Activity of daptomycin or linezolid in combination with rifampin or gentamicin against biofilm-forming Enterococcus faecalis or E. faecium in an in vitro pharmacodynamic model using simulated endocardial vegetations and an in vivo survival assay using Galleria mellonella larvae

From Polymeric Nanoformulations to Polyphenols-Strategies for Enhancing the Efficacy and Drug Delivery of Gentamicin

Gentamicin is an essential broad-spectrum aminoglycoside antibiotic that is used in over 40 clinical conditions and has shown activity against a wide range of nosocomial, biofilm-forming, multi-drug resistant bacteria. Nevertheless, the low cellular penetration and serious side effects of gentamicin, as well as the fear of the development of antibacterial resistance, has led to a search for ways to circumvent these obstacles. This review provides an overview of the chemical and pharmacological properties of gentamicin and offers six different strategies (the isolation of specific types of gentamicin, encapsulation in polymeric nanoparticles, hydrophobization of the gentamicin molecule, and combinations of gentamicin with other antibiotics, polyphenols, and natural products) that aim to enhance the drug delivery and antibacterial activity of gentamicin. In addition, factors influencing the synthesis of gentamicin-loaded polymeric (poly (lactic-co-glycolic acid) (PLGA) and chitosan) nanoparticles and the methods used in drug release studies are discussed. Potential research directions and future perspectives for gentamicin-loaded drug delivery systems are given.

Synergistic properties of linezolid against Enterococcus spp. isolates: a systematic review from in vitro studies

PURPOSE

Vancomycin-resistant enterococci (VRE) are a leading cause of hospital-acquired infections with limited therapeutic options. Combination of at least two antimicrobials is a possible strategy to obtain rapid and sustained bactericidal effects and overcome the emergence of resistance. We revised the literature on linezolid synergistic properties from in vitro studies to assess its activity in combination with molecules belonging to other antibiotic classes against Enterococcus spp.

METHODS

We performed a systematic review of the literature from three peer-reviewed databases including papers evaluating linezolid synergistic properties in vitro against Enterococcus spp. isolates.

RESULTS

We included 206 Enterococcus spp. isolates (92 E. faecalis, 90 E. faecium, 2 E. gallinarum, 3 E. casseliflavus, 19 Enterococcus spp.) from 24 studies. When an isolate was tested with different combinations, each combination was considered independently for further analysis. The most frequent interaction was indifferent effect (247/343, 72% of total interactions). The highest synergism rates were observed when linezolid was tested in combination with rifampin (10/49, 20.4% of interactions) and fosfomycin (16/84, 19.0%, of interactions). Antagonistic effect accounted for 7/343 (2.0%) of total interactions.

CONCLUSION

Our study reported overall limited synergistic in vitro properties of linezolid with other antibiotics when tested against Enterococcus spp. The clinical choice of linezolid in combination with other antibiotics should be guided by reasoned empiric therapy in the suspicion of a polymicrobial infection or targeted therapy on microbiological results, rather than on an intended synergistic effect of the linezolid-based combination.

Fosfomycin Enhances the Inhibition Ability of Linezolid Against Biofilms of Vancomycin-Resistant Enterococcus faecium in vitro

Purpose

We explored the inhibition ability of linezolid/fosfomycin combination against biofilms of vancomycin-resistant Enterococcus faecium (VREfm) and tried to provide a theoretical basis for the treatment of VREfm biofilm-associated infections.

Methods

Four clinical isolates of VREfm (No.2, No.4, No.5, and No.6) were used for this study, which were collected from the First Affiliated Hospital of Anhui Medical University. The checkerboard method was used to assess the synergistic effect of linezolid and fosfomycin. The inhibition ability of biofilm biomass was evaluated by crystal violet staining, and the metabolic activity was tested by an Alamar blue cell viability assay. Changes in biofilm formation-related genes of the strains after incubating with drugs were investigated via the quantitative real-time polymerase chain reaction (RT-qPCR).

Results

The fractional inhibitory concentration index (FICI) showed that linezolid combined with fosfomycin had a synergistic effect on all four VREfm isolates. Compared with linezolid monotherapy, linezolid combined with fosfomycin led to a significant decrease in biofilm biomass and metabolic activity, especially in the mature biofilm. The results of RT-qPCR showed linezolid combined with fosfomycin inhibition biofilm formation through the inhibition of cylA, ebpA, and gelE transcription in VREfm in the initial and mature stages. To the mature biofilm, the combination also reduced the expression of asa1, atlA, and esp.

Conclusion

The combination of linezolid and fosfomycin represented stronger inhibitory effect on the biofilm formation of VREfm than linezolid alone.

Daptomycin-Impregnated PMMA Cement against Vancomycin-Resistant Germs: Dosage, Handling, Elution, Mechanical Stability, and Effectiveness

BACKGROUND

The number of periprosthetic joint infections caused by vancomycin-resistant pathogens is increasing. Currently, no PMMA cement is commercially available to cover VRE. Daptomycin shows promising results in treating infection, offering a good safety profile and a reduced risk of developing resistance. The purpose of this in vitro study was to investigate the mechanical stability, handling properties, elution behavior, and antimicrobial effectiveness of PMMA cement loaded with three different daptomycin concentrations in comparison to commercially available antibiotic-loaded bone cement (ALBC).

METHODS

Mechanical properties and handling characteristics (ISO 5833, DIN 53435), HPLC elution, antimicrobial effectiveness with proliferation assay (DIN 17025), and inhibition zone testing were investigated.

RESULTS

All tested daptomycin concentrations met the ISO and DIN standards for mechanical strength. Loading of 40 g of PMMA cement with 0.5 g of daptomycin did not show any antimicrobial effectiveness, in contrast to 1.0 g and 1.5 g. PMMA cement with 1.5 g of daptomycin was the best in terms of elution and effectiveness, and it showed good ISO mechanical strength; ISO doughing was sticky for a little longer and setting was faster compared to the vancomycin-containing reference cement.

CONCLUSION

PMMA cement containing 0.5 g of gentamicin and 1.5 g of daptomycin could be a good alternative to the already established COPAL® (Wehrheim, Germany) G+V for the treatment of PJIs caused by VRE.

Daptomycin as an option for lock therapy: a systematic literature review

Aim: To review preclinical and clinical data relevant to daptomycin lock therapy in catheter-related bloodstream infection (CRBSI). Methods: Systematic review in PubMed, Scopus and clinical trial registries. Results: Preclinical data demonstrate daptomycin lock solution stability and compatibility with heparin, good biofilm penetration, bactericidal activity against biofilm-embedded bacteria, and high efficacy in vitro and in animal catheter infection models. Clinical data remain limited (two case reports and five case series totaling n = 65 CRBSI episodes), albeit promising (successful catheter salvage in about 80% of cases). Conclusion: Despite theoretical advantages of daptomycin, clinical data remain scarce. Comparative studies versus alternative lock solutions are needed, as well as studies to define optimal daptomycin lock regimen (including optimal concentration, dwell time and lock duration).

Therapeutics for Vancomycin-Resistant Enterococcal Bloodstream Infections

Vancomycin-resistant enterococci (VRE) are common causes of bloodstream infections (BSIs) with high morbidity and mortality rates. They are pathogens of global concern with a limited treatment pipeline. Significant challenges exist in the management of VRE BSI, including drug dosing, the emergence of resistance, and the optimal treatment for persistent bacteremia and infective endocarditis. Therapeutic drug monitoring (TDM) for antimicrobial therapy is evolving for VRE-active agents; however, there are significant gaps in the literature for predicting antimicrobial efficacy for VRE BSIs. To date, TDM has the greatest evidence for predicting drug toxicity for the three main VRE-active antimicrobial agents daptomycin, linezolid, and teicoplanin. This article presents an overview of the treatment options for VRE BSIs, the role of antimicrobial dose optimization through TDM in supporting clinical infection management, and challenges and perspectives for the future.

