also analyzed. Last but not least, efflux pump inhibitors, especially those of plant origin, are discussed.

Gain and loss of antibiotic resistant genes in multidrug resistant bacteria: One Health perspective

The emergence of multidrug resistance (MDR) has become a global health threat due to the increasing unnecessary use of antibiotics. Multidrug resistant bacteria occur mainly by accumulating resistance genes on mobile genetic elements (MGEs), made possible by horizontal gene transfer (HGT). Humans and animal guts along with natural and engineered environments such as wastewater treatment plants and manured soils have proven to be the major reservoirs and hotspots of spreading antibiotic resistance genes (ARGs). As those environments support the dissemination of MGEs through the complex interactions that take place at the human-animal-environment interfaces, a growing One Health challenge is for multiple sectors to communicate and work together to prevent the emergence and spread of MDR bacteria. However, maintenance of ARGs in a bacterial chromosome and/or plasmids in the environments might place energy burdens on bacterial fitness in the absence of antibiotics, and those unnecessary ARGs could eventually be lost. This review highlights and summarizes the current investigations into the gain and loss of ARG genes in MDR bacteria among human-animal-environment interfaces. We also suggest alternative treatments such as combinatory therapies or sequential use of different classes of antibiotics/adjuvants, treatment with enzyme-inhibitors, and phage therapy with antibiotics to solve the MDR problem from the perspective of One Health issues.

Repurposing the anti-viral drug zidovudine (AZT) in combination with meropenem as an effective treatment for infections with multi-drug resistant, carbapenemase-producing strains of Klebsiella pneumoniae

Multi-drug resistant (MDR) Klebsiella pneumoniae represent a global threat to healthcare due to lack of effective treatments and high mortality rates. The aim of this research was to explore the potential of administering zidovudine (AZT) in combination with an existing antibiotic to treat resistant K. pneumoniae infections. Two MDR K. pneumoniae strains were employed, producing either the NDM-1 or KPC-3 carbapenemase. Efficacy of combinations of AZT with meropenem were compared with monotherapies against infections in Galleria mellonella larvae by measuring larval mortality and bacterial burden. The effect of the same combinations in vitro was determined via checkerboard and time-kill assays. In vitro, both K. pneumoniae strains were resistant to meropenem but were susceptible to AZT. In G. mellonella, treatment with either AZT or meropenem alone offered minimal therapeutic benefit against infections with either strain. In contrast, combination therapy of AZT with meropenem presented significantly enhanced efficacy compared to monotherapies. This was correlated with prevention of bacterial proliferation within the larvae but not elimination. Checkerboard assays showed that the interaction between AZT and meropenem was not synergistic but indifferent. In summary, combination therapy of AZT with meropenem represents a potential treatment for carbapenemase-producing MDR K. pneumoniae and merits further investigation.

Activity of carbonyl cyanide-3-chlorophenylhydrazone on biofilm formation and antimicrobial resistance in Pseudomonas aeruginosa using quantum dots-meropenem conjugates as nanotools

Hospital infections associated with multidrug-resistant (MDR) Pseudomonas aeruginosa are a worldwide public health problem. Efflux systems and biofilm formation are mechanisms related to resistance to carbapenemics. In this study, quantum dots (QDs) were used to evaluate the effect of carbonyl cyanide-3-chlorophenylhydrazone (CCCP), an efflux pump system inhibitor, on biofilm formation and antimicrobial resistance profile of P. aeruginosa strains. For this, QDs were covalently conjugated to meropenem (MPM) and incubated with a P. aeruginosa resistant isolate (P118) or a control sensitive strain (ATCC Pa27853). P118 was also analyzed with conjugates after previous CCCP efflux inhibitor incubation. Fluorescence microscopy images showed that both sensitive and resistant bacteria were efficiently labeled. Nevertheless, P118 isolates presented fluorescent cell agglomerates, suggesting biofilm formation. The addition of the CCCP changed the labeling profile of the resistant isolate, and the absence of agglomerates was observed, indicating no biofilm formation. Genetic assays revealed the presence of MexA and MexE genes encoding channel proteins from efflux pump systems in both resistant and sensitive strains. Disk-diffusion and broth microdilution tests determined drug susceptibility profiles in the presence and absence of CCCP for P118 isolates. We verified that the CCCP efflux system inhibitor may contribute to P. aeruginosa resistant phenotype reduction for some antimicrobials. This study verified the efficiency of QD-MPM conjugates to trigger and study biofilm formation, or its inhibition, before and after CCCP addition. QDs conjugated to antimicrobials can be used as nanotools to investigate multidrug-resistant bacterial strains on biofilm formation.

