In vitro activity of telavancin in combination with colistin versus Gram-negative bacterial pathogens

Polymyxin combination therapy for multidrug-resistant, extensively-drug resistant, and difficult-to-treat drug-resistant gram-negative infections: is it superior to polymyxin monotherapy?

INTRODUCTION

The increasing prevalence of infections with multidrug-resistant (MDR), extensively-drug resistant (XDR) or difficult-to-treat drug resistant (DTR) Gram-negative bacilli (GNB), including Pseudomonas aeruginosa, Acinetobacter baumannii, Klebsiella pneumoniae, Enterobacter species, and Escherichia coli poses a severe challenge.

AREAS COVERED

The rapid growing of multi-resistant GNB as well as the considerable deceleration in development of new anti-infective agents have made polymyxins (e.g. polymyxin B and colistin) a mainstay in clinical practices as either monotherapy or combination therapy. However, whether the polymyxin-based combinations lead to better outcomes remains unknown. This review mainly focuses on the effect of polymyxin combination therapy versus monotherapy on treating GNB-related infections. We also provide several factors in designing studies and their impact on optimizing polymyxin combinations.

EXPERT OPINION

An abundance of recent in vitro and preclinical in vivo data suggest clinical benefit for polymyxin-drug combination therapies, especially colistin plus meropenem and colistin plus rifampicin, with synergistic killing against MDR, XDR, and DTR P. aeruginosa, K. pneumoniae and A. baumannii. The beneficial effects of polymyxin-drug combinations (e.g. colistin or polymyxin B + carbapenem against carbapenem-resistant K. pneumoniae and carbapenem-resistant A. baumannii, polymyxin B + carbapenem + rifampin against carbapenem-resistant K. pneumoniae, and colistin + ceftolozan/tazobactam + rifampin against PDR-P. aeruginosa) have often been shown in clinical setting by retrospective studies. However, high-certainty evidence from large randomized controlled trials is necessary. These clinical trials should incorporate careful attention to patient's sample size, characteristics of patient's groups, PK/PD relationships and dosing, rapid detection of resistance, MIC determinations, and therapeutic drug monitoring.

Combination Therapy versus Monotherapy in the Treatment of Stenotrophomonas maltophilia Infections: A Systematic Review and Meta-Analysis

Stenotrophomonas maltophilia is a multidrug-resistant bacterium that is difficult to treat in hospitals worldwide, leading to high mortality. Published data describing the use of monotherapy or combination therapy and which one is better is still unclear. We aimed to investigate the efficacy of monotherapy and combination therapy in the treatment of S. maltophilia infections. We performed a systematic review of combination therapy and additionally a systematic review and meta-analysis to determine the effects of monotherapy versus combination therapy on mortality in S. maltophilia infections. Electronic databases: Cochrane Library, PubMed, Embase, ClinicalTrials.gov, Scopus, and OpenGrey were accessed. Of the 5030 articles identified, 17 studies were included for a systematic review of combination therapy, of which 4 cohort studies were finally included for meta-analysis. We found there is a trend of favorable outcomes with respect to mortality in the use of combination therapy to treat complex or severe S. maltopholia infections. A meta-analysis of monotherapy showed a statistical significance in the decreasing rate of mortality in hospital-acquired pneumonia (hazard ratio 1.42; 95% confidence interval, 1.04-1.94) compared to combination therapy, but not significant in bacteremia (hazard ratio 0.76; 95% confidence interval, 0.18-3.18). Further studies should continue to explore this association.

Synthesis of an amphiphilic vancomycin aglycone derivative inspired by polymyxins: overcoming glycopeptide resistance in Gram-positive and Gram-negative bacteria in synergy with teicoplanin in vitro

Gram-negative bacteria possess intrinsic resistance to glycopeptide antibiotics so these important antibacterial medications are only suitable for the treatment of Gram-positive bacterial infections. At the same time, polymyxins are peptide antibiotics, structurally related to glycopeptides, with remarkable activity against Gram-negative bacteria. With the aim of breaking the intrinsic resistance of Gram-negative bacteria against glycopeptides, a polycationic vancomycin aglycone derivative carrying an n-decanoyl side chain and five aminoethyl groups, which resembles the structure of polymyxins, was prepared. Although the compound by itself was not active against the Gram-negative bacteria tested, it synergized with teicoplanin against Escherichia coli, Pseudomonas aeruginosa and Acinetobacter baumannii, and it was able to potentiate vancomycin against these Gram-negative strains. Moreover, it proved to be active against vancomycin- and teicoplanin-resistant Gram-positive bacteria.

In vitro and in vivo efficacy of minocycline-based therapy for Elizabethkingia anophelis and the impact of reduced minocycline susceptibility

OBJECTIVES

Elizabethkingia anophelis is inherently resistant to multiple antibiotics, except minocycline. This study aimed to determine the in vitro and in vivo efficacy of minocycline monotherapy and combination therapy against susceptible strains and the impact of reduced minocycline susceptibility.

METHODS

Three clinical isolates and one laboratory-induced mutant with reduced minocycline susceptibility were included. Time-kill and checkerboard assays were used to assess in vitro efficacy and synergy, respectively. Galleria mellonella infection and mouse pneumonia models were used to assess in vivo efficacy, and a mouse thigh infection model was used to determine the bacterial load.

RESULTS

Minocycline monotherapy exerted a modest inhibitory effect on three clinical minocycline-susceptible E. anophelis isolates in vitro, but delayed G. mellonella death and improved infected mouse survival; it also significantly reduced the in vivo bacterial load. Minocycline had decreased efficacy on G. mellonella and mice infected by the mutant with reduced minocycline susceptibility. Genome comparison revealed several spontaneous mutations associated with reduced minocycline susceptibility. Among eight antibiotics tested in combination with minocycline, rifampin consistently showed in vitro synergy. The addition of rifampin (1 mg/L) reduced the mutant prevention concentration of minocycline from 2-4 mg/L to < 0.5 mg/L. However, compared with monotherapy, the combination of rifampin and minocycline did not further reduce the bacterial load or improve the survival of G. mellonella or mice.

CONCLUSION

Minocycline monotherapy was in vivo effective against susceptible E. anophelis. Reduced minocycline susceptibility due to spontaneous mutation decreased its therapeutic efficacy. In combination with rifampin, it prevented the in vitro emergence of reduced susceptibility but did not provide additional in vivo survival benefit.