Eradication of Staphylococcus epidermidis within Biofilms: Comparison of Systemic versus Supratherapeutic Concentrations of Antibiotics

Biofilm-forming bacterial infections result in clinical failure, recurring infections, and high health care costs. The antibiotic concentrations needed to eradicate biofilm require further research. We aimed to model an in vitro prosthetic joint infection (PJI) to elucidate the activity of traditional systemic concentrations versus supratherapeutic concentrations to eradicate a Staphylococcus epidermidis biofilm PJI. We evaluated S. epidermidis high-biofilm-forming (ATCC 35984) and low-biofilm-forming (ATCC 12228) isolates in an in vitro pharmacodynamic biofilm reactor model with chromium cobalt coupons to simulate prosthetic joint infection. Vancomycin, daptomycin, levofloxacin, and minocycline were used alone and combined with rifampin to evaluate the effect of biofilm eradication. We simulated three exposures: (i) humanized systemic dosing alone, (ii) supratherapeutic doses (1,000× MIC), and (iii) and dosing in combination with rifampin. Resistance development was monitored throughout the study. Simulated humanized systemic doses of a lipoglycopeptide (daptomycin), a fluoroquinolone (levofloxacin), a tetracycline (minocycline), and a glycopeptide (vancomycin) alone failed to eradicate a formed S. epidermidis biofilm. Supratherapeutic doses of vancomycin (2,000 μg/mL) and minocycline (15 μg/mL) with or without rifampin (15 μg/mL) failed to eradicate biofilms. However, a levofloxacin supratherapeutic dose (125 μg/mL) with rifampin eradicated the high-biofilm-producing isolate by 48 h. Interestingly, supratherapeutic-dose exposures of daptomycin (500 μg/mL) alone eradicated high- and low-biofilm-forming isolates in established biofilms. The concentrations needed to eradicate biofilms on foreign materials are not obtained with systemic dosing regimens. The failure of systemic dosing regimens to eradicate biofilms validates clinical findings with recurring infections. The addition of rifampin to supratherapeutic dosing regimens does not result in synergy. Supratherapeutic daptomycin dosing may be effective at the site of action to eradicate biofilms. Further studies are needed.

Streptococcus pneumoniae serotype 19A from carriers and invasive disease: virulence gene profile and pathogenicity in a Galleria mellonella model

PURPOSE

This study aimed to evaluate and compare the presence of genes related to surface proteins between isolates of Streptococcus pneumoniae from healthy carriers (HC) and invasive pneumococcal disease (IPD) with a particular focus on serotype 19A.

METHODS

The presence of these genes was identified by real-time PCR. Subsequently, we employed the Galleria mellonella larval infection model to study their effect on pathogenicity in vivo.

RESULTS

The percentage of selected virulence genes was similar between the HC and IPD groups (p > 0.05), and the genes lytA, nanB, pavA, pcpA, phtA, phtB, phtE, rrgA, and sipA were all present in both groups. However, the virulence profile of the isolates differed individually between HC and IPD groups. The highest lethality in G. mellonella was for IPD isolates (p < 0.01), even when the virulence profile was the same as compared to the HC isolates or when the nanA, pspA, pspA-fam1, and pspC genes were not present.

CONCLUSIONS

The occurrence of the investigated virulence genes was similar between HC and IPD S. pneumoniae serotype 19A groups. However, the IPD isolates showed a higher lethality in the alternative G. mellonella model than the HC isolates, regardless of the virulence gene composition, indicating that other virulence factors may play a decisive role in virulence. Currently, this is the first report using the in vivo G. mellonella model to study the virulence of clinical isolates of S. pneumoniae.

The Virtuous Galleria mellonella Model for Scientific Experimentation

The first research on the insect Galleria mellonella was published 85 years ago, and the larva is now widely used as a model to study infections caused by bacterial and fungal pathogens, for screening new antimicrobials, to study the adjacent immune response in co-infections or in host-pathogen interaction, as well as in a toxicity model. The immune system of the G. mellonella model shows remarkable similarities with mammals. Furthermore, results from G. mellonella correlate positively with mammalian models and with other invertebrate models. Unlike other invertebrate models, G. mellonella can withstand temperatures of 37 °C, and its handling and experimental procedures are simpler. Despite having some disadvantages, G. mellonella is a virtuous in vivo model to be used in preclinical studies, as an intermediate model between in vitro and mammalian in vivo studies, and is a great example on how to apply the bioethics principle of the 3Rs (Replacement, Reduction, and Refinement) in animal experimentation. This review aims to discuss the progress of the G. mellonella model, highlighting the key aspects of its use, including experimental design considerations and the necessity to standardize them. A different score in the “cocoon” category included in the G. mellonella Health Index Scoring System is also proposed.

Penicillin-Binding Proteins and Alternative Dual-Beta-Lactam Combinations for Serious Enterococcus faecalis Infections with Elevated Penicillin MICs

Ampicillin-ceftriaxone has become a first-line therapy for Enterococcus faecalis endocarditis. We characterized the penicillin-binding protein (PBP) profiles of various E. faecalis strains and tested for synergy to better inform beta-lactam options for the treatment of E. faecalis infections. We assessed the affinity of PBP2B from elevated-MIC strain E. faecalis LS4828 compared to type strain JH2-2 using the fluorescent beta-lactam Bocillin FL. We also characterized pbp4 and pbpA structures and PBP4 and PBP2B expression and used deletion and complementation studies to assess the impact of PBP2B on the levels of resistance. We tested penicillin-susceptible and -resistant E. faecalis isolates against ceftriaxone or ceftaroline combinations with other beta-lactams in 24-h time-kill studies. Two penicillin-susceptible strains (JH2-2 and L2052) had identical pbp sequences and similar PBP expression levels. One reduced-penicillin-susceptibility strain (L2068) had pbp sequences identical to those of the susceptible strains but expressed more PBP4. The second decreased-penicillin-susceptibility strain (LS4828) had amino acid substitutions in both PBP4 and PBP2B and expressed increased quantities of both proteins. PBP2B did not appear to contribute significantly to the elevated beta-lactam MICs. No synergy was demonstrable against the strains with both mutated PBPs and increased expression (L2068 and LS4828). Meropenem plus ceftriaxone or ertapenem plus ceftriaxone demonstrated the most consistent synergistic activity. PBP2B of strain LS4828 does not contribute significantly to reduced penicillin susceptibility. Neither the MIC nor the level of PBP expression correlated directly with the identified synergistic combinations when tested at static subinhibitory concentrations.

Daptomycin synergistic properties from in vitro and in vivo studies: a systematic review

INTRODUCTION

Daptomycin is a bactericidal lipopeptide antibiotic approved for the treatment of systemic infections (i.e. skin and soft tissue infections, bloodstream infections, infective endocarditis) caused by Gram-positive cocci. It is often prescribed in association with a partner drug to increase its bactericidal effect and to prevent the emergence of resistant strains during treatment; however, its synergistic properties are still under evaluation.

METHODS

We performed a systematic review to offer clinicians an updated overview of daptomycin synergistic properties from in vitro and in vivo studies. Moreover, we reported all in vitro and in vivo data evaluating daptomycin in combination with other antibiotic agents, subdivided by antibiotic classes, and a summary graph presenting the most favourable combinations at a glance.

RESULTS

A total of 92 studies and 1087 isolates (723 Staphylococcus aureus, 68 Staphylococcus epidermidis, 179 Enterococcus faecium, 105 Enterococcus faecalis, 12 Enterococcus durans) were included. Synergism accounted for 30.9% of total interactions, while indifferent effect was the most frequently observed interaction (41.9%). Antagonistic effect accounted for 0.7% of total interactions. The highest synergistic rates against S. aureus were observed with daptomycin in combination with fosfomycin (55.6%). For S. epidermidis and Enterococcus spp., the most effective combinations were daptomycin plus ceftobiprole (50%) and daptomycin plus fosfomycin (63.6%) or rifampicin (62.8%), respectively.