Antibiotic resistance breakers: current approaches and future directions

Infections of antibiotic-resistant pathogens pose an ever-increasing threat to mankind. The investigation of novel approaches for tackling the antimicrobial resistance crisis must be part of any global response to this problem if an untimely reversion to the pre-penicillin era of medicine is to be avoided. One such promising avenue of research involves so-called antibiotic resistance breakers (ARBs), capable of re-sensitising resistant bacteria to antibiotics. Although some ARBs have previously been employed in the clinical setting, such as the β-lactam inhibitors, we posit that the broader field of ARB research can yet yield a greater diversity of more effective therapeutic agents than have been previously achieved. This review introduces the area of ARB research, summarises the current state of ARB development with emphasis on the various major classes of ARBs currently being investigated and their modes of action, and offers a perspective on the future direction of the field.

Delivery of antibacterial silver nanoclusters to Pseudomonas aeruginosa using species-specific DNA aptamers

Introduction. The use of silver as an antimicrobial therapeutic is limited by its toxicity to host cells compared with that required to kill bacterial pathogens.Aim. To use aptamer targeting of DNA scaffolded silver nanoclusters as an antimicrobial agent for treating Pseudomonas aeruginosa infections.Methodology. Antimicrobial activity was assessed in planktonic cultures and in vivo using an invertebrate model of infection.Results. The aptamer conjugates that we call aptabiotics have potent antimicrobial activity. Targeted silver nanoclusters were more effective at killing P. aeruginosa than the equivalent quantity of untargeted silver nanoclusters. The aptabiotics have an IC₅₀ of 1.3-2.6 µM against planktonically grown bacteria. Propidium iodide staining showed that they rapidly depolarize bacterial cells to kill approximately 50 % of the population within 10 min following treatment. In vivo testing in the Galleria mellonella model of infection prolonged survival from an otherwise lethal infection.Conclusion. Using P. aeruginosa as a model, we show that targeting of DNA-scaffolded silver nanoclusters with an aptamer has effective fast-acting antimicrobial activity in vitro and in an in vivo animal model.

Infection-based chemical screens uncover host-pathogen interactions

Bacterial pathogens must resist host innate immunity to cause disease. While Gram-negative bacteria have a protective outer membrane, this membrane is subject to host-induced damage that makes these pathogens vulnerable. We developed a high content screening platform that identifies compounds that cause the killing of the bacterial pathogen Salmonella enterica in macrophages. This platform enables the rapid discovery of compounds that work in concert with the macrophage to prevent pathogen survival, as most hit compounds are not active in standard microbiological media and are not pro-drugs. We describe within the platform and the compounds it has found, and consider how they may help us discover new ways to fight infection.

Dual β-lactam combination therapy for multi-drug resistant Pseudomonas aeruginosa infection: enhanced efficacy in vivo and comparison with monotherapies of penicillin-binding protein inhibition

The aim of the study was to determine the efficacy of dual β-lactam combination treatments derived from eight approved drugs against Galleria mellonella larvae infected with MDR strains of P. aeruginosa. Carbapenem-resistant P. aeruginosa NCTC 13437 and an unrelated clinical isolate were used to infect G. mellonella larvae and the efficacy of twenty-eight dual β-lactam combination therapies were compared to their constituent monotherapies. For the most potent combinations identified, penicillin-binding protein (PBP) inhibition profiles were measured and compared with each constituent antibiotic. Five of the dual β-lactam combinations resulted in greater than 70% survival of infected G. mellonella. Two combinations showed potent, enhanced efficacy versus both strains − ceftazidime + meropenem and aztreonam + meropenem. Comparison of PBP inhibition profiles revealed that the enhanced efficacy of these two dual β-lactam combinations could not be explained by more potent inhibition of PBPs or inhibition of a broader range of PBPs. A possible contribution to the enhanced efficacy of the combinations could be stimulation of innate immunity via increased haemocyte numbers compared to their constituent monotherapies. Combinations of β-lactam antibiotics show promise in overcoming MDR P. aeruginosa and are worthy of additional study and development.

Effect of Staphylococcus aureus Tet38 native efflux pump on in vivo response to tetracycline in a murine subcutaneous abscess model

Objectives

Staphylococcus aureus native efflux pump Tet38 confers resistance to tetracycline when overexpressed. tet38 expression is selectively upregulated in infection sites. This study evaluated the effect of Tet38 on tetracycline response in a murine subcutaneous abscess model.

