Bactericidal activity of gentamicin against Enterococcus faecalis in vitro and in vivo

Phytosynthesis of Silver Nanoparticles Using Leonurus cardiaca L. Extracts

The present work describes, for the first time in the literature, the phytosynthesis of silver nanoparticles using Leonurus cardiaca L. extracts. The influence of the extraction method (classical temperature extraction and microwave extraction), as well as of the extract concentration on the characteristics of the nanoparticles, was studied using analytical methods, such as UV-Vis spectrometry, X-ray diffraction, dynamic light scattering, and transmission electron microscopy. Experimental data suggest that use of lower extract concentration leads to smaller dimensions nanoparticles, the same effect using the extract obtained by microwave-assisted extraction. The smallest recorded crystallite sizes (by X-ray diffraction) were under 3 nm. The antioxidant properties (determined by the DPPH assay) and the antimicrobial potential (determined against Gram-negative and Gram-positive strains) are enhanced by the phytosynthesis process (as demonstrated by the comparison of the nanoparticles' properties with the parent extracts). The present work could also represent an important step in obtaining nanoparticles with enhanced properties and controlled morphologies, but also offers information on the phytosynthesis of metallic nanoparticles using low extract concentrations.

Prevalence of Class 1 Integron and In Vitro Effect of Antibiotic Combinations of Multidrug-Resistant Enterococcus Species Recovered from the Aquatic Environment in the Eastern Cape Province, South Africa

Enterococci are regarded as a better indication of faecal pollution in freshwater and marine waters. Their levels in seawater are positively connected with swimming-related gastrointestinal disorders. This study used an Enterococcus-specific polymerase chain reaction (PCR) to characterize the isolates. Classes 1 and 2 integrons were examined for environmental Enterococcus isolates using a standard biological procedure. All strains were assessed against a panel of 12 antibiotics from various classes using disc diffusion methods. The microdilution method was used to work out the minimum inhibitory concentration (MIC) according to the CLSI guiding principles. The combination therapy of the resistant drugs was evaluated using a checkerboard assay and a time-dependent test for assessing their bactericidal or bacteriostatic activity. The gene diversity of the tested organisms was analyzed with the aid of Enterobacterial Repetitive Intergenic Consensus (ERIC) PCR. In total, 57 Enterococcus spp. environmental samples were recovered, in which Enterococcus faecalis (33.33%) and Enterococcus faecium (59.65%) were the dominant species. Resistance to linezolid, ciprofloxacin, erythromycin, gentamicin, vancomycin, rifampicin, and tetracycline was prevalent. Fifty (50) strains tested positive for class 1 integron, more frequent in Enterococcus faecium and Enterococcus faecalis isolates, with no gene cassette array discovered. A combination of gentamicin (MIC 4 µg/mL) with vancomycin (MIC 256 µg/mL) antibiotics against Enterococcus faecalis showed antibacterial activity. In contrast, the combination of ciprofloxacin (1 µg/mL) with Ampicillin (16 µg/mL) antibiotics against Enterococcus faecalis showed a bacteriostatic effect. The ERIC-PCR analysis pointed out that most of the assessed isolates have close genetic similarities.

Molecular detection and antibiotyping of multi-drug resistant Enterococcus faecium from healthy broiler chickens in Bangladesh

BACKGROUND

Enterococcus faecium is a ubiquitously distributed member of the intestinal microbiota of both humans and animals. Antibiotic resistant E. faecium are a major public health concern.

OBJECTIVES

This study aimed to detect multi-drug resistant (MDR) E. faecium and their antibiotic resistance genes from broiler chickens in Bangladesh.

METHODS

A total of 100 faecal samples of healthy broilers were screened by conventional methods and polymerase chain reaction (PCR) to detect E. faecium and their resistance genes. Disk diffusion test was employed to determine antibiotic profiles.