FUTURE PERSPECTIVES

We believe this systematic review could be useful for the future updates of guidelines on systemic infections where daptomycin plays a key role.

Meropenem plus Ceftaroline Is Active against Enterococcus faecalis in an In Vitro Pharmacodynamic Model Using Humanized Dosing Simulations

The standard of care for serious Enterococcus faecalis infections is ampicillin plus ceftriaxone. Ampicillin's inconvenient dosing schedule, drug instability, allergy potential, along with ceftriaxone's high risk for Clostridioides difficile infection and its promotion of vancomycin-resistant enterococci (VRE), led our team to explore alternative options. This work aimed to understand the role of carbapenems in combination with cephalosporins in these infections. We selected two ampicillin and penicillin susceptible E. faecalis strains (AMP-MIC 0.5-2 μg/mL; PCN-MIC 2 μg/mL) and simulated human therapeutic dosing regimens in a 48-h in vitro pharmacodynamic model (IVPD) with ampicillin (2g q4h), ertapenem (1g q24h), meropenem (2g q8h), ceftriaxone (2g q12h), and ceftaroline (600 mg q8h). As expected, ampicillin plus ceftriaxone demonstrated enhanced activity compared with ampicillin monotherapy with no MIC increases in either isolate. Meropenem and ceftaroline demonstrated significant kill against both isolates, with no regrowth or MIC increases occurring. Meropenem plus ceftriaxone also demonstrated significant kill, and while no MIC increases were identified for meropenem, there was minor regrowth and larger standard deviations. Ertapenem combined with either ceftriaxone or ceftaroline enhanced activity at 24 h, but at 48 h, regrowth occurred, and ertapenem MIC increases were noted. Meropenem-based combination therapy against E. faecalis may provide clinicians with another regimen to treat severe E. faecalis infections. Meropenem plus ceftaroline was as active as the standard of care treatment (ampicillin plus ceftriaxone) and may serve as an alternative for serious E. faecalis infections. Further studies are warranted to determine the clinical efficacy.

Flavaspidic acid BB combined with mupirocin improves its anti-bacterial and anti-biofilm activities against Staphylococcus epidermidis

BACKGROUND

The increase in drug-resistant opportunistic pathogenic bacteria, especially of antibiotic-resistant Staphylococcus epidermidis (S. epidermidis), has led to difficulties in the treatment of skin and soft tissue infections (SSTI). The major reason for bacterial resistance is the formation of bacterial biofilm. Here, we report a promising combination therapy of flavaspidic acid BB (BB) and mupirocin, which can effectively eradicate the biofilm of S. epidermidis and eliminate its drug resistance.

RESULT

The susceptibility test showed that the combination of BB and mupirocin has good antibacterial and antibiofilm activities, and the fractional inhibitory concentration index (FICI) of BB combined with mupirocin was 0.51 ± 0.00 ~ 0.75 ± 0.05, showing synergistic effect. Moreover, the time-kill curve assay results indicated that the combination of drugs can effectively inhibit the planktonic S. epidermidis. After drugs treatment, the drug-combination showed significantly inhibitory effects on the metabolic activity and total biomass in each stage of biofilm formation. The synergistic effect is likely related to the adhesion between bacteria, which is confirmed by field emission scanning electron microscope. And the expression level of aap, sarA and agrA genes were detected by real-time quantitative PCR (qRT-PCR).

CONCLUSION

Our study provides the experimental data for the use of BB for the clinical treatment of skin infections and further demonstrate the potential of BB as a novel biofilm inhibitor.

Combination Therapy of Chloramphenicol and Daptomycin for the Treatment of Infective Endocarditis Secondary to Multidrug Resistant Enterococcus faecium

A 38-years-old female with an aortic valve replacement presented to an outside hospital (OSH) with fevers and malaise. Blood cultures revealed VRE which was resistant to linezolid, resistant to ampicillin, non-susceptible to daptomycin (MIC of 8 mcg/mL), and exhibited susceptibility to gentamicin. The patient was therefore initiated on intravenous (IV) daptomycin 6 mg/kg q24h and gentamicin IV 1 mg/kg q8h. However, after 14 days of therapy with daptomycin and gentamicin, the patient was transferred to our institution for the management of cardiogenic shock and hypoxemic respiratory failure. Given the concern for treatment failure, her antimicrobial regimen was changed to IV chloramphenicol 12.5 mg/kg every 6 hours with IV daptomycin 10 mg/kg every 48 hours given an estimated creatinine clearance of 30 mL/minutes. In vitro susceptibilities for chloramphenicol were performed which confirmed susceptibility. A transesophageal echocardiogram revealed a possible abscess at the left coronary cusp and aortic valve dehiscence. The patient underwent aortic valve replacement with aortic root reconstruction. The aortic valve culture grew VRE susceptible to linezolid but resistant to ampicillin and doxycycline. The patient was deemed clinically cured after 42 days of combination therapy with daptomycin and chloramphenicol. After 6 years of follow-up, the patient has not had a recurrent VRE infection. To our knowledge, this is the first case of endocarditis secondary to VRE that was successfully managed with the combination of daptomycin and chloramphenicol.

Comparison of Anti-Microbic and Anti-Biofilm Activity Among Tedizolid and Radezolid Against Linezolid-Resistant Enterococcus faecalis Isolates

Background

The emergence and spread of linezolid-resistant Enterococcus faecalis (E. faecalis) have emerged as a serious threat to human health globally. Therefore, this study aims to compare the anti-microbic as well as the anti-biofilm activity of linezolid, tedizolid, and radezolid against linezolid-resistant E. faecalis.

Methods

A total of 2128 E. faecalis isolates were assessed from the First Affiliated Hospital of Wenzhou Medical University from 2011 to 2019. Antibiotic sensitivity was evaluated using the micro broth dilution method. Oxazolidinone-resistant chromosomal and plasmid-borne genes such as cfr, cfr(A), cfr(B), cfr(C), cfr(D), optrA, and poxtA were detected by PCR and then sequenced to detect the presence of mutations in the domain V of the 23S rRNA and the ribosomal proteins L3, L4, and L22. Conjugation experiments were conducted using the broth method. The inhibition and eradication of biofilm were evaluated through crystal violet staining, whereas the efflux pump activities were detected by agar dilution.

Results

Out of 2128 isolated E. faecalis, 71 (3.34%) were linezolid-resistant isolates in which the MICs of tedizolid and radezolid ranged from 1 to 4 μg/mL and 0.5–1 μg/mL, respectively. The MIC₅₀/MIC₉₀ of tedizolid and radezolid were 4 and 8-fold lower than the linezolid, respectively. Out of 71 resistant isolates, 57 (80.28%) carried optrA, 1 (1.41%) carried cfr, 4 (5.63%) carried optrA and cfr, and 6 (8.45%) carried optrA and cfr(D), with no mutations of 23S rRNA gene and ribosomal proteins L3, L4, and L22. Besides, the transfer rate of the optrA, cfr, and cfr(D) was 17.91%, 0% and 0%, respectively. Radezolid showed more effectiveness in eradicating biofilm (8 × MIC). However, tedizolid was more effective than radezolid and linezolid in inhibiting the biofilm formation (1/4 MIC, 1/8MIC, and 1/16MIC). Additionally, in combination with CCCP, the MICs of radezolid in all linezolid-resistant isolates decreased ≥4-fold.

Conclusion

Radezolid showed greater antimicrobial activity than tedizolid and linezolid against linezolid-resistant E. faecalis. However, both tedizolid and radezolid showed differential activity on biofilm inhibition, eradication, and efflux pump compared to linezolid. Thus, our study might bring important clinical value in the application of these drugs for resistant pathogenic strains.