Methods

S. aureus MW2 and its tet38 mutant were injected subcutaneously on the opposite flanks of each mouse. The infected mice were treated with tetracycline (10 mg/kg) or PBS (control) intraperitoneally every 12 h. The efficacy of tetracycline against S. aureus was measured by the relative change in viable bacteria in the abscesses 24 h after infection compared with the initial inoculum. Plasmid-based tet38-complemented strains were created and used to infect the mice followed by tetracycline or PBS treatment.

Results

The increased bacterial loads of S. aureus MW2 and its tet38 mutant in the abscess after 24 h were similar. Tetracycline produced significant decreases of both MW2 and the tet38 mutant compared with control. Although the tetracycline MICs for MW2 and the tet38 mutant did not differ in vitro, the antibacterial effect of tetracycline was significantly 2-fold greater in the tet38 mutant compared with the MW2 parental strain in vivo with a decrease of 0.67 ± 0.21 and 0.35 ± 0.19 log10 cfu/abscess, respectively (P < 0.05). The increased tetracycline activity in the tet38 mutant was complemented by plasmid-encoded tet38.

Conclusions

This study demonstrated that selective increased expression of tet38 in an abscess can affect tetracycline efficacy against S. aureus in vivo, highlighting an effect of native efflux pumps on response to therapy not reflected by testing in vitro.

A Structural View on Medicinal Chemistry Strategies against Drug Resistance

The natural phenomenon of drug resistance is a widespread issue that hampers the performance of drugs in many major clinical indications. Antibacterial and antifungal drugs are affected, as well as compounds for the treatment of cancer, viral infections, or parasitic diseases. Despite the very diverse set of biological targets and organisms involved in the development of drug resistance, the underlying molecular mechanisms have been identified to understand the emergence of resistance and to overcome this detrimental process. Detailed structural information on the root causes for drug resistance is nowadays frequently available, so next-generation drugs can be designed that are anticipated to suffer less from resistance. This knowledge-based approach is essential for fighting the inevitable occurrence of drug resistance.

An α-Lipoic acid derivative, and anti-ROS agent, prevents the acquisition of multi-drug resistance in clinical isolates of Pseudomonas aeruginosa

Pseudomonas aeruginosa is one of the most common causes of nosocomial infections, and its multi-drug resistance has been a serious problem worldwide. The aim of this study was to evaluate whether exposure to piperacillin and reactive oxygen species (ROS) could lead to multi-drug resistance for clinical isolates of P. aeruginosa. The inhibition of this acquired resistance by the anti-ROS agent was also examined. In vitro inducement of multi-drug resistance was performed against 20 clinical isolates. These strains were incubated for 24 h and transferred 5 times after being exposed to 1 mM H₂O₂ (ROS) in addition to a sub-MIC of piperacillin by the agar dilution method. Each MIC of piperacillin and levofloxacin was determined. As the mechanism of levofloxacin resistance, mutation of QRDR was investigated. The expression level of genes encoding efflux pumps; mexA, mexY, mexC, and D2 porin; oprD were determined by real-time PCR. Multi-resistance to both piperacillin and levofloxacin was induced with 4 of 20 strains (20%). No amino acid change was confirmed in QRDR. These strains showed overexpression of mexA, mexY, mexC, and another one showed decrease of oprD expression. Resistance development in 4 strains was inhibited by the same method including the anti-ROS agent, sodium zinc histidine dithiooctanamide (DHL-His-Zn). In conclusion, stimulation by ROS promoted acquisition of multi-drug resistance in 20% of isolates of P. aeruginosa, and DHL-His-Zn completely inhibited this acquisition of resistance. Therefore, this anti-ROS agent may be useful to assist antimicrobial chemotherapy by preventing multi-drug resistance.

Steroidal alkaloids and conessine from the medicinal plant Holarrhena antidysenterica restore antibiotic efficacy in a Galleria mellonella model of multidrug-resistant Pseudomonas aeruginosa infection

Background

This study aimed to evaluate the efficacy of combinations of steroidal alkaloids and conessine from the Thai medicinal plant Holarrhena antidysenterica with antibiotics against Pseudomonas aeruginosa strains possessing different efflux-pump-mediated multidrug-resistant (MDR) phenotypes in a Galleria mellonella infection model.