RESULTS

By PCR, among 100 samples, 45% [95% confidence interval (CI): 35.62%-54.76%] were positive for E. faecium. Based on antibiogram, all the E. faecium isolates were found resistant to ampicillin, and frequently (93.33%-55.56%) resistant to ceftriaxone, cefotaxime, streptomycin, erythromycin, and imipenem; moderate to lower (26.67%-4.44%) resistance to tetracycline, ciprofloxacin, norfloxacin, chloramphenicol, gentamicin, and vancomycin. Interestingly, 80% (95% CI: 66.18%-89.10%) E. faecium isolates were MDR in nature. In addition, the indices of multiple antibiotic resistance (MAR) ranged from 0.08 to 0.83. By bivariate analysis, high positive significant correlations were observed between resistance profiles of erythromycin and imipenem, ciprofloxacin and norfloxacin, erythromycin and streptomycin, ceftriaxone and cefotaxime, tetracycline and chloramphenicol, and streptomycin and imipenem. Furthermore, the prevalence of resistance genes of E. faecium was 58.33% (tetA), 33.33% (tetB), 35.56% (blaTEM ), 60% (CITM), 13.33% (aadA1), and 12% (SHV).

CONCLUSIONS

To the best of our knowledge, this is the first study in Bangladesh to detect MDR and MAR E. faecium and their associated resistance genes. The detection of MDR and MAR E. faecium and their corresponding resistance genes from healthy broilers is of public health concern because of their potential to enter into the food chain.

Seventy-Five Years of Research on Protein Binding

This review summarizes evidence that the impact of protein binding of the activity of antibiotics is multifaceted and more complex than indicated by the numerical value of protein binding alone. A plethora of studies has proven that protein binding of antibiotics matters, as the free fraction only is antibacterially active and governs pharmacokinetics. Several studies have indicated that independent from protein binding of immunoglobulin G, albumin, α1-acid-glycoprotein, and pulmonary surfactant acted synergistically with antibacterial agents, thus suggesting that some intrinsic properties of serum proteins may have mediated serum-antibiotic synergisms. It has been demonstrated that IgG and albumin permeabilized Gram-negative and Gram-positive bacteria and facilitated the uptake of poorly penetrating antibiotics. Alpha-1-acid-glycoprotein and pulmonary surfactant also exerted a permeabilizing activity, but proof that this property results in a sensitizing effect is missing. The permeabilizing effect of serum proteins may explain why serum-antibiotic synergisms do not represent a general phenomenon but are limited to specific drug-bug associations only. Although evidence has been generated to support the hypothesis that native serum proteins interact synergistically with antibiotics, systematic and well-controlled studies have to be performed to substantiate this phenomenon. The interactions between serum proteins and bacterial surfaces are driven by physicochemical forces. However, preparative techniques, storage conditions, and incubation methods have a significant impact on the intrinsic activities of these serum proteins affecting serum-antibiotic synergisms, so these techniques have to be standardized; otherwise, contradictory data or even artifacts will be generated.

Therapeutic drug monitoring of anti-tumour necrosis factor-α agents in inflammatory bowel disease

INTRODUCTION

Anti-TNFα therapy has revolutionised treatment of inflammatory bowel disease, however primary non-response and secondary loss of response are a significant problem. Therapeutic drug monitoring (TDM) has recently emerged as a means of optimising use of anti-TNFα agents. Areas covered: TDM of anti-TNFα agents can guide clinical decisions during treatment failure events, prevent treatment failure events, and potentially result in significant healthcare cost saving. TDM for anti-TNFα agent involves measurement of drug levels and anti-drug antibodies, and can be performed reactively or proactively. Reactive TDM reserves testing for treatment failure events, while proactive TDM also consists of periodic TDM for patients responding to anti-TNFα therapy to allow treatment optimisation. Generation of anti-drug antibodies is recognised as one important mechanism of treatment failure and adverse events. Expert opinion: Evidence strongly supports TDM at time of treatment failure, while studies employing proactive TDM have demonstrated conflicting results. TDM can also help better select patients likely to remain in clinical remission on anti-TNFα treatment interruption. Currently TDM is used to optimise anti-TNFα treatment, but it is not used by most clinicians to prevent adverse reactions to anti-TNFα agents.