Alternatives to Fight Vancomycin-Resistant Staphylococci and Enterococci

Gram positive pathogens are a significant cause of healthcare-associated infections, with Staphylococci and Enterococci being the most prevalent ones. Vancomycin, a last resort glycopeptide, is used to fight these bacteria but the emergence of resistance against this drug leaves some patients with few therapeutic options. To counter this issue, new generations of antibiotics have been developed but resistance has already been reported. In this article, we review the strategies in place or in development to counter vancomycin-resistant pathogens. First, an overview of traditional antimicrobials already on the market or in the preclinical or clinical pipeline used individually or in combination is summarized. The second part focuses on the non-traditional antimicrobials, such as antimicrobial peptides, bacteriophages and nanoparticles. The conclusion is that there is hitherto no substitute equivalent to vancomycin. However, promising strategies based on drugs with multiple mechanisms of action and treatments based on bacteriophages possibly combined with conventional antibiotics are hoped to provide treatment options for vancomycin-resistant Gram-positive pathogens.

New Quinone Antibiotics against Methicillin-Resistant S. aureus

There is an urgent need for the development of new antibiotics. Here, we describe the inhibitory activity of new quinone compounds against methicillin-resistant Staphylococcus aureus (ATCC^® 43300), methicillin-sensitive S. aureus (ATCC^® 29213), and two clinical isolates from Chile (ISP-213 and ISP-214). We observed 99.9% reduction in viability within 2 h of exposure without the cultures exhibiting any post-antibiotic effect, which was twice the kinetics to that observed with vancomycin. These clinical isolates did not acquire resistance to these quinone derivatives during the course of our study. We found that these compounds protected larvae of the greater wax moth, sp. Galleria mellonella, from infection by these MRSA clinical strains as effectively as vancomycin. These quinone derivatives are potential drug candidates worth further development.

In vitro and in vivo Antibacterial Activity of Linezolid Plus Fosfomycin Against Staphylococcus aureus with Resistance to One Drug

Objective

The purpose of this study is to assess the in vitro/vivo activities of linezolid plus fosfomycin against Staphylococcus aureus (S. aureus) isolates with varying susceptibility to the study drugs.

Methods

The increasing concentration stepwise method was used to induce S. aureus resistant strains. The in vitro antibacterial activity of linezolid combined with fosfomycin against S. aureus in vitro was studied by time-kill curve and PAE. The transmission electron microscopy (TEM) was employed to observe the cell morphology of bacteria treated with drug, and the changes of cell wall thickness were recorded. The Galleria mellonella infection model was established to demonstrate the in vivo efficacy of linezolid and fosfomycin against S. aureus with varying susceptibility.

Results

The antibiotic combination showed excellent synergistic or additive effects on the original and the linezolid-resistant strain, but showed indifferent effect for fosfomycin-resistant strain. TEM images showed that fosfomycin alone and in combined could reduce the cell wall thickness of the strains resistant to linezolid and cell lysis, while linezolid increases the cell wall thickness of the strains resistant to fosfomycin. In the Galleria mellonella infection model, the survival rate of the antibiotic combined was improved compared with that of the single drug. There was a good correlation between in vivo efficacy and in vitro susceptibility.

Conclusion

The type of interaction expressed in the test combination was highly dependent on fosfomycin resistance.

Minocycline Alone and in Combination with Polymyxin B, Meropenem, and Sulbactam against Carbapenem-Susceptible and -Resistant Acinetobacter baumannii in an In Vitro Pharmacodynamic Model

Acinetobacter baumannii is recognized as an urgent public health threat by the Centers for Disease Control and Prevention (CDC). Current treatment options are scarce, particularly against carbapenem-resistant Acinetobacter baumannii (CRAB). We simulated the impact of minocycline standard (200 mg load + 100 mg Q12h) and high (700 mg load + 350 mg Q12h) doses, polymyxin B (2.5 mg/kg Q12h), sulbactam (1 g Q6h and 9 g/24 h as continuous infusion), and meropenem (intermittent 1 or 2 g Q8h and 6 g/24 h as continuous infusion) alone or in combination against CRAB and non-CRAB isolates by simulating human therapeutic dosing regimens in a 72-h, in vitro pharmacodynamic (IVPD) model. There were no monotherapy regimens that demonstrated bactericidal activity against the tested non-CRAB and CRAB strains. Resistance development was common in monotherapy regimens. Against the CRAB isolate, the triple combination of high-dose minocycline (fAUC/MIC 21.2), polymyxin B (fAUC/MIC 15.6), and continuous-infusion sulbactam (67% T _>MIC) was the most consistently active regimen. Against non-CRAB, the triple therapy regimen of high-dose minocycline (fAUC/MIC 84.8) with continuous-infusion meropenem (100% T _>MIC) and continuous-infusion sulbactam (83% T _>MIC), as well as the double therapy of high-dose minocycline (fAUC/MIC 84.8) with continuous-infusion meropenem (100% T _>MIC), resulted in persistently bactericidal activity. In conclusion, triple therapy with high-dose minocycline, continuous-infusion sulbactam, and polymyxin B produced the most significant kill against the carbapenem-resistant Acinetobacter baumannii, with no regrowth and minimal resistance development.

Galleria mellonella larvae exhibit a weight-dependent lethal median dose when infected with methicillin-resistant Staphylococcus aureus

Galleria mellonella is a recognised model to study antimicrobial efficacy; however, standardisation across the scientific field and investigations of methodological components are needed. Here, we investigate the impact of weight on mortality following infection with Methicillin-resistant Staphylococcus aureus (MRSA). Larvae were separated into six weight groups (180-300 mg at 20 mg intervals) and infected with a range of doses of MRSA to determine the 50% lethal dose (LD50), and the 'lipid weight' of larvae post-infection was quantified. A model of LD50 values correlated with weight was developed. The LD50 values, as estimated by our model, were further tested in vivo to prove our model. We establish a weight-dependent LD50 in larvae against MRSA and demonstrate that G. mellonella is a stable model within 180-260 mg. We present multiple linear models correlating weight with: LD50, lipid weight, and larval length. We demonstrate that the lipid weight is reduced as a result of MRSA infection, identifying a potentially new measure in which to understand the immune response. Finally, we demonstrate that larval length can be a reasonable proxy for weight. Refining the methodologies in which to handle and design experiments involving G. mellonella, we can improve the reliability of this powerful model.

Synergistic activity of fosfomycin and chloramphenicol against vancomycin-resistant Enterococcus faecium (VREfm) isolates from bloodstream infections

Vancomycin-resistant Enterococcus faecium (VREfm) infections are increasing. Current anti-VREfm options (linezolid and daptomycin) are suboptimal. Fosfomycin maintains good efficacy against VREfm and chloramphenicol is active against ≥ 90% of VREfm. We tested chloramphenicol + fosfomycin (CAF+FOS) against 10 VREfm isolated from blood. MICs were 64 to 512 µg/mL for fosfomycin and 8 to 16 µg/mL for chloramphenicol. The combination decreased both MICs, with a synergic effect in 50% of the isolates and an additive effect in the remaining 50%. Time-kill assays performed on fractional inhibitory concentration index ≤ 0.5 strains confirmed the synergism. The antibiotic combination at ¼ of minimum inhibitory concentrations (MICs) caused a ≥ 2 log₁₀ reduction compared to the two antibiotics alone. Finally, we provided a proof of concept of the in vitro efficacy of CAF+FOS in G. mellonella. The survival of G. mellonella larvae treated with the combination was significantly higher. The activity of fosfomycin and chloramphenicol against VREfm increases when they are used in combination.

In vivo synergism of ampicillin, gentamicin, ceftaroline and ceftriaxone against Enterococcus faecalis assessed in the Galleria mellonella infection model

BACKGROUND

The unfavourable safety profile of aminoglycosides and the synergistic effects observed in vitro have prompted the development of novel dual β-lactam therapies, e.g. ampicillin/ceftriaxone or ampicillin/ceftaroline, for the treatment of Enterococcus faecalis endocarditis.

