In-vitro activity of the combination of ampicillin and arbekacin against high-level gentamicin-resistant enterococci

Management of multidrug-resistant enterococcal infections

Enterococci are organisms with a remarkable ability to adapt to the environment and acquire antibiotic resistance determinants. The evolution of antimicrobial resistance in these organisms poses enormous challenges for clinicians when faced with patients affected with severe infections. The increased prevalence and dissemination of multidrug-resistant Enterococcus faecium worldwide has resulted in a major decrease in therapeutic options because the majority of E. faecium isolates are now resistant to ampicillin and vancomycin, and exhibit high-level resistance to aminoglycosides, which are three of the traditionally most useful anti-enterococcal antibiotics. Newer antibiotics such as linezolid, daptomycin and tigecycline have good in vitro activity against enterococcal isolates, although their clinical use may be limited in certain clinical scenarios as a result of reduced rates of success, possible underdosing for enterococci and low serum levels, respectively, and also by the emergence of resistance. The experimental agent oritavancin may offer some hope for the treatment of vancomycin-resistant enterococci but clinical data are still lacking. Thus, optimal therapies for the treatment of multidrug-resistant enterococcal infections continue to be based on empirical observations and extrapolations from in vitro and animal data. Clinical studies evaluating new strategies, including combination therapies, to treat severe vancomycin-resistant E. faecium infections are urgently needed.

Genotypic diversity and epidemiology of high-level gentamicin resistant Enterococcus in a Chinese hospital

OBJECTIVE

To investigate the antibiotics resistance of Enterococcus, the aminoglycoside-modifying enzymes (AME) and homology of high-level gentamicin resistant (HLGR) Enterococcus in clinical specimens for the implementation of effective infection control measures.

METHODS

The resistance of 13 antimicrobial agents was determined by Kirby-Bauer (K-B) or agar dilution method. And the HLGR and high-level streptomycin resistant (HLSR) isolates were screened by agar screen. Production of beta-lactamases was tested by the nitrocefin disc method. The aminoglycoside-modifying enzyme genes were detected by polymerase chain reaction (PCR). Pulsed-field gel electrophoresis (PFGE) was used to analyze the homology of HLGR isolates from in-patients.

RESULTS

No isolates resistant to linezolid, vancomycin and teicoplanin were found. Ampicillin-resistant isolates did not produce beta-lactamases and 68 HLGR isolates were screened at the rate of 64.2%. The positive rate of aac(6')-Ie-aph(2'')-Ia was 86.8% and 3 isolates had the new AME gene designated aph(2'')-Ie mostly similar to aph(2'')-Id. Among 51 HLGR isolates from in-patients, PFGE grouped 17 Enterococcus faecalis (E. faecalis) isolates into 4 clusters (A-D), and 33 Enterococcus faecium (E. faecium) isolates into 8 clusters (A-H), of which the A cluster is the main.

CONCLUSIONS

HLGR has become the important antibiotic resistance pathogen causing nocosomial infection. And the aac(6')-Ie-aph(2'')-Ia gene was the main aminoglycoside-modifying enzyme gene leading to HLGR.

Synergistic Effect of [10]-Gingerol and Aminoglycosides against Vancomycin-Resistant Enterococci (VRE)

An extract from ginger (root of Zingiber officinale) reduced the minimum inhibitory concentrations (MICs) of aminoglycosides in vancomycin-resistant enterococci (VRE). The effective compound was isolated and identified as [10]-gingerol. In the presence of [10]-gingerol at 1/10 concentration of its own MIC, the MIC of arbekacin was lowered by 1/32 to 1/16. [10]-Gingerol also reduced the MICs of other aminoglycosides, and of bacitracin and polymixin B, but not of other antimicrobial agents tested. Because [10]-gingerol reduced the MICs of several aminoglycosides both in strains possessing or lacking aminoglycoside-modification enzymes, it seems that the effect of [10]-gingerol is not related to these enzymes, which mainly confer bacterial resistance against aminoglycosides. It seemed that a detergent-like effect of [10]-gingerol potentiated the antimicrobial activity of the aminoglycosides. In fact, some detergents such as sodium dodecyl sulfate (SDS) and Triton X-100 reduced the MICs of aminoglycosides, bacitracin and polymixin B in VRE. Since the intrinsic resistance to aminoglycosides in enterococci is due to low level of entry of the drugs into the cells, increase in the membrane permeability caused by [10]-gingerol will enhance the influx of aminoglycosides into enterococcal cells.

Biological counterstrike: antibiotic resistance mechanisms of Gram-positive cocci

The development of antibiotic resistance by bacteria is an evolutionary inevitability, a convincing demonstration of their ability to adapt to adverse environmental conditions. Since the emergence of penicillinase-producing Staphylococcus aureus in the 1940s, staphylococci, enterococci and streptococci have proved themselves adept at developing or acquiring mechanisms that confer resistance to all clinically available antibacterial classes. The increasing problems of methicillin-resistant S. aureus and coagulase-negative staphylococci (MRSA and MRCoNS), glycopeptide-resistant enterococci and penicillin-resistant pneumococci in the 1980s, and recognition of glycopeptide-intermediate S. aureus in the 1990s and, most recently, of fully vancomycin-resistant isolates of S. aureus have emphasised our need for new anti-Gram-positive agents. Antibiotic resistance is one of the major public health concerns for the beginning of the 21st century. The pharmaceutical industry has responded with the development of oxazolidinones, lipopeptides, injectable streptogramins, ketolides, glycylcyclines, second-generation glycopeptides and novel fluoroquinolones. However, clinical use of these novel agents will cause new selective pressures and will continue to drive the development of resistance. This review describes the various antibiotic resistance mechanisms identified in isolates of staphylococci, enterococci and streptococci, including mechanisms of resistance to recently introduced anti-Gram-positive agents.

Molecular characterization of gentamicin-resistant Enterococci in the United States: evidence of spread from animals to humans through food

We evaluated the molecular mechanism for resistance of 360 enterococci for which the gentamicin MICs were >/=128 micro g/ml. The aac(6')-Ie-aph(2")-Ia, aph(2")-Ic, and aph(2")-Id genes were identified by PCR in isolates from animals, food, and humans. The aph(2")-Ib gene was not identified in any of the isolates. Two Enterococcus faecalis