Methods

P. aeruginosa strains with defined mutations that result in the overexpression of the MexAB-OprM, MexCD-OprJ and MexEF-OprN efflux pumps, and a strain with all three of these pumps deleted, were used. In vitro, the effect of combinations of steroidal alkaloids and conessine with antibiotics was compared with antibiotic treatment alone via MIC determination and time-kill assays. Efficacy of combinations of the steroidal alkaloids and conessine with levofloxacin were compared with monotherapies against infections in G. mellonella larvae by measuring larval mortality and bacterial burden.

Results

Combination therapies of conessine or steroidal alkaloids with levofloxacin enhanced bacterial inhibition in vitro and restored antibiotic efficacy in vivo compared to the constituent monotherapies. Neither conessine nor the steroidal alkaloids induced any detectable toxicity in G. mellonella larvae. The enhanced efficacy of the combination treatments was most pronounced with conessine and correlated with reduced larval burden of infecting P. aeruginosa. Notably, the enhanced efficacy of conessine/levofloxacin combinations was only detected in the parent strain and strains that overexpressed the MexAB-OprM or MexEF-OprN efflux systems.

Conclusions

Steroidal alkaloids from Holarrhena antidysenterica, and particularly the principal active ingredient conessine, restored levofloxacin efficacy against resistant P. aeruginosa strains possessing efflux-mediated MDR phenotypes. The compounds should be investigated further as a potential novel therapy.

Electronic supplementary material

The online version of this article (10.1186/s12906-018-2348-9) contains supplementary material, which is available to authorized users.

Standardization of G. mellonella Larvae to Provide Reliable and Reproducible Results in the Study of Fungal Pathogens

In the past decade, Galleria mellonella (wax moth) larvae have become widely used as a non-mammalian infection model. However, the full potential of this infection model has yet to be realised, limited by the variable quality of larvae used and the lack of standardised procedures. Here, we review larvae suitable for research, protocols for dosing larvae, and methods for scoring illness in larvae infected with fungal pathogens. The development of standardised protocols for carrying out our experimental work will allow high throughput screens to be developed, changing the way in which we evaluate panels of mutants and strains. It will also enable the in vivo screening of potential antimicrobials at an earlier stage in the research and development cycle.

Establishment and Validation of Galleria mellonella as a Novel Model Organism To Study Mycobacterium abscessus Infection, Pathogenesis, and Treatment

Treatment of Mycobacterium abscessus infections is extremely challenging due to its intrinsic resistance to most antibiotics, and research of pathogenesis is limited due to a lack of a practical in vivo model of infection. The objective of this study was to establish a simple in vivo model for M. abscessus infection, virulence, and drug testing in Galleria mellonella larvae. We inoculated larvae with M. abscessus bacteria and assessed histopathology, CFU count, and mortality with and without antibiotic treatment. We also constructed a luminescent, recombinant M. abscessus mutant, mDB158, and imaged infected larvae using the IVIS in vivo imaging system. M. abscessus proliferated and induced granuloma-like responses in infected larvae, leading to larval mortality. The G. mellonella model was further validated successfully by demonstration of the expected favorable antimicrobial effect of treatment with meropenem and the superiority of combination treatment (meropenem and tigecycline) over that with single agents. We then used IVIS imaging of larvae infected with luminescent M. abscessus, allowing live real-time assessment of bacterial load. We used this method to compare the antimicrobial effects of various antibiotics (meropenem, amikacin, linezolid, levofloxacin, etc.) on bacterial proliferation and larval survival. Meropenem and amikacin had the most favorable effects, correlating well with common clinical practice guidelines. These findings suggest G. mellonella to be an excellent in vivo model for research of M. abscessus infection, pathogenesis, and treatment. Luminescent M. abscessus and IVIS imaging further facilitates this model. Results obtained in this model clearly substantiated common clinical practice, thus validating the model as a predictor of treatment efficacy and outcome.

Effect of efflux pump inhibition on Pseudomonas aeruginosa transcriptome and virulence

Efflux pumps of the resistance-nodulation-cell-division (RND) family increase antibiotic resistance in many bacterial pathogens, representing candidate targets for the development of antibiotic adjuvants. RND pumps have also been proposed to contribute to bacterial infection, implying that efflux pump inhibitors (EPIs) could also act as anti-virulence drugs. Nevertheless, EPIs are usually investigated only for their properties as antibiotic adjuvants, while their potential anti-virulence activity is seldom taken into account. In this study it is shown that RND efflux pumps contribute to Pseudomonas aeruginosa PAO1 pathogenicity in an insect model of infection, and that the well-characterized EPI Phe-Arg-β-naphthylamide (PAβN) is able to reduce in vivo virulence of the P. aeruginosa PAO1 laboratory strain, as well as of clinical isolates. The production of quorum sensing (QS) molecules and of QS-dependent virulence phenotypes is differentially affected by PAβN, depending on the strain. Transcriptomic and phenotypic analyses showed that the protection exerted by PAβN from P. aeruginosa PAO1 infection in vivo correlates with the down-regulation of key virulence genes (e.g. genes involved in iron and phosphate starvation). Since PAβN impacts P. aeruginosa virulence, anti-virulence properties of EPIs are worthy to be explored, taking into account possible strain-specificity of their activity.