OBJECTIVES

For comparison with in vitro chequerboard assay results, a partial chequerboard setup of ampicillin/gentamicin, ampicillin/ceftriaxone and ampicillin/ceftaroline against E. faecalis was established in the Galleria mellonella larval infection model.

METHODS

Discrimination of synergistic and additive interactions was based on the evaluation of larval survival, bacterial quantity in the haemolymph and a pathology score index (internal to the workgroup). Single and multiple dosing schemes based on the half-life of ampicillin were applied. Pharmacokinetic data of the antibiotics in the larvae were determined via agar plate diffusion assays.

RESULTS

Ampicillin and ceftriaxone exhibited strain-specific synergistic interactions in the larvae under both dosing regimens, while the other two combinations showed additive effects. Ampicillin/ceftaroline was inferior to ampicillin/ ceftriaxone. Not all synergistic effects observed in vitro could be replicated in the larvae.

CONCLUSIONS

Our results suggest superior efficacy of ampicillin/ceftriaxone for the treatment of high-inoculum enterococcal infections, for at least some strains, but question the benefit of the current standard of adding the nephrotoxic gentamicin compared with the safer ceftriaxone. This is the first study to develop a scheme for differentiation between additive and synergistic effects in larvae and apply a multiple-antibiotic dosing scheme based on the pharmacokinetics of ampicillin. The model allows the analysis of synergistic effects of antimicrobials in an in vivo setting, but the clinical correlation warrants further study.

Treatment Considerations for CNS Infections Caused by Vancomycin-Resistant Enterococcus faecium: A Focused Review of Linezolid and Daptomycin

Objective:

To review the current literature describing pharmacology, pharmacokinetics/pharmacodynamics (PK/PD), efficacy, and safety of linezolid and daptomycin for the treatment of central nervous system (CNS) infections caused by vancomycin-resistant Enterococcus (VRE) faecium.

Data Sources:

A literature search of PubMed/MEDLINE databases was conducted (from 1950 to April 2020) utilizing the following key terms: vancomycin-resistant Enterococcus, VRE, meningitis, ventriculitis, CNS infection, daptomycin, and linezolid.

Study Selection and Data Extraction:

All relevant studies and case reports describing the treatment of VRE faecium from the CNS with linezolid or daptomycin were included.

Data Synthesis:

A total of 17 reports describing 22 cases were identified. There were 15 of 19 cases involving linezolid that reported clinical cure, of which 53.3% were monotherapy. Only 5 of 9 cases involving intravenous (IV) daptomycin resulted in cure; all 4 cases reporting daptomycin administration via the intrathecal or intraventricular route achieved clearance from the cerebrospinal fluid (CSF).

Relevance to Patient Care and Clinical Practice:

The preferred treatment option for VRE faecium infections involving the CNS remains unclear. Supporting evidence through observational case reports have described varying outcomes with linezolid and daptomycin. This review compares reported outcomes between the 2 agents and provides a thorough discussion on drug- and patient-specific variables to consider.

Conclusions:

Linezolid monotherapy appears to be safe and effective for the treatment of susceptible-VRE faecium CNS infections, with consideration of therapeutic drug monitoring in special populations and with prolonged treatment duration. Daptomycin is an effective treatment option via intrathecal or intraventricular administration when neurosurgical access is available. The role of IV daptomycin remains inconclusive.

Galleria mellonella as an infection model for the multi-host pathogen Streptococcus agalactiae reflects hypervirulence of strains associated with human invasive disease

Streptococcus agalactiae, or group B Streptococcus (GBS), infects diverse hosts including humans and economically important species such as cattle and fishes. In the context of human health, GBS is a major cause of neonatal infections and an emerging cause of invasive disease in adults and of foodborne disease in Southeast Asia. Here we show that GBS is able to establish a systemic infection in Galleria mellonella larvae that is associated with extensive bacterial replication and dose-dependent larval survival. This infection model is suitable for use with GBS isolates from both homeothermic and poikilothermic hosts. Hypervirulent sequence types (ST) associated with invasive human disease in neonates (ST17) or adults (ST283) show increased virulence in this model, indicating it may be useful in studying GBS virulence determinants, albeit with limitations for some host-specific virulence factors. In addition, we demonstrate that larval survival can be afforded by antibiotic treatment and so the model may also be useful in the development of novel anti-GBS strategies. The use of G. mellonella in GBS research has the potential to provide a low-cost infection model that could reduce the number of vertebrates used in the study of GBS infection.

Radezolid Is More Effective Than Linezolid Against Planktonic Cells and Inhibits Enterococcus faecalis Biofilm Formation

The aim of this study was to compare the effects of radezolid and linezolid on planktonic and biofilm cells of Enterococcus faecalis. A total of 302 E. faecalis clinical isolates were collected, and the minimum inhibitory concentrations (MICs) of radezolid and linezolid were determined by the agar dilution method. Changes in the transcriptome of a high-level, in vitro-induced linezolid-resistant isolate were assessed by RNA sequencing and RT-qPCR, and the roles of efflux pump-related genes were confirmed by overexpression analysis. Biofilm biomass was evaluated by crystal violet staining and the adherent cells in the biofilms were quantified according to CFU numbers. The MIC₅₀/MIC₉₀ values of radezolid (0.25/0.50 mg/L) against the 302 E. faecalis clinical isolates were eightfold lower than those of linezolid (2/4 mg/L). The radezolid MICs against the high-level linezolid-resistant isolates (linezolid MICs ≥ 64 mg/L) increased to ≥ 4 mg/L with mutations in the four copies of the V domain of the 23S rRNA gene. The mRNA expression level of OG1RF_12220 (mdlB2, multidrug ABC superfamily ATP-binding cassette transporter) increased in the high-level linezolid-resistant isolates, and radezolid and linezolid MICs against the linezolid-sensitive isolate increased with overexpression of OG1RF_12220. Radezolid (at 1/4 or 1/8× the MIC) inhibited E. faecalis biofilm formation to a greater extent than linezolid, which was primarily achieved through the inhibition of ahrC, esp, relA, and relQ transcription in E. faecalis. In conclusion, radezolid is more effective than linezolid against planktonic E. faecalis cells and inhibits biofilm formation by this bacterium.

Pharmacodynamics Of Linezolid-Plus-Fosfomycin Against Vancomycin-Susceptible And -Resistant Enterococci In Vitro And In Vivo Of A Galleria mellonella Larval Infection Model

Objectives

To explore the in vitro and in vivo antibacterial activity of linezolid/fosfomycin combination against vancomycin-susceptible and -resistant enterococci (VSE and VRE), and provide a theoretical basis for the treatment of VRE.

Methods

The checkerboard method and time-kill curve study were used to evaluate the efficacy of linezolid combined with fosfomycin against VSE and VRE. The transmission electron microscopy (TEM) was employed to observe the cell morphology of bacteria treated with each drug alone or in combination, which further elucidate the mechanism of action of antibiotic combination therapy. The Galleria mellonella infection model was constructed to demonstrate the in vivo efficacy of linezolid plus fosfomycin for VSE and VRE infection.

Results

The fractional inhibitory concentration index (FICI) values of all strains suggested that linezolid showed synergy or additivity in combination with fosfomycin against five of the six strains. Time-kill experiments demonstrated that the combination of linezolid-fosfomycin at 1×MIC or 2×MIC led to higher degree of bacterial killing without regrowth for all isolates tested than each monotherapy. TEM images showed that the combination treatment damaged the bacterial cell morphology more obviously than each drug alone. In the Galleria mellonella infection model, the enhanced survival rate of the combination treatment compared with linezolid monotherapy (P<0.05) was revealed.