Staphylococcus aureus inactivates daptomycin by releasing membrane phospholipids

Daptomycin is a bactericidal antibiotic of last resort for serious infections caused by methicillin-resistant Staphylococcus aureus (MRSA)^1,2. Although resistance is rare, treatment failure can occur in more than 20% of cases^3,4 and so there is a pressing need to identify and mitigate factors that contribute to poor therapeutic outcomes. Here, we show that loss of the Agr quorum-sensing system, which frequently occurs in clinical isolates, enhances S. aureus survival during daptomycin treatment. Wild-type S. aureus was killed rapidly by daptomycin, but Agr-defective mutants survived antibiotic exposure by releasing membrane phospholipids, which bound and inactivated the antibiotic. Although wild-type bacteria also released phospholipid in response to daptomycin, Agr-triggered secretion of small cytolytic toxins, known as phenol soluble modulins, prevented antibiotic inactivation. Phospholipid shedding by S. aureus occurred via an active process and was inhibited by the β-lactam antibiotic oxacillin, which slowed inactivation of daptomycin and enhanced bacterial killing. In conclusion, S. aureus possesses a transient defence mechanism that protects against daptomycin, which can be compromised by Agr-triggered toxin production or an existing therapeutic antibiotic.

Gentamicin improves the activities of daptomycin and vancomycin against Enterococcus faecalis in vitro and in an experimental foreign-body infection model

For enterococcal implant-associated infections, the optimal treatment regimen has not been defined. We investigated the activity of daptomycin, vancomycin, and gentamicin (and their combinations) against Enterococcus faecalis in vitro and in a foreign-body infection model. Antimicrobial activity was investigated by time-kill and growth-related heat production studies (microcalorimetry) as well as with a guinea pig model using subcutaneously implanted cages. Infection was established by percutaneous injection of E. faecalis in the cage. Antibiotic treatment for 4 days was started 3 h after infection. Cages were removed 5 days after end of treatment to determine the cure rate. The MIC, the minimal bactericidal concentration (MBC) in the logarithmic phase, and the MBC in the stationary phase were 1.25, 5, and >20 μg/ml for daptomycin, 1, >64, and >64 μg/ml for vancomycin, and 16, 32, and 4 μg/ml for gentamicin, respectively. In vitro, gentamicin at subinhibitory concentrations improved the activity against E. faecalis when combined with daptomycin or vancomycin in the logarithmic and stationary phases. In the animal model, daptomycin cured 25%, vancomycin 17%, and gentamicin 50% of infected cages. In combination with gentamicin, the cure rate for daptomycin increased to 55% and that of vancomycin increased to 33%. In conclusion, daptomycin was more active than vancomycin against adherent E. faecalis, and its activity was further improved by the addition of gentamicin. Despite a short duration of infection (3 h), the cure rates did not exceed 55%, highlighting the difficulty of eradicating E. faecalis from implants already in the early stage of implant-associated infection.

The Enterococcus faecalis superoxide dismutase is essential for its tolerance to vancomycin and penicillin

OBJECTIVES

Enterococcus faecalis is a human commensal that has the ability to become a pathogen. Because of its ruggedness, it can persist in the hospital setting and cause serious nosocomial infections. E. faecalis can acquire multiple drug resistance determinants but is also intrinsically tolerant to a number of antibiotics, such as penicillin or vancomycin, meaning that these usually bactericidal drugs only exhibit a bacteriostatic effect. Recently, evidence has been presented that exposure to bactericidal antibiotics induced the production of reactive oxygen species in bacteria. Here, we studied the role of enzymes involved in the oxidative stress response in the survival of E. faecalis after antibiotic treatment.

METHODS

Mutants defective in genes encoding oxidative stress defence activities were tested by time-kill curves for their contribution to antibiotic tolerance in comparison with the E. faecalis JH2-2 wild-type (WT).

RESULTS

In killing assays, WT cultures lost 0.2 +/- 0.1 and 1.3 +/- 0.2 log(10) cfu/mL after 24 h of vancomycin or penicillin exposure, respectively. A deletion mutant of the superoxide dismutase gene (DeltasodA) exhibited a lack of tolerance as cultures lost 4.1 +/- 0.5 and 4.8 +/- 0.7 log(10) cfu/mL after 24 h of exposure to the same drugs. Complementation of DeltasodA re-established the tolerant phenotype. Bacterial killing was an oxygen-dependent process and a model is presented implicating the superoxide anion as the mediator of this killing. As predicted from the model, a mutant defective in peroxidase activities excreted hydrogen peroxide at an elevated rate.