Emerging Roles of Glycopeptide Antibiotics: Moving beyond Gram-Positive Bacteria

Glycopeptides, a class of cell wall biosynthesis inhibitors, have been the antibiotics of choice against drug-resistant Gram-positive bacterial infections. Their unique mechanism of action involving binding to the substrate of cell wall biosynthesis and substantial longevity in clinics makes this class of antibiotics an attractive choice for drug repurposing and reprofiling. However, resistance to glycopeptides has been observed due to alterations in the substrate, cell wall thickening, or both. The emergence of glycopeptide resistance has resulted in the development of synthetic and semisynthetic glycopeptide analogues to target acquired resistance. Recent findings demonstrate that these derivatives, along with some of the FDA approved glycopeptides have been shown to have antimicrobial activity against Gram-negative bacteria, Mycobacteria, and viruses thus expanding their spectrum of activity across the microbial kingdom. Additional mechanisms of action and identification of novel targets have proven to be critical in broadening the spectrum of activity of glycopeptides. This review focuses on the applications of glycopeptides beyond their traditional target group of Gram-positive bacteria. This will aid in making the scientific community aware about the nontraditional activity profiles of glycopeptides, identify the existing loopholes, and further explore this antibiotic class as a potential broad-spectrum antimicrobial agent.

In vitro and in vivo comparison of eravacycline- and tigecycline-based combination therapies for tigecycline-resistant Acinetobacter baumannii

Several antimicrobial combination therapies are used to treat multiple drug resistant (MDR) and extensively drug resistant (XDR) Acinetobacter baumannii infections. A novel antibiotic, eravacycline, shows a higher potency than tigecycline. The efficacies of eravacycline-based therapies have not yet been evaluated. We demonstrated the effectiveness of eravacycline- and tigecycline-based combination therapies in XDR and especially tigecycline resistant A. baumannii. Thirteen eligible isolates were selected from 642 non-duplicate Acinetobacter blood isolates from four medical centres in 2010-2014. Tigecycline/imipenem and eravacycline/imipenem combinations were simultaneously effective against some isolates in vitro with fractional inhibitory concentration index of 0.5. In contrast, eravacycline- and tigecycline-based combination therapies provided no additional benefits in mouse survival compared to those for monotherapy. In summary, colistin is still the final resort for XDR-A. baumannii treatment according to the sensitivities. Owning to rapid development of resistance in A. baumannii, novel antibiotics are urgently needed.

Antimicrobial Treatment Strategies for Stenotrophomonas maltophilia: A Focus on Novel Therapies

Stenotrophomonas maltophilia is an urgent global threat due to its increasing incidence and intrinsic antibiotic resistance. Antibiotic development has focused on carbapenem-resistant Enterobacteriaceae, Pseudomonas, and Acinetobacter, with approved antibiotics in recent years having limited activity for Stenotrophomonas. Accordingly, novel treatment strategies for Stenotrophomonas are desperately needed. We conducted a systemic literature review and offer recommendations based on current evidence for a treatment strategy of Stenotrophomonas infection.

Antisense inhibition of lpxB gene expression in Acinetobacter baumannii by peptide-PNA conjugates and synergy with colistin

BACKGROUND

LpxB is an enzyme involved in the biosynthesis pathway of lipid A, a component of LPS.

OBJECTIVES

To evaluate the lpxB gene in Acinetobacter baumannii as a potential therapeutic target and to propose antisense agents such as peptide nucleic acids (PNAs) as a tool to combat bacterial infection, either alone or in combination with known antimicrobial therapies.

METHODS

RNA-seq analysis of the A. baumannii ATCC 17978 strain in a murine pneumonia model was performed to study the in vivo expression of lpxB. Protein expression was studied in the presence or absence of anti-lpxB (KFF)3K-PNA (pPNA). Time-kill curve analyses and protection assays of infected A549 cells were performed. The chequerboard technique was used to test for synergy between pPNA and colistin. A Galleria mellonella infection model was used to test the in vivo efficacy of pPNA.

RESULTS

The lpxB gene was overexpressed during pneumonia. Treatment with a specific pPNA inhibited LpxB expression in vitro, decreased survival of the ATCC 17978 strain and increased the survival rate of infected A549 cells. Synergy was observed between pPNA and colistin in colistin-susceptible strains. In vivo assays confirmed that a combination treatment of anti-lpxB pPNA and colistin was more effective than colistin in monotherapy.

CONCLUSIONS

The lpxB gene is essential for A. baumannii survival. Anti-lpxB pPNA inhibits LpxB expression, causing bacterial death. This pPNA showed synergy with colistin and increased the survival rate in G. mellonella. The data suggest that antisense pPNA molecules blocking the lpxB gene could be used as antibacterial agents.

Treatment Outcomes of Stenotrophomonas maltophilia Bacteremia in Critically Ill Children: A Multicenter Experience

OBJECTIVES

Stenotrophomonas maltophilia is a gram-negative opportunistic bacterium that may cause a myriad of clinical diseases in immunocompromised individuals. We aimed to describe the clinical characteristics, risk factors, mortality, and treatment of S. maltophilia bacteremia in critically ill children, a topic on which data are sparse.

DESIGN

A multicenter observational retrospective study in which medical charts of critically ill children with S. maltophilia bacteremia were reviewed between 2012 and 2017.

SETTING

Data were collected from each of the four largest PICUs nationwide, allocated in tertiary medical centers to which children with complex conditions are referred regularly.

PATIENTS

A total of 68 suitable cases of S. maltophilia bacteremia were retrieved and reviewed.

MEASUREMENTS AND MAIN RESULTS

The total occurrence rate of S. maltophilia isolation had increased significantly during the study period (r = 0.65; p = 0.02). The crude mortality was 42%, and the attributed mortality was 18%. Significant risk factors for mortality were a longer length of hospital stay prior to infection (33 d in nonsurvivors vs 28 in survivors; p = 0.03), a nosocomial source of infection (p = 0.02), presentation with septic shock (p < 0.001), and treatment with chemotherapy (p = 0.007) or carbapenem antibiotics (p = 0.05) prior to culture retrieval. On multivariate analysis, septic shock (odds ratio, 14.6; 95% CI, 1.45-147.05; p = 0.023) and being treated with chemotherapy prior to infection (odds ratio, 5.2; 95% CI, 1.59-17.19; p = 0.006)] were associated with mortality. The combination of ciprofloxacin, trimethoprim-sulfamethoxazole, and minocycline resulted in the longest survival time (p < 0.01).