Conclusion

Our data manifested that the combination of linezolid and fosfomycin was a potential therapeutic regimen for VRE infection. The combination displayed excellent bacterial killing and inhibited amplification of fosfomycin-resistant subpopulations.

A Review of Combination Antimicrobial Therapy for Enterococcus faecalis Bloodstream Infections and Infective Endocarditis

Enterococci, one of the most common causes of hospital-associated infections, are responsible for substantial morbidity and mortality. Enterococcus faecalis, the more common and virulent species, causes serious high-inoculum infections, namely infective endocarditis, that are associated with cardiac surgery and mortality rates that remained unchanged for the last 30 years. The best cures for these infections are observed with combination antibiotic therapy; however, optimal treatment has not been fully elucidated. It is the purpose of this review to highlight treatment options and their limitations, and provide direction for future investigative efforts to aid in the treatment of these severe infections. While ampicillin plus ceftriaxone has emerged as a preferred treatment option, mortality rates continue to be high, and from a safety standpoint, ceftriaxone, unlike other cephalosporins, promotes colonization with vancomycin resistant-enterococci due to high biliary concentrations. More research is needed to improve patient outcomes from this high-mortality disease.

Antivirulence activity of auranofin against vancomycin-resistant enterococci: in vitro and in vivo studies

INTRODUCTION

Vancomycin-resistant enterococci (VRE) are a leading cause of nosocomial infections because of the limited number of effective therapeutic options. In an effort to repurpose FDA-approved drugs against antibiotic-resistant bacteria, auranofin has been identified as a potent drug against VRE.

METHODS AND RESULTS

The present study determined that auranofin's antibacterial activity was not affected when evaluated against a higher inoculum size of VRE (~10⁷ CFU/mL), and auranofin successfully reduced the burden of stationary phase VRE cells via a time-kill assay. In addition, auranofin reduced VRE production of key virulence factors, including proteases, lipase and haemagglutinin. The promising features of auranofin prompted evaluation of its in vivo efficacy in a lethal mouse model of VRE septicaemia. All mice receiving auranofin at 0.125 mg/kg orally, 0.125 mg/kg subcutaneously (SC) or 0.0625 mg/kg (SC) survived the lethal VRE challenge. Additionally, auranofin was superior to linezolid, the current drug of choice, in reducing VRE burden in the liver, kidneys and spleen of mice. Remarkably, auranofin successfully reduced VRE below the limit of detection in murine internal organs after 4 days of oral or subcutaneous treatment.

CONCLUSION

These results indicate that auranofin warrants further investigation as a new treatment for systemic VRE infections.

Fluopsin C for Treating Multidrug-Resistant Infections: In vitro Activity Against Clinically Important Strains and in vivo Efficacy Against Carbapenemase-Producing Klebsiella pneumoniae

The increasing emergence of multidrug-resistant (MDR) organisms in hospital infections is causing a global public health crisis. The development of drugs with effective antibiotic action against such agents is of the highest priority. In the present study, the action of Fluopsin C against MDR clinical isolates was evaluated under in vitro and in vivo conditions. Fluopsin C was produced in cell suspension culture of Pseudomonas aeruginosa LV strain, purified by liquid adsorption chromatography and identified by mass spectrometric analysis. Bioactivity, bacterial resistance development risk against clinically important pathogenic strains and toxicity in mammalian cell were initially determined by in vitro models. In vivo toxicity was evaluated in Tenebrio molitor larvae and mice. The therapeutic efficacy of intravenous Fluopsin C administration was evaluated in a murine model of Klebsiella pneumoniae (KPC) acute sepsis, using six different treatments. The in vitro results indicated MIC and MBC below 2 μg/mL and low bacterial resistance development frequency. Electron microscopy showed that Fluopsin C may have altered the exopolysaccharide matrix and caused disruption of the cell wall of MDR bacteria. Best therapeutic results were achieved in mice treated with a single dose of 2 mg/kg and in mice treated with two doses of 1 mg/kg, 8 h apart. Furthermore, acute and chronic histopathological studies demonstrated absent nephrotoxicity and moderate hepatotoxicity. The results demonstrated the efficacy of Fluopsin C against MDR organisms in in vitro and in vivo models, and hence it can be a novel therapeutic agent for the control of severe MDR infections.

Vancomycin-resistant enterococcal infection in a Thai university hospital: clinical characteristics, treatment outcomes, and synergistic effect

Purpose

The incidence of infections with vancomycin-resistant enterococci (VRE) is increasing, with associated high mortality rates and limited therapeutic choices. We investigated the clinical characteristics and treatment outcomes of VRE infection and also determined the in vitro effect of monotherapy and combined antimicrobials against clinical VRE isolates.

Patients and methods

Clinical data and bacterial isolates obtained from patients with VRE infections between January 2014 and April 2018 at Phramongkutklao Hospital were reviewed. The clinical outcomes included in-hospital mortality, 30-day mortality, and microbiological eradication. Clonal relationships were assessed by random amplified polymorphic DNA analysis. In vitro activity of linezolid, tigecycline, fosfomycin, gentamicin, chloramphenicol, and ampicillin were determined by minimum inhibitory concentration (MIC) values. Tests of synergy of fosfomycin- or gentamicin-based combinations by the checkerboard method were reported with the fractional inhibitory concentration index or MIC reduction, respectively.

Results

Among 26 cases of VRE infection, nosocomial and gastrointestinal infections were the most common. There were various treatment regimens, but linezolid-containing regimens were generally used. In-hospital and 30-day mortality were 73.1% and 57.7%, respectively. Higher mortality was significantly associated with illness severity. The VRE isolates tested were universally susceptible to linezolid and tigecycline. A synergistic or additive effect was observed for fosfomycin combined with linezolid (100%) and with tigecycline (83.3%). Fourfold or greater MIC reduction was observed for linezolid or fosfomycin plus gentamicin at concentrations 1 (58.3%, 62.5%), 2 (83.3%, 62.5%), and 4 μg/mL (91.6%, 62.5%).

Conclusion

In-hospital mortality among patients with VRE infection was high. Linezolid remains a treatment of choice. However, combination therapy such as linezolid plus fosfomycin and linezolid plus gentamicin should be considered in cases of serious infection.

Comparison of linezolid and vancomycin lock solutions with and without heparin against biofilm-producing bacteria

PURPOSE

The activity of linezolid and vancomycin lock solutions against biofilm-producing strains of Staphylococcus aureus, S. epidermidis, and Enterococcus faecalis was studied.

METHODS

Two strains each of methicillin-susceptible S. aureus (MSSA), methicillin-resistant S. aureus (MRSA), and S. epidermidis, and 1 strain of vancomycin-susceptible E. faecalis and vancomycin-resistant E. faecalis were tested against vancomycin and linezolid to assess prevention of biofilm formation and eradication of these pathogens within a formed biofilm. Activity was also tested in a 72-hour in vitro central venous catheter (CVC) model. After 24 hours of biofilm growth in a CVC, a lock solution containing vancomycin (2 or 5 mg/mL) or linezolid (1 or 2 mg/mL) alone or in combination with heparin sodium (5,000 units/mL with benzyl alcohol 0.45%) was instilled and incubated at 35 °C for 72 hr. Heparin and 0.9% sodium chloride injection were also tested.

RESULTS

Linezolid and vancomycin prevented biofilm formation below the minimum inhibitory concentration for 88% and 25% of isolates tested, respectively. The addition of preservative-containing heparin decreased the activity of vancomycin and linezolid lock solutions against all strains. Vancomycin 2- and 5-mg/mL lock solutions had the most activity against MSSA and E. faecalis strains (p < 0.01). Linezolid 2 mg/mL was the most active lock solution against the MRSA strains tested (p < 0.01). There were no significant differences in vancomycin or linezolid lock solution activity against S. epidermidis.