CONCLUSIONS

SodA is central to the intrinsic ability of E. faecalis to withstand drug-induced killing, and the superoxide anion seems to be the key effector of bacterial death.

Acquired gentamicin resistance by permeability impairment in Enterococcus faecalis

Enterococci are intrinsically resistant to low levels of aminoglycosides. We previously selected in vitro and in vivo Enterococcus faecalis with intermediate-level resistance to gentamicin that did not abolish synergism with a cell-wall-active agent (E. Aslangul et al., Antimicrob. Agents Chemother. 49:4144-4148, 2005). The aim of this study was to investigate the mechanism of resistance to gentamicin in the 1688-G3 third-step mutant (MIC, 512 microg/ml) of E. faecalis JH2-2. No mutations were found in the genes for L6 ribosomal protein and the four copies of 16S rRNA. Production of a known aminoglycoside-modifying enzyme was unlikely due to the distinct resistance phenotype and absence of the corresponding genes. Efflux was also unlikely since ethidium bromide MICs were similar for JH2-2 and 1688-G3 and since the pump inhibitors reserpine and verapamil had no effect on gentamicin resistance in both strains. To study gentamicin accumulation, we developed a nonisotopic method based on a fluorescent polarization immunoassay. Impaired gentamicin accumulation was observed in 1688-G3 compared to JH2-2 and was only partially reversible by the N,N'-dicyclohexylcarbodiimide (DCCD) uncoupler agent. The lower sensitivity of 1688-G3 to DCCD suggested alteration of the FoF1-ATPase. However, no mutations were detected in the structural genes (atp) for the Fo channel and no difference in transcript levels of atpB and atpE was found between 1688-G3 and JH2-2. Our data are compatible with acquisition of intermediate-level gentamicin resistance by uptake impairment in E. faecalis.

Relationship between the level of acquired resistance to gentamicin and synergism with amoxicillin in Enterococcus faecalis

In enterococci, intrinsic low-level resistance to gentamicin does not abolish synergism with a cell wall-active antibiotic while high-level resistance due to acquired aminoglycoside-modifying enzymes does. To study the impact of intermediate levels of resistance to gentamicin (64 < MIC < 500 microg/ml), we selected in vitro three consecutive generations of mutants of Enterococcus faecalis JH2-2 with MICs of gentamicin at 128 microg/ml for G1-1477, 256 microg/ml for G2-1573, and 512 microg/ml for G3-1688. E. faecalis 102, which is highly resistant to gentamicin by enzymatic inactivation was used as control. In in vitro killing curves experiments, gentamicin concentrations allowing bactericidal activity and synergism in combination with amoxicillin increased from 4 microg/ml (1/16th the MIC), 16 microg/ml (one-eighth the MIC), 64 microg/ml (one-quarter the MIC), and 256 microg/ml (one-half the MIC) for strains JH2-2, G1-1477, G2-1573 and G3-1688, respectively. As expected, no bactericidal effect of the combination or synergism could be obtained with strain 102. In rabbits with aortic endocarditis caused by strain G1-1477 or G2-1573, combination therapy with amoxicillin and gentamicin was significantly more active than amoxicillin alone (P < 0.05) but not in those infected with the strains G3-1688 and 102. Thus, intermediate levels of resistance to gentamicin was not associated with a loss of a beneficial effect of the gentamicin-amoxicillin combination in vivo even though higher concentrations of gentamicin were necessary to achieve in vitro synergism. Therefore, the use of an MIC of 500 microg/ml as a clinical cutoff limit to predict in vivo benefit of the combination remains a simple and effective tool.

Simulation of human gentamicin pharmacokinetics in an experimental Enterococcus faecalis endocarditis model

Significant differences between animal and human pharmacokinetics may be responsible for the conflicting results of experimental studies. This study determined the impact of human pharmacokinetic simulation (HPS) on gentamicin activity in an Enterococcus faecalis endocarditis model. The decrease in bacterial counts was greater with HPS than with a dose-equivalent regimen without HPS.