CONCLUSIONS

The significant attributed mortality associated with S. maltophilia bacteremia in critically ill children calls for an aggressive therapeutic approach. The findings of this investigation favor a combination of trimethoprim-sulfamethoxazole, ciprofloxacin, and minocycline.

Expanding the potential of NAI-107 for treating serious ESKAPE pathogens: synergistic combinations against Gram-negatives and bactericidal activity against non-dividing cells

Objectives

To characterize NAI-107 and related lantibiotics for their in vitro activity against Gram-negative pathogens, alone or in combination with polymyxin, and against non-dividing cells or biofilms of Staphylococcus aureus. NAI-107 was also evaluated for its propensity to select or induce self-resistance in Gram-positive bacteria.

Methods

We used MIC determinations and chequerboard experiments to establish the antibacterial activity of the examined compounds against target microorganisms. Time-kill assays were used to evaluate killing of exponential and stationary-phase cells. The effects on biofilms (growth inhibition and biofilm eradication) were evaluated using biofilm-coated pegs. The frequency of spontaneous resistant mutants was evaluated by either direct plating or by continuous sub-culturing at 0.5 × MIC levels, followed by population analysis profiles.

Results

The results showed that NAI-107 and its brominated variant are highly active against Neisseria gonorrhoeae and some other fastidious Gram-negative pathogens. Furthermore, all compounds strongly synergized with polymyxin against Acinetobacter baumannii, Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa, and showed bactericidal activity. Surprisingly, NAI-107 alone was bactericidal against non-dividing A. baumannii cells. Against S. aureus, NAI-107 and related lantibiotics showed strong bactericidal activity against dividing and non-dividing cells. Activity was also observed against S. aureus biofilms. As expected for a lipid II binder, no significant resistance to NAI-107 was observed by direct plating or serial passages.

Conclusions

Overall, the results of the current work, along with previously published results on the efficacy of NAI-107 in experimental models of infection, indicate that this lantibiotic represents a promising option in addressing the serious threat of antibiotic resistance.

Management of Stenotrophomonas maltophilia Infections in Critically Ill Children

BACKGROUND

Stenotrophomonas maltophilia is a life-threatening nosocomial pathogen with profound multidrug-resistant attributes. It is associated with high mortality, particularly in immunocompromised patients. Data on therapy for S. maltophilia infections are scarce, especially in children hospitalized in intensive care settings (pediatric intensive care unit).

METHODS

A retrospective chart review of pediatric patients with isolates of S. maltophilia hospitalized over a 5-year period in 2 pediatric intensive care units.

RESULTS

Thirty-one patients and 91 isolates from blood, respiratory secretions and soft tissues were identified and reviewed. The overall incidence of S. maltophilia infections increased during the study period (P = 0.003). The all-cause crude mortality was 61%, and the attributed mortality was approximately 16%. Risk factors associated with mortality included longer hospitalization before infection (P = 0.002), septic shock (P = 0.003), mechanical ventilation (P = 0.004), an indwelling central vein catheter (P = 0.03) and prior use of steroids (P = 0.04) and carbapenems (P = 0.004). On multivariate analysis, mortality was associated with mechanical ventilation (P = 0.02) and preinfection hospitalization days (P = 0.01). Combination treatment of trimethoprim and sulfamethoxazole, ciprofloxacin and/or minocycline significantly extended survival time (P < 0.001). The method of treatment did not significantly affect the interval between S. maltophilia isolation to resolution of infection (P = 0.200).

CONCLUSIONS

Combinations of trimethoprim and sulfamethoxazole, ciprofloxacin and minocycline are proposed for pediatric intensive care unit patients harboring S. maltophilia. Meticulous evaluation of central vascular access and prior treatment with carbapenems are indicated, especially for mechanically ventilated and septic children.

Pneumonia infection in mice reveals the involvement of the feoA gene in the pathogenesis of Acinetobacter baumannii

Acinetobacter baumannii has emerged in the last decade as an important nosocomial pathogen. To identify genes involved in the course of a pneumonia infection, gene expression profiles were obtained from A. baumannii ATCC 17978 grown in mouse infected lungs and in culture medium. Gene expression analysis allowed us to determine a gene, the A1S_0242 gene (feoA), over-expressed during the pneumonia infection. In the present work, we evaluate the role of this gene, involved in iron uptake. The inactivation of the A1S_0242 gene resulted in an increase susceptibility to oxidative stress and a decrease in biofilm formation, in adherence to A549 cells and in fitness. In addition, infection of G. mellonella and pneumonia in mice showed that the virulence of the Δ0242 mutant was significantly attenuated. Data presented in this work indicated that the A1S_0242 gene from A. baumannii ATCC 17978 strain plays a role in fitness, adhesion, biofilm formation, growth, and, definitively, in virulence. Taken together, these observations show the implication of the feoA gene plays in the pathogenesis of A. baumannii and highlight its value as a potential therapeutic target.

Better living through chemistry: Addressing emerging antibiotic resistance

The increasing emergence of multidrug-resistant bacteria is recognized as a major threat to human health worldwide. While the use of small molecule antibiotics has enabled many modern medical advances, it has also facilitated the development of resistant organisms. This minireview provides an overview of current small molecule drugs approved by the US Food and Drug Administration (FDA) for use in humans, the unintended consequences of antibiotic use, and the mechanisms that underlie the development of drug resistance. Promising new approaches and strategies to counter antibiotic-resistant bacteria with small molecules are highlighted. However, continued public investment in this area is critical to maintain an edge in our evolutionary "arms race" against antibiotic-resistant microorganisms. Impact statement The alarming increase in antibiotic-resistant microorganisms is a rapidly emerging threat to human health throughout the world. Historically, small molecule drugs have played a major role in controlling bacterial infections and they continue to offer tremendous potential in countering resistant organisms. This minireview provides a broad overview of the relevant issues, including the diversity of FDA-approved small molecule drugs and mechanisms of drug resistance, unintended consequences of antibiotic use, the current state of development for small molecule antibacterials and financial challenges that impact progress towards novel therapies. The content will be informative to diverse stakeholders, including clinicians, basic scientists, translational scientists and policy makers, and may be used as a bridge between these key players to advance the development of much-needed therapeutics.

Recent advances and perspectives in the design and development of polymyxins

Covering: 1947-early 2017, particularly from 2005-early 2017The rise of bacterial pathogens with acquired resistance to almost all available antibiotics is becoming a serious public health issue. Polymyxins, antibiotics that were mostly abandoned a few decades ago because of toxicity concerns, are ultimately considered as a last-line therapy to treat infections caused by multi-drug resistant Gram-negative bacteria. This review surveys the progress in understanding polymyxin structure, and their chemistry, mechanisms of antibacterial activity and nephrotoxicity, biomarkers, synergy and combination with other antimicrobial agents and antibiofilm properties. An update of recent efforts in the design and development of a new generation of polymyxin drugs is also discussed. A novel approach considering the modification of the scaffold of polymyxins to integrate metabolism and detoxification issues into the drug design process is a promising new line to potentially prevent accumulation in the kidneys and reduce nephrotoxicity.

A review on cell wall synthesis inhibitors with an emphasis on glycopeptide antibiotics

Cell wall biosynthesis inhibitors (CBIs) have historically been one of the most effective classes of antibiotics. They are the most extensively used class of antibiotics and their importance is exemplified by the β-lactams and glycopeptide antibiotics. However, this class of antibiotics has not received impunity from resistance development. In the wake of this predicament, this review presents the progress of CBIs, especially glycopeptide derivatives as antibiotics to confront antibacterial resistance. The various strategies used for the development of CBIs, their clinical status and possible directions in which this field can evolve have also been discussed.

Clinical translation of polymyxin-based combination therapy: Facts, challenges and future opportunities

The emergence and spread of multidrug resistant Gram-negative bacteria has led to a resurgence in the clinical use of polymyxin antibiotics. However, the prevalence of polymyxin resistance is on the rise at an alarming rate, motivating the idea of combination therapy to sustain the revival of these "old" antibiotics. Although ample evidence in favor of combination therapy has emerged, it seems impracticable and confusing to find a promising combination from the diverse reports or gain adequate information on the efficacy and safety profile. With a stagnating discovery pipeline of novel antimicrobials, there is a clear need to fill the knowledge gaps in translating these basic research data to beneficial clinical practice. In this review, we examined the factors and ambiguities that stand as major hurdles in bringing polymyxin combination therapy to bedside care, highlighting the importance and urgency of incorporating translational research insights into areas of difficulty. We also discussed future research priorities that are essential to gather the necessary evidence and insights for promoting the best possible use of polymyxins in combination therapy.

Synergistic combinations of polymyxins

The proliferation of extensively drug-resistant Gram-negative pathogens has necessitated the therapeutic use of colistin and polymyxin B. However, treatment failures with polymyxin monotherapies and the emergence of polymyxin resistance have catalysed the search for polymyxin combinations that synergistically kill polymyxin-susceptible and -resistant organisms. This mini-review examines recent (2011-2016) in vitro and in vivo studies that have attempted to identify synergistic polymyxin combinations against Pseudomonas aeruginosa, Klebsiella pneumoniae and Acinetobacter baumannii. Clinical evidence for the use of combination regimens is also discussed.

Investigation of C-5 alkynyl (alkynyloxy or hydroxymethyl) and/or N-3 propynyl substituted pyrimidine nucleoside analogs as a new class of antimicrobial agents

The resurgence of mycobacterial infections and the emergence of drug-resistant strains urgently require a new class of agents that are distinct than current therapies. A group of 5-ethynyl (6-10), 5-(2-propynyloxy) (16, 18, 20, 22, 24), 5-(2-propynyloxy)-3-N-(2-propynyl) (17, 19, 21, 23, 25) and 5-hydroxymethyl-3-N-(2-propynyl) (30-33) derivatives of pyrimidine nucleosides were synthesized and evaluated against mycobacteria [Mycobacterium tuberculosis (Mtb), Mycobacterium bovis (BCG) and Mycobacterium avium], gram-positive bacteria (Staphylococcus aureus and Enterococcus faecalis) and gram-negative bacteria (Escherichia coli, Salmonella typhimurium and Pseudomonas aeruginosa) alone and in combination with existing drugs in in vitro assays. Although several compounds exhibited marked inhibitory activity at a higher concentration against Mtb, M. bovis, S. aureus and E. faecalis, they displayed unexpected synergistic and additive interactions at their lower concentrations with antitubercular drugs isoniazid and rifampicin, and antibacterial drug gentamicin. The active analogues were also found to inhibit intracellular Mtb in a human monocytic cell line infected with H37Ra. Oral administration of 5-hydroxymethyl-3-N-(2-propynyl)-3'-azido-2',3'-dideoxyuridine (32) and 5-hydroxymethyl-3-N-(2-propynyl)-2',3'-dideoxyuridine (33) at a dose of 100mg/kg for two weeks showed promising in vivo effects in mice infected with Mtb (H37Ra). No in vitro cytotoxicity of the test compounds was observed up to the highest concentration tested (CC50>300μg/mL).

In Vivo and In Vitro Efficacy of Minocycline-Based Combination Therapy for Minocycline-Resistant Acinetobacter baumannii

Minocycline-based combination therapy has been suggested to be a possible choice for the treatment of infections caused by minocycline-susceptible Acinetobacter baumannii, but its use for the treatment of infections caused by minocycline-resistant A. baumannii is not well established. In this study, we compared the efficacy of minocycline-based combination therapy (with colistin, cefoperazone-sulbactam, or meropenem) to that of colistin in combination with meropenem for the treatment of minocycline-resistant A. baumannii infection. From 2006 to 2010, 191 (17.6%) of 1,083 A. baumannii complex isolates not susceptible to minocycline from the Taiwan Surveillance of Antimicrobial Resistance program were collected. Four representative A. baumannii isolates resistant to minocycline, amikacin, ampicillin-sulbactam, ceftazidime, ciprofloxacin, cefepime, gentamicin, imipenem, levofloxacin, meropenem, and piperacillin-tazobactam were selected on the basis of the diversity of their pulsotypes, collection years, health care setting origins, and geographic areas of origination. All four isolates had tetB and overexpressed adeABC, as revealed by quantitative reverse transcription-PCR. Among all minocycline-based regimens, only the combination with colistin produced a fractional inhibitory concentration index comparable to that achieved with meropenem combined with colistin. Minocycline (4 or 16 μg/ml) in combination with colistin (0.5 μg/ml) also synergistically killed minocycline-resistant isolates in time-kill studies. Minocycline (50 mg/kg of body weight) in combination with colistin (10 mg/kg) significantly improved the survival of mice and reduced the number of bacteria present in the lungs of mice compared to the results of monotherapy. However, minocycline (16 μg/ml)-based therapy was not effective at reducing biofilm-associated bacteria at 24 or 48 h when its effectiveness was compared to that of colistin (0.5 μg/ml) and meropenem (8 μg/ml). The clinical use of minocycline in combination with colistin for the treatment of minocycline-resistant A. baumannii may warrant further investigation.

Pseudomonas aeruginosa: targeting cell-wall metabolism for new antibacterial discovery and development

Pseudomonas aeruginosa is a leading cause of hospital-acquired infections and is resistant to most antibiotics. With therapeutic options against P. aeruginosa dwindling, and the lack of new antibiotics in advanced developmental stages, strategies for preserving the effectiveness of current antibiotics are urgently required. β-Lactam antibiotics are important agents for treating P. aeruginosa infections, thus, adjuvants that potentiate the activity of these compounds are desirable for extending their lifespan while new antibiotics - or antibiotic classes - are discovered and developed. In this review, we discuss recent research that has identified exploitable targets of cell-wall metabolism for the design and development of compounds that hinder resistance and potentiate the activity of antipseudomonal β-lactams.

Enhanced efficacy of imipenem-colistin combination therapy against multiple-drug-resistant Enterobacter cloacae: in vitro activity and a Galleria mellonella model

BACKGROUND/PURPOSE

To investigate the in vitro and in vivo activity of imipenem-colistin combination against multidrug-resistant Enterobacter cloacae infections in order to determine whether it should be explored further.

METHODS

The antimicrobial activity of colistin alone and in combination with imipenem was assessed versus an imipenem-susceptible isolate, E. cloacae GN1059, or an imipenem-resistant strain, E. cloacae GN0791, isolated in Anhui, China. The potential synergy of imipenem-colistin was evaluated using a checkerboard assay, as well as static time-kill experiments at 1× and 2× minimum inhibitory concentration (MIC). A simple invertebrate model (Galleria mellonella) was developed to assess the in vivo efficacy of imipenem-colistin in treating E. cloacae infection.

RESULTS

In checkerboard assays, synergy (defined as a fractional inhibitory concentration index of ≤ 0.5) was observed between imipenem and colistin for both isolates tested. In time-kill assays, the combination of imipenem-colistin at 1× or 2× MIC resulted in complete killing of both strains. In the G. mellonella larvae model infected with lethal doses of E. cloacae, the combination therapy led to significantly increased survival of the larvae as compared with imipenem or colistin monotherapy alone (p < 0.05).

CONCLUSION

This is the first report demonstrating the efficacy of antimicrobial agents in the G. mellonella larvae model of infections caused by E. cloacae. Our study suggested that imipenem-colistin combination was highly active against E. cloacae both in vitro and in the simple invertebrate model, and provided preliminary in vivo evidence that such combination might be useful therapeutically.

Activity of levofloxacin in combination with colistin against Acinetobacter baumannii: In vitro and in a Galleria mellonella model

BACKGROUND/PURPOSE

Treatment of Acinetobacter baumannii infections is challenging owing to widespread multidrug-resistant A. baumannii (MDR-AB) and the lack of novel agents. Although recent data suggest that levofloxacin (LVX) may have unique activity against MDR-AB in combination with colistin (CST), further preclinical work is needed.

METHODS

We used a A. baumannii type strain ATCC19606, a CST-resistant strain AB19606R, and two clinical isolates (GN0624 and GN1115) of MDR-AB to investigate the in vitro and in vivo efficacy of LVX-CST combination. Synergy studies were performed using the microtiter plate chequerboard assay and time-kill methodology. Inhibitory activity of antibiotics against biofilms and the mutant prevention concentrations were also studied in vitro. A simple invertebrate model (Galleria mellonella) has been used to assess the in vivo activity of antimicrobial therapies.

RESULTS

The LVX-CST combination was bactericidal against the CST-susceptible clinical isolate (GN0624). In checkerboard assays, synergy (defined as a fractional inhibitory concentration index of < 0.5) was observed between CST and LVX in GN0624. The combination had antibiofilm properties on the preformed biofilms of four tested strains and could prevent the emergence of CST-resistant A. baumanni. Treatment of G. mellonella larvae infected with lethal doses of A. baumannii resulted in significantly enhanced survival rates when LVX was given with CST compared with CST treatment alone (p < 0.05).

CONCLUSION

In summary, a synergistic or additive effect between CST and LVX was observed in vitro and in vivo against CST-susceptible A. baumannii strains, although not against CST-resistant ones.

The role of vancomycin in addition with colistin and meropenem against colistin-sensitive multidrug resistant Acinetobacter baumannii causing severe infections in a Paediatric Intensive Care Unit

Background

Acinetobacter baumannii has been associated with high morbidity and mortality rates, even in pediatric patients. Therapeutic options are limited, especially when the strain is multidrug resistant.

Methods

Clinical and microbiological analyses of 4 cases of systemic infections caused by multi drug resistant A. baumannii treated with colistin/vancomycin combination at a Pediatric Intensive Care Unit were performed in order to explore the potential synergistic activity of colistin plus vancomycin. All the patients were treated with colistin, meropenem and vancomycin.

Results

Four severe infections due to MDR A. baumannii were observed. All patients treated with colistin/vancomycin combination had a positive outcome with no infection relapses. Most importantly, no significant adverse events related to the simultaneous administration of COL plus VAN were observed. In our in-vitro experiments, the synergistic effect of the combination COL plus VAN showed an early bactericidal activity even at VAN concentration of 16 mg/L, which reflects the serum trough concentrations obtained in patients.

Discussion

An antimicrobial strategy based on the activity of colistin plus vancomycin was in-vitro and in-vivo effective in life-threatening infections caused by multidrug-resistant A. baumannii in a Pediatric Intensive Care Unit, in the absence of adverse effects. Colistin plus vancomycin were highly synergic and bactericidal against carbapenem-resistant, colistin sensitive A. baumannii whereas the addition of meropenem did not enhance the in-vitro activity of colistin plus vancomycin.

Conclusions

Our results confirm existing data on the potential synergistic activity of a therapeutic strategy including colistin plus vancomycin and provide important new clinical information for its potential use as a therapeutic option against MDR A. baumannii infections, especially in the pediatric population.

Electronic supplementary material

The online version of this article (doi:10.1186/s12879-015-1133-3) contains supplementary material, which is available to authorized users.

Update on infections caused by Stenotrophomonas maltophilia with particular attention to resistance mechanisms and therapeutic options

Stenotrophomonas maltophilia is a Gram-negative, biofilm-forming bacterium. Although generally regarded as an organism of low virulence, S. maltophilia is an emerging multi-drug resistant opportunistic pathogen in hospital and community settings, especially among immunocompromised hosts. Risk factors associated with S. maltophilia infection include underlying malignancy, cystic fibrosis, corticosteroid or immunosuppressant therapy, the presence of an indwelling central venous catheter and exposure to broad spectrum antibiotics. In this review, we provide a synthesis of information on current global trends in S. maltophilia pathogenicity as well as updated information on the molecular mechanisms contributing to its resistance to an array of antimicrobial agents. The prevalence of S. maltophilia infection in the general population increased from 0.8-1.4% during 1997-2003 to 1.3-1.68% during 2007-2012. The most important molecular mechanisms contributing to its resistance to antibiotics include β-lactamase production, the expression of Qnr genes, and the presence of class 1 integrons and efflux pumps. Trimethoprim/sulfamethoxazole (TMP/SMX) is the antimicrobial drug of choice. Although a few studies have reported increased resistance to TMP/SMX, the majority of studies worldwide show that S. maltophilia continues to be highly susceptible. Drugs with historically good susceptibility results include ceftazidime, ticarcillin-clavulanate, and fluoroquinolones; however, a number of studies show an alarming trend in resistance to those agents. Tetracyclines such as tigecycline, minocycline, and doxycycline are also effective agents and consistently display good activity against S. maltophilia in various geographic regions and across different time periods. Combination therapies, novel agents, and aerosolized forms of antimicrobial drugs are currently being tested for their ability to treat infections caused by this multi-drug resistant organism.

Optimum treatment strategies for carbapenem-resistant Acinetobacter baumannii bacteremia

Carbapenem-resistant Acinetobacter baumannii (CRAB) constitutes an increasing problem worldwide. CRAB bacteremia is associated with a high fatality rate and its optimal treatment has not been established. Early institution of appropriate therapy is shown to improve survival of patients with CRAB bloodstream infection. Regrettably, treatment options are limited. Little information exists about the efficacy of sulbactam for the treatment of CRAB bacteremia. Colistin and tigecycline possess good in vitro activity and represent in many cases the only therapeutic options although clinical data are scarce. The need for a loading dose of colistin has been recently demonstrated to rapidly achieve therapeutic levels. The use of combination therapy is also a matter of debate but current evidence do not support its routine use.

A Review of Novel Combinations of Colistin and Lipopeptide or Glycopeptide Antibiotics for the Treatment of Multidrug-Resistant Acinetobacter baumannii

As antibiotic resistance continues to increase among Gram-negative organisms such as Acinetobacter baumannii there is a growing need for novel therapies to overcome these resistance mechanisms. Antibiotics active against multidrug-resistant A. baumannii (MDRAB) are few, and agents in development are primarily active against other multidrug-resistant Gram-negative organisms. The combinations of colistin and antimicrobials such as glycopeptides and lipopeptides are unique potential treatment modalities against MDRAB. For both lipopeptides and glycopeptides in vitro data have demonstrated significant synergy, resulting in rapid bactericidal activity in time-kill curves. Several invertebrate in vivo models have also demonstrated increased survival compared to colistin alone. Currently, very little clinical data have focused on using these combinations for infections caused exclusively by multidrug-resistant Gram-negatives. The combination of vancomycin and colistin has been studied with conflicting results regarding both improved outcomes and risk of nephrotoxicity. Although in vitro and in vivo models have proved promising, further investigation is required to provide clinical data necessary to support the use of these combinations. The objective of this review is to summarize literature currently available for the novel combination of lipopeptides or glycopeptides with colistin for the treatment of A. baumannii, in particular MDRAB.

Molecular mechanisms of antibiotic resistance

Antibiotic-resistant bacteria that are difficult or impossible to treat are becoming increasingly common and are causing a global health crisis. Antibiotic resistance is encoded by several genes, many of which can transfer between bacteria. New resistance mechanisms are constantly being described, and new genes and vectors of transmission are identified on a regular basis. This article reviews recent advances in our understanding of the mechanisms by which bacteria are either intrinsically resistant or acquire resistance to antibiotics, including the prevention of access to drug targets, changes in the structure and protection of antibiotic targets and the direct modification or inactivation of antibiotics.

In vivo activity of daptomycin/colistin combination therapy in a Galleria mellonella model of Acinetobacter baumannii infection

Antimicrobial treatment of multidrug-resistant Acinetobacter baumannii (MDR-AB) infections continues to pose significant challenges. With limited options, clinicians have been pushed towards using unorthodox combinations of licensed antibiotics. Although daptomycin/colistin combination appears to be a promising treatment option based on in vitro data, further preclinical work is needed. In this study, the A. baumannii-Galleria mellonella system was employed to study the in vivo efficacy of this combination in order to determine whether it should be explored further for the treatment of MDR-AB infections. The antimicrobial activity of colistin alone and in combination with daptomycin was assessed versus an A. baumannii type strain (ATCC 19606) and a MDR-AB clinical strain (GN2231) isolated in Anhui, China. Synergy studies were performed using the microtitre plate chequerboard assay and time-kill methodology. The in vivo activity of daptomycin/colistin combination was assessed using a G. mellonella larvae model. The combination of daptomycin and colistin was bactericidal against both strains tested. In chequerboard assays, daptomycin was highly active against A. baumannii when combined with colistin [fractional inhibitory concentration index (FICI) of <0.5]. Treatment of G. mellonella larvae infected with lethal doses of A. baumannii resulted in significantly enhanced survival rates when daptomycin was given with colistin compared with colistin treatment alone (P<0.05). This work suggests that daptomycin/colistin combination is highly active against A. baumannii both in vitro and in a simple invertebrate model of infection.