CONCLUSION

Heparin reduced activity of vancomycin and linezolid lock solutions against S. aureus, S. epidermidis, and E. faecalis biofilms. While linezolid or vancomycin lock solution reduced overall biofilm burden, it did not completely eradicate the bacteria at tested concentrations.

In vivo daptomycin efficacy against experimental vancomycin-resistant Enterococcus faecium endocarditis

Objectives

Daptomycin has become a first-line therapeutic option for vancomycin-resistant Enterococcus faecium infective endocarditis (IE). Although high doses (≥8 mg/kg) are often recommended, optimal doses, particularly for strains with MICs close to the susceptibility breakpoint (4 mg/L), are still debated.

Methods

Daptomycin efficacy at doses equivalent to 8 mg/kg daptomycin (DAP8) and 12 mg/kg daptomycin (DAP12) in humans was evaluated in a rabbit model of aortic valve IE induced by 108 cfu of E. faecium reference strain Aus0004 (daptomycin MIC = 2 mg/L) or its in vitro mutant strain Mut4 (daptomycin MIC = 4 mg/L). Treatment began 48 h post-inoculation and lasted 5 days.

Results

With Aus0004, the median log10 cfu/g of vegetations was significantly lower after DAP8 and DAP12 versus controls [6.05 (n = 12) and 2.15 (n = 10) versus 9.14 (n = 11), respectively; P < 0.001], with DAP12 being more effective than DAP8 concerning vegetation bacterial load (P < 0.001) and percentages of sterile vegetations (100% versus 0%, respectively; P < 0.001). Daptomycin-resistant Aus0004 mutants were detected in 8.3% of DAP8-treated vegetations. With Mut4, the median log10 cfu/g of vegetations was significantly lower after DAP8 and DAP12 versus controls [7.7 (n = 11) and 6.95 (n = 10) versus 9.59 (n = 11), respectively; P = 0.001 and P = 0.002], without any between-dose difference, but no vegetation was sterile. Moreover, 7 of 11 (63.6%) and 7 of 9 (77.8%) vegetations contained resistant mutants after DAP8 and DAP12, respectively.

Conclusions

DAP12 was the most successful strategy against IE due to a WT E. faecium strain (daptomycin MIC = 2 mg/L). To treat IE strains with MIC = 4 mg/L, DAP8 or DAP12 monotherapy was poorly effective with the risk of resistant mutant emergence. Reassessment of the daptomycin susceptibility breakpoint for enterococci seems necessary.

Evaluation of Phage Therapy in the Context of Enterococcus faecalis and Its Associated Diseases

Bacteriophages (phages) or bacterial viruses have been proposed as natural antimicrobial agents to fight against antibiotic-resistant bacteria associated with human infections. Enterococcus faecalis is a gut commensal, which is occasionally found in the mouth and vaginal tract, and does not usually cause clinical problems. However, it can spread to other areas of the body and cause life-threatening infections, such as septicemia, endocarditis, or meningitis, in immunocompromised hosts. Although E. faecalis phage cocktails are not commercially available within the EU or USA, there is an accumulated evidence from in vitro and in vivo studies that have shown phage efficacy, which supports the idea of applying phage therapy to overcome infections associated with E. faecalis. In this review, we discuss the potency of bacteriophages in controlling E. faecalis, in both in vitro and in vivo scenarios. E. faecalis associated bacteriophages were compared at the genome level and an attempt was made to categorize phages with respect to their suitability for therapeutic application, using orthocluster analysis. In addition, E. faecalis phages have been examined for the presence of antibiotic-resistant genes, to ensure their safe use in clinical conditions. Finally, the domain architecture of E. faecalis phage-encoded endolysins are discussed.

In vitro activities of daptomycin combined with fosfomycin or rifampin on planktonic and adherent linezolid-resistant isolates of Enterococcus faecalis

PURPOSE

This study aimed to explore daptomycin combined with fosfomycin or rifampin against the planktonic and adherent linezolid-resistant isolates of Enterococcus faecalis.

METHODOLOGY

Four linezolid-resistant and four linezolid-sensitive isolates of E. faecalis which formed biofilms were collected for this study. Biofilm biomasses were detected by crystal violet staining and the adherent cells in the mature biofilms were quantified by c.f.u. determination.

RESULTS

Daptomycin alone, or combined with fosfomycin or rifampin (4×MIC) demonstrated bactericidal activities on the planktonic cells, and daptomycin combined with fosfomycin killed more planktonic cells (at least 1-log10 c.f.u. ml^-1) than daptomycin or fosfomycin alone. Daptomycin alone (16×MIC) showed anti-biofilm activities against the mature biofilms and bactericidal activities on the adherent cells, while daptomycin combined with fosfomycin (16×MIC) demonstrated significantly more anti-biofilm activities than daptomycin or fosfomycin alone and effectively killed the adherent cells in the mature biofilms. The high concentration of daptomycin (512 mg l^-1 ) combined with fosfomycin indicated more bactericidal activities on the adherent cells and more anti-biofilm activities against the mature biofilms than daptomycin 64 mg l^-1 (16×MIC) combined with fosfomycin. The addition of rifampin increased the anti-biofilm and bactericidal activities of daptomycin against the mature biofilms and the adherent cells of two isolates, however, which was not observed in other isolates.

CONCLUSIONS

Daptomycin combined with fosfomycin demonstrated better effect on the planktonic and adherent linezolid-resistant isolates of E. faecalis than daptomycin or fosfomycin alone. The role of rifampin in the treatment of E. faecalis isolates is discrepant and needs more studies.

Combatting resistant enterococcal infections: a pharmacotherapy review

INTRODUCTION

The role of enterococci in infectious diseases has evolved from a gut and urinary commensal to a major pathogen of concern. Few options exist for resistant enterococci, and appropriate use of the available agents is crucial.

AREAS COVERED

Herein, the authors discuss antibiotics with clinically useful activity against Enterococcus faecalis and E. faecium. The article specifically discusses: antibiotics active against enterococci and their mechanism of resistance, pharmacokinetic and pharmacodynamic principles, in vitro combinations, and clinical studies which focus on urinary tract, intra-abdominal, central nervous system, and bloodstream infections due to enterococci.

EXPERT OPINION

Aminopenicillins are preferred over all other agents when enterococci are susceptible and patients can tolerate them. Daptomycin and linezolid have demonstrated clinical efficacy against vancomycin-resistant enterococci (VRE). Synergistic combinations are often warranted in complex infections of high inoculum and biofilms while monotherapies are generally appropriate for uncomplicated infections. Although active against resistant enterococci, the pharmacokinetics, efficacy and safety of tigecycline and quinupristin/dalfopristin can problematical for severe infections. For cystitis, amoxicillin, nitrofurantoin, or fosfomycin are ideal. Recently, approved agents such as tedizolid and oritavancin have good in vitro activity against VRE but clinical studies against other resistant enterococci are lacking.

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.

Evaluation of daptomycin combinations with cephalosporins or gentamicin against Streptococcus mitis group strains in an in vitro model of simulated endocardial vegetations (SEVs)

Objectives

Among viridans group streptococcal infective endocarditis (IE), the Streptococcus mitis group is the most common aetiological organism. Treatment of IE caused by the S. mitis group is challenging due to the high frequency of β-lactam resistance, drug allergy and intolerability of mainstay antimicrobial agents such as vancomycin or gentamicin. Daptomycin has been suggested as an alternative therapeutic option in these scenarios based on its excellent susceptibility profile against S. mitis group strains . However, the propensity of many S. mitis group strains to rapidly evolve stable, high-level daptomycin resistance potentially limits this approach.

Methods

We evaluated the activity of 6 mg/kg/day daptomycin alone or in combination with gentamicin, ceftriaxone or ceftaroline against two daptomycin-susceptible S. mitis group strains over 96 h in a pharmacokinetic/pharmacodynamic model of simulated endocardial vegetations.