A Novel Restraint Device for Injection of Galleria mellonella Larvae that Minimizes the Risk of Accidental Operator Needle Stick Injury

Larvae of the insect Galleria mellonella are increasingly being used for studying pathogenic microbes and their virulence mechanisms, and as a rapid model for screening novel antimicrobial agents. The larvae (waxworms) are most frequently infected by injection of pathogenic organisms into the haemocoel through the insect's prolegs. The mostly widely used method for restraining the waxworms for injection is by grasping them between the operator's fingers, which puts the operator at risk of needle stick injury, an important consideration when working with highly pathogenic and/or drug-resistant microorganisms. While use of a stab proof glove can reduce this risk of injury, it does so at the loss of manual dexterity and speed, resulting in a more labor-intensive, and cumbersome assay. We describe a simple cost effective device (the so-called “Galleria Grabber”) for restraining waxworms for injection that keeps the operator's fingers clear of the needle thus reducing the risk of injury.

Inhibition of Pseudomonas aeruginosa by Peptide-Conjugated Phosphorodiamidate Morpholino Oligomers

Pseudomonas aeruginosa is a highly virulent, multidrug-resistant pathogen that causes significant morbidity and mortality in hospitalized patients and is particularly devastating in patients with cystic fibrosis. Increasing antibiotic resistance coupled with decreasing numbers of antibiotics in the developmental pipeline demands novel antibacterial approaches. Here, we tested peptide-conjugated phosphorodiamidate morpholino oligomers (PPMOs), which inhibit translation of complementary mRNA from specific, essential genes in P. aeruginosa PPMOs targeted to acpP, lpxC, and rpsJ, inhibited P. aeruginosa growth in many clinical strains and activity of PPMOs could be enhanced 2- to 8-fold by the addition of polymyxin B nonapeptide at subinhibitory concentrations. The PPMO targeting acpP was also effective at preventing P. aeruginosa PAO1 biofilm formation and at reducing existing biofilms. Importantly, treatment with various combinations of a PPMO and a traditional antibiotic demonstrated synergistic growth inhibition, the most effective of which was the PPMO targeting rpsJ with tobramycin. Furthermore, treatment of P. aeruginosa PA103-infected mice with PPMOs targeting acpP, lpxC, or rpsJ significantly reduced the bacterial burden in the lungs at 24 h by almost 3 logs. Altogether, this study demonstrates that PPMOs targeting the essential genes acpP, lpxC, or rpsJ in P. aeruginosa are highly effective at inhibiting growth in vitro and in vivo These data suggest that PPMOs alone or in combination with antibiotics represent a novel approach to addressing the problems associated with rapidly increasing antibiotic resistance in P. aeruginosa.

Insights into host-pathogen interactions from state-of-the-art animal models of respiratory Pseudomonas aeruginosa infections

Pseudomonas aeruginosa is an important opportunistic pathogen that can cause acute respiratory infections in immunocompetent patients or chronic infections in immunocompromised individuals and in patients with cystic fibrosis. When acquiring the chronic infection state, bacteria are encapsulated within biofilm structures enabling them to withstand diverse environmental assaults, including immune reactions and antimicrobial therapy. Understanding the molecular interactions within the bacteria, as well as with the host or other bacteria, is essential for developing innovative treatment strategies. Such knowledge might be accumulated in vitro. However, it is ultimately necessary to confirm these findings in vivo. In the present Review, we describe state-of-the-art in vivo models that allow studying P. aeruginosa infections in molecular detail. The portrayed mammalian models exclusively focus on respiratory infections. The data obtained by alternative animal models which lack lung tissue, often provide molecular insights that are easily transferable to mammals. Importantly, these surrogate in vivo systems reveal complex molecular interactions of P. aeruginosa with the host. Herein, we also provide a critical assessment of the advantages and disadvantages of such models.

Dithiazole thione derivative as competitive NorA efflux pump inhibitor to curtail multi drug resistant clinical isolate of MRSA in a zebrafish infection model

Multi drug resistant (MDR) pathogens pose a serious threat to public health since they can easily render most potent drugs ineffective. Efflux pump inhibitors (EPI) can be used to counter the MDR phenotypes arising due to increased efflux. In the present study, a series of dithiazole thione derivatives were synthesized and checked for its antibacterial and efflux pump inhibitory (EPI) activity. Among 10 dithiazole thione derivatives, real-time efflux studies revealed that seven compounds were potent EPIs relative to CCCP. Zebrafish toxicity studies identified four non-toxic putative EPIs. Both DTT3 and DTT9 perturbed membrane potential and DTT6 was haemolytic. Among DTT6 and DTT10, the latter was less toxic as evidenced by histopathology studies. Since DTT10 was non-haemolytic, did not affect the membrane potential, and was least toxic, it was chosen further for in vivo study, wherein DTT10 potentiated effect of ciprofloxacin against clinical strain of MRSA and reduced bacterial burden in muscle and skin tissue of infected zebrafish by ~ 1.7 and 2.5 log fold respectively. Gene expression profiling of major efflux transport proteins by qPCR revealed that clinical isolate of MRSA, in the absence of antibiotic, upregulated NorA, NorB and MepA pump, whereas it downregulates NorC and MgrA relative to wild-type strain of Staphylococcus aureus. In vitro studies with NorA mutant strains and substrate profiling revealed that at higher concentrations DTT10 is likely to function as a competitive inhibitor of NorA efflux protein in S. aureus, whereas at lower concentrations it might inhibit ciprofloxacin efflux through NorB and MepA as implied by docking studies. A novel non-toxic, non-haemolytic dithiazole thione derivative (DTT10) was identified as a potent competitive inhibitor of NorA efflux pump in S. aureus using in silico, in vitro and in vivo studies. This study also underscores the importance of using zebrafish infection model to screen and evaluate putative EPI for mitigating MDR strains of S. aureus.

The analysis of the antibiotic resistome offers new opportunities for therapeutic intervention

Most efforts in the development of antimicrobials have focused on the screening of lethal targets. Nevertheless, the constant expansion of antimicrobial resistance makes the antibiotic resistance determinants themselves suitable targets for finding inhibitors to be used in combination with antibiotics. Among them, inhibitors of antibiotic inactivating enzymes and of multidrug efflux pumps are suitable candidates for improving the efficacy of antibiotics. In addition, the application of systems biology tools is helping to understand the changes in bacterial physiology associated to the acquisition of resistance, including the increased susceptibility to other antibiotics displayed by some antibiotic-resistant mutants. This information is useful for implementing novel strategies based in metabolic interventions or combination of antibiotics for improving the efficacy of antibacterial therapy.

Effective immunosuppression with dexamethasone phosphate in the Galleria mellonella larva infection model resulting in enhanced virulence of Escherichia coli and Klebsiella pneumoniae

The aim was to evaluate whether immunosuppression with dexamethasone 21-phosphate could be applied to the Galleria mellonella in vivo infection model. Characterised clinical isolates of Escherichia coli or Klebsiella pneumoniae were employed, and G. mellonella larvae were infected with increasing doses of each strain to investigate virulence in vivo. Virulence was then compared with larvae exposed to increasing doses of dexamethasone 21-phosphate. The effect of dexamethasone 21-phosphate on larval haemocyte phagocytosis in vitro was determined via fluorescence microscopy and a burden assay measured the growth of infecting bacteria inside the larvae. Finally, the effect of dexamethasone 21-phosphate treatment on the efficacy of ceftazidime after infection was also noted. The pathogenicity of K. pneumoniae or E. coli in G. mellonella larvae was dependent on high inoculum numbers such that virulence could not be attributed specifically to infection by live bacteria but also to factors associated with dead cells. Thus, for these strains, G. mellonella larvae do not constitute an ideal infection model. Treatment of larvae with dexamethasone 21-phosphate enhanced the lethality induced by infection with E. coli or K. pneumoniae in a dose- and inoculum size-dependent manner. This correlated with proliferation of bacteria in the larvae that could be attributed to dexamethasone inhibiting haemocyte phagocytosis and acting as an immunosuppressant. Notably, prior exposure to dexamethasone 21-phosphate reduced the efficacy of ceftazidime in vivo. In conclusion, demonstration of an effective immunosuppressant regimen can improve the specificity and broaden the applications of the G. mellonella model to address key questions regarding infection.