In vitro synergy of polymyxins with other antibiotics for Acinetobacter baumannii: a systematic review and meta-analysis

In order to provide preliminary guidance for rational antibiotic combination therapy in the clinic, a systematic review and meta-analysis was performed to evaluate the in vitro synergistic activity of polymyxins combined with other antibiotics against Acinetobacter baumannii. An extensive literature search was undertaken without restriction according to region, publication type or language. All available in vitro synergy tests on antibiotic combinations consisting of polymyxins were included. The primary outcome assessed was the in vitro activity of combination therapy on bacterial kill or inhibition. In total, 70 published studies and 31 conference proceedings reporting testing of polymyxins in combination with 11 classes consisting of 28 antibiotic types against 1484 A. baumannii strains were included in the analysis. In time-kill studies, high in vitro synergy and bactericidal activity were found for polymyxins combined with several antibiotic classes such as carbapenems and glycopeptides. Carbapenems or rifampicin combination could efficiently suppress the development of colistin resistance and displayed a >50% synergy rate against colistin-resistant strains. Synergy rates of chequerboard microdilution and Etest methods in most antibiotic combinations were generally lower than those of time-kill assays. The benefits of these antibiotic combinations should be further demonstrated by well-designed clinical studies.

Infections Caused by Stenotrophomonas maltophilia in Recipients of Hematopoietic Stem Cell Transplantation

Stenotrophomonas maltophilia (S. maltophilia) is a globally emerging Gram-negative bacillus that is widely spread in environment and hospital equipment. Recently, the incidence of infections caused by this organism has increased, particularly in patients with hematological malignancy and in recipients of hematopoietic stem cell transplantation (HSCT) having neutropenia, mucositis, diarrhea, central venous catheters or graft versus host disease and receiving intensive cytotoxic chemotherapy, immunosuppressive therapy, or broad-spectrum antibiotics. The spectrum of infections in HSCT recipients includes pneumonia, urinary tract and surgical site infection, peritonitis, bacteremia, septic shock, and infection of indwelling medical devices. The organism exhibits intrinsic resistance to many classes of antibiotics including carbapenems, aminoglycosides, most of the third-generation cephalosporins, and other β-lactams. Despite the increasingly reported drug resistance, trimethoprim-sulfamethoxazole is still the drug of choice. However, the organism is still susceptible to ticarcillin-clavulanic acid, tigecycline, fluoroquinolones, polymyxin-B, and rifampicin. Genetic factors play a significant role not only in evolution of drug resistance but also in virulence of the organism. The outcome of patients having S. maltophilia infections can be improved by: using various combinations of novel therapeutic agents and aerosolized aminoglycosides or colistin, prompt administration of in vitro active antibiotics, removal of possible sources of infection such as infected indwelling intravascular catheters, and application of strict infection control measures.

Activity of the type I signal peptidase inhibitor MD3 against multidrug-resistant Gram-negative bacteria alone and in combination with colistin

OBJECTIVES

Effective treatment of Gram-negative bacterial infections is increasingly challenging due to the spread of multidrug-resistant strains and a lack of new antimicrobials in development. Bacterial type I signal peptidases (SPases) represent a highly conserved and essential target for inhibition by novel compounds. SPases are required for the effective processing of membrane translocated proteins involved in core functions related to metabolism, virulence and resistance. In this study we assessed the biochemical and functional activity of a novel synthetic inhibitor (MD3) of SPases against a wide range of Gram-negative pathogens.

METHODS

The activity and specificity of MD3 for recombinant Pseudomonas aeruginosa SPase (LepB) and a genetically engineered LepB-regulatable strain were investigated. Antimicrobial activity of the compound alone and in combination with outer membrane-permeabilizing agents (sodium hexametaphosphate, colistin) was also determined against a collection of P. aeruginosa, Acinetobacter baumannii, Klebsiella pneumoniae and Stenotrophomonas maltophilia isolates.

RESULTS

MD3 was found to inactivate the P. aeruginosa LepB protein (IC50 10 μM), resulting in antimicrobial effects potentiated in the presence of colistin. MD3 also demonstrated potent activity against wild-type and multidrug-resistant strains of A. baumannii and S. maltophilia with MICs ranging from 0.5 to 14 mg/L in the presence of subinhibitory concentrations of colistin.

CONCLUSIONS

MD3 is a novel inhibitor of bacterial SPase in a range of non-fermentative Gram-negative bacteria. The antimicrobial activity is potentiated in combination with colistin and suggests that SPase inhibition warrants further exploration as a basis for future mono or combination therapies.

Efficacy of the small molecule inhibitor of Lipid II BAS00127538 against Acinetobacter baumannii

Objective

To test the activity of a small molecule compound that targets Lipid II against Acinetobacter baumannii.

Methods

Susceptibility to small molecule Lipid II inhibitor BAS00127538 was assessed using carbapenem- and colistin-resistant clinical isolates of A. baumannii. In addition, synergy between colisitin and this compound was assessed.

Results

Small molecule Lipid II inhibitor BAS00127538 potently acts against A. baumannii and acts synergistically with colistin.

Conclusion

For the first time, a compound that targets Lipid II is described that acts against multi-drug resistant isolates of A. baumannii. The synergy with colistin warrants further lead development of BAS00127538.

Infections Caused by Acinetobacter baumannii in Recipients of Hematopoietic Stem Cell Transplantation

Acinetobacter baumannii (A. baumannii) is a Gram-negative, strictly aerobic, non-fermentative coccobacillus, which is widely distributed in nature. Recently, it has emerged as a major cause of health care-associated infections (HCAIs) in addition to its capacity to cause community-acquired infections. Risk factors for A. baumannii infections and bacteremia in recipients of hematopoietic stem cell transplantation include: severe underlying illness such as hematological malignancy, prolonged use of broad-spectrum antibiotics, invasive instrumentation such as central venous catheters or endotracheal intubation, colonization of respiratory, gastrointestinal, or urinary tracts in addition to severe immunosuppression caused by using corticosteroids for treating graft versus host disease. The organism causes a wide spectrum of clinical manifestations, but serious complications such as bacteremia, septic shock, ventilator-associated pneumonia, extensive soft tissue necrosis, and rapidly progressive systemic infections that ultimately lead to multi-organ failure and death are prone to occur in severely immunocompromised hosts. The organism is usually resistant to many antimicrobials including penicillins, cephalosporins, trimethoprim-sulfamethoxazole, almost all fluoroquinolones, and most of the aminoglycosides. The recently increasing resistance to carbapenems, colistin, and polymyxins is alarming. Additionally, there are geographic variations in the resistance patterns and several globally and regionally resistant strains have already been described. Successful management of A. baumannii infections depends upon appropriate utilization of antibiotics and strict application of preventive and infection control measures. In uncomplicated infections, the use of a single active beta-lactam may be justified, while definitive treatment of complicated infections in critically ill individuals may require drug combinations such as colistin and rifampicin or colistin and carbapenem. Mortality rates in patients having bacteremia or septic shock may reach 70%. Good prognosis is associated with presence of local infection, absence of multidrug resistant strain, and presence of uncomplicated infection while poor outcome is associated with severe underlying medical illness, bacteremia, septic shock, multi-organ failure, HCAIs, admission to intensive care facilities for higher levels of care, and culture of certain aggressive genotypes of A. baumannii.

Combination therapy in severe Acinetobacter baumannii infections: an update on the evidence to date

ABSTRACT: 

Acinetobacter baumannii is a drug-resistant Gram-negative pathogen increasingly causing hospital-acquired infections in critically ill patients. In this review, we summarize the current mechanisms of antimicrobial resistance in A. baumannii and describe in detail recent in vitro and in vivo experimental data on the activity of antimicrobial combinations against this microorganism. We then introduce the rationale for the use of combination antibiotic therapy in resistant A. baumannii infections. Finally, we present and critically discuss both uncontrolled clinical studies and the few randomized clinical trials of combination antimicrobial therapy for these infections, with a special focus on ongoing multinational trials and optimal approach to future research in this field.

Clinical efficacy and safety of the combination of colistin plus vancomycin for the treatment of severe infections caused by carbapenem-resistant Acinetobacter baumannii

A potent synergy of a glycopeptide-colistin combination against Acinetobacter baumannii has recently been described. We set out to assess the efficacy and safety of this combination in a retrospective study including episodes of ventilator-associated pneumonia or bacteremia caused by carbapenem-resistant A. baumannii. We compared 29 patients (group I) treated with colistin plus vancomycin with 28 patients treated with colistin alone (group II). Group I received vancomycin (for empirical or targeted therapy) at the onset of colistin administration and both antimicrobials coincided for at least 5 days. Baseline characteristics, clinical cure, microbiological eradication, and mortality were similar in both groups but the rate of acute kidney injury was higher in group I (55.2 vs. 28%; p = 0.04). In critically ill patients with carbapenem-resistant A. baumannii infections, clinical outcomes do not differ in patients treated with colistin plus vancomycin from those receiving colistin without vancomycin. This combination significantly increases the risk of renal failure.

Synergistic effect between colistin and bacteriocins in controlling Gram-negative pathogens and their potential to reduce antibiotic toxicity in mammalian epithelial cells

Pathogens resistant to most conventional antibiotics are a harbinger of the need to discover novel antimicrobials and anti-infective agents and develop innovative strategies to combat them. The aim of this study was to assess the in vitro activity of colistin alone or in combination with two bacteriocins, nisin A and pediocin PA-1/AcH, against Salmonella choleraesuis ATCC 14028, Pseudomonas aeruginosa ATCC 27853, Yersinia enterocolitica ATCC 9610, and Escherichia coli ATCC 35150 (O157:H7). The strain most sensitive to colistin was enterohemorrhagic E. coli O157:H7, which was inhibited at a concentration of about 0.12 μg/ml. When nisin A (1.70 μg/ml) or pediocin PA-1/AcH (1.56 μg/ml) was combined with colistin, the concentrations required to inhibit E. coli O157:H7 were 0.01 and 0.03 μg/ml, respectively. The in vitro antigenotoxic effect of colistin was determined by using the comet assay method to measure the level of DNA damage in freshly isolated human peripheral blood leukocytes (PBLs) incubated with colistin for 1 h at 37°C. Changes in the tail extents of PBLs of about 69.29 ± 0.08 μm were observed at a final colistin concentration of about 550 ng/ml. Besides the synergistic effect, the combination of colistin (1 mg/ml) and nisin (2 mg/ml) permitted us to re-evaluate the toxic effect of colistin on Vero (monkey kidney epithelial) cells.

Novel derivatives of polymyxins

Polymyxin B and colistin (polymyxin E) are bactericidal pentacationic lipopeptides that act specifically on Gram-negative bacteria, first by disrupting their outermost permeability barrier, the outer membrane (OM), and then damaging the cytoplasmic membrane. Both were discovered in the mid-1950s and subsequently used in intravenous therapy, but soon largely abandoned because of nephrotoxicity. The emergence of extremely multiresistant strains has now forced clinicians to reinstate them in the therapy of severe infections caused by such strains. This article reviews recent attempts to develop novel derivatives of polymyxins that exhibit less toxicity and greater potency than the existing drugs. In addition, studies of novel des-fatty acyl-polymyxin derivatives that display activity against Pseudomonas aeruginosa are included. The review also covers recent studies of derivatives that lack potent bactericidal action, but which disrupt the OM, which increases bacterial permeability to other antibiotics, facilitating their entry into the cell.

In vivo efficacy of telavancin/colistin combination therapy in a Galleria mellonella model of Acinetobacter baumannii infection

Treatment of Acinetobacter baumannii infections is challenging owing to widespread multidrug resistance and the lack of novel agents. There is now considerable interest in the potential of unorthodox combination therapies such as colistin and glycopeptides (e.g. vancomycin and teicoplanin), since potent synergy can be demonstrated in vitro. A simple invertebrate model (Galleria mellonella) has been developed to assess the in vivo activity of antimicrobial therapies and was used to investigate the efficacy of colistin combined with the lipoglycopeptide telavancin in the treatment of A. baumannii infection. Galleria mellonella larvae were inoculated with 10⁵ CFU/larvae of A. baumannii type strain ATCC 19606 or the multidrug-resistant clinical isolate AB210. Infected caterpillars were treated with either telavancin (10 mg/kg), colistin (2.5 mg/kg) or a telavancin/colistin combination. Larvae were incubated at 37 °C for up to 96 h and were scored daily. Survival curves were plotted and analysed using the log-rank test. The telavancin/colistin combination was effective in the treatment of larvae infected with both strains but was superior to colistin monotherapy in the treatment of A. baumannii AB210 (P<0.001). The combination of telavancin and colistin was effective in a simple invertebrate model of A. baumannii infection. This is in agreement with a previous in vitro study and provides preliminary in vivo evidence that such a combination might be useful therapeutically.