Results

Daptomycin alone was not bactericidal and high-level daptomycin resistance evolved at 96 h in both organisms. Combinations of daptomycin + ceftriaxone and daptomycin + ceftaroline demonstrated enhanced killing activity compared with each antibiotic alone and prevented emergence of daptomycin resistance at 96 h. Use of gentamicin as an adjunctive agent neither improved the efficacy of daptomycin nor prevented the development of daptomycin resistance.

Conclusions

Addition of ceftriaxone or ceftaroline to daptomycin improves the bactericidal activity against S. mitis group strains and prevents daptomycin resistance emergence. Further investigation with combinations of daptomycin and β-lactams in a large number of strains is warranted to fully elucidate the clinical implications of such combinations for treatment of S. mitis group IE.

Vancomycin-resistant enterococcus infection in the hematopoietic stem cell transplant recipient: an overview of epidemiology, management, and prevention

Vancomycin-resistant enterococcus (VRE) is now one of the leading causes of nosocomial infections in the United States. Hematopoietic stem cell transplantation (HSCT) recipients are at increased risk of VRE colonization and infection. VRE has emerged as a major cause of bacteremia in this population, raising important clinical questions regarding the role and impact of VRE colonization and infection in HSCT outcomes as well as the optimal means of prevention and treatment. We review here the published literature and scientific advances addressing these thorny issues and provide a rational framework for their approach.

Model systems for the study of Enterococcal colonization and infection

Enterococcus faecalis and Enterococcus faecium are common inhabitants of the human gastrointestinal tract, as well as frequent opportunistic pathogens. Enterococci cause a range of infections including, most frequently, infections of the urinary tract, catheterized urinary tract, bloodstream, wounds and surgical sites, and heart valves in endocarditis. Enterococcal infections are often biofilm-associated, polymicrobial in nature, and resistant to antibiotics of last resort. Understanding Enterococcal mechanisms of colonization and pathogenesis are important for identifying new ways to manage and intervene with these infections. We review vertebrate and invertebrate model systems applied to study the most common E. faecalis and E. faecium infections, with emphasis on recent findings examining Enterococcal-host interactions using these models. We discuss strengths and shortcomings of each model, propose future animal models not yet applied to study mono- and polymicrobial infections involving E. faecalis and E. faecium, and comment on the significance of anti-virulence strategies derived from a fundamental understanding of host-pathogen interactions in model systems.

Vancomycin-Resistant Enterococci: Therapeutic Challenges in the 21st Century

Vancomycin-resistant enterococci are serious health threats due in part to their ability to persist in rugged environments and their propensity to acquire antibiotic resistance determinants. Enterococci have now established a home in our hospitals and possess mechanisms to defeat most currently available antimicrobials. This article reviews the history of the struggle with this pathogen, what is known about the traits associated with its rise in the modern medical environment, and the current understanding of therapeutic approaches in severe infections caused by these microorganisms. As the 21st century progresses, vancomycin-resistant enterococci continue to pose a daunting clinical challenge.

Bacteriophage Lysin CF-301, a Potent Antistaphylococcal Biofilm Agent

Biofilms pose a unique therapeutic challenge because of the antibiotic tolerance of constituent bacteria. Treatments for biofilm-based infections represent a major unmet medical need, requiring novel agents to eradicate mature biofilms. Our objective was to evaluate bacteriophage lysin CF-301 as a new agent to target Staphylococcus aureus biofilms. We used minimum biofilm-eradicating concentration (MBEC) assays on 95 S. aureus strains to obtain a 90% MBEC (MBEC₉₀) value of ≤0.25 μg/ml for CF-301. Mature biofilms of coagulase-negative staphylococci, Streptococcus pyogenes, and Streptococcus agalactiae were also sensitive to disruption, with MBEC₉₀ values ranging from 0.25 to 8 μg/ml. The potency of CF-301 was demonstrated against S. aureus biofilms formed on polystyrene, glass, surgical mesh, and catheters. In catheters, CF-301 removed all biofilm within 1 h and killed all released bacteria by 6 h. Mixed-species biofilms, formed by S. aureus and Staphylococcus epidermidis on several surfaces, were removed by CF-301, as were S. aureus biofilms either enriched for small-colony variants (SCVs) or grown in human synovial fluid. The antibacterial activity of CF-301 was further demonstrated against S. aureus persister cells in exponential-phase and stationary-phase populations. Finally, the antibiofilm activity of CF-301 was greatly improved in combinations with the cell wall hydrolase lysostaphin when tested against a range of S. aureus strains. In all, the data show that CF-301 is highly effective at disrupting biofilms and killing biofilm bacteria, and, as such, it may be an efficient new agent for treating staphylococcal infections with a biofilm component.

Role of Combination Antimicrobial Therapy for Vancomycin-Resistant Enterococcus faecium Infections: Review of the Current Evidence

Enterococcus species are the second most common cause of nosocomial infections in the United States and are particularly concerning in critically ill patients with preexisting comorbid conditions. Rising resistance to antimicrobials that were historically used as front-line agents for treatment of enterococcal infections, such as ampicillin, vancomycin, and aminoglycosides, further complicates the treatment of these infections. Of particular concern are Enterococcus faecium strains that are associated with the highest rate of vancomycin resistance. The introduction of antimicrobial agents with specific activity against vancomycin-resistant Enterococcus (VRE) faecium including daptomycin, linezolid, quinupristin-dalfopristin, and tigecycline did not completely resolve this clinical dilemma. In this review, the mechanisms of action and resistance to currently available anti-VRE antimicrobial agents including newer agents such as oritavancin and dalbavancin will be presented. In addition, novel combination therapies including β-lactams and fosfomycin, and the promising results from in vitro, animal studies, and clinical experience in the treatment of VRE faecium will be discussed.

Emergence of linezolid resistance in hepatobiliary infections caused by Enterococcus faecium

Enterococcal infections are common in liver transplantation and hepatopancreaticobiliary (HPB) surgery. Linezolid is frequently used to treat not only vancomycin-resistant Enterococcus (VRE), but also vancomycin-sensitive Enterococcus (VSE) infections, and resistance can develop. This study evaluated all the Liver Unit patients who developed infections with linezolid-resistant Enterococcus (LRE) in order to elicit the association with prior linezolid usage, to explore possible risk factors for infection, and to better understand the epidemiology of these isolates in this patient group. Between 2010 and 2015, infections with LRE developed in 10 patients (8 following liver transplantation and 2 following HPB surgery) after 22-108 days of treatment. Selected pulsed-field gel electrophoresis demonstrated that 2 out of 10 patients were cocolonized with different strains and indicated that cross-transmission may have occurred. In conclusion, in this group of patients with complex hepatobiliary infections, the optimal antibiotic strategies for the treatment of Enterococcus faecium infections are not clearly defined, and there is a significant risk of emergence of resistance to linezolid in E. faecium after exposure to this agent in patients, especially in the presence of a deep source of infection on a background of hepatic artery insufficiency. Caution is needed when using prolonged courses of linezolid in this setting, and further studies are necessary to determine the optimum treatment.

Galleria mellonella infection models for the study of bacterial diseases and for antimicrobial drug testing

Galleria mellonella (greater wax moth or honeycomb moth) has been introduced as an alternative model to study microbial infections. G. mellonella larvae can be easily and inexpensively obtained in large numbers and are simple to use as they don't require special lab equipment. There are no ethical constraints and their short life cycle makes them ideal for large-scale studies. Although insects lack an adaptive immune response, their innate immune response shows remarkable similarities with the immune response in vertebrates. This review gives a current update of what is known about the immune system of G. mellonella and provides an extensive overview of how G. mellonella is used to study the virulence of Gram-positive and Gram-negative bacteria. In addition, the use of G. mellonella to evaluate the efficacy of antimicrobial agents and experimental phage therapy are also discussed. The review concludes with a critical assessment of the current limitatons of G. mellonella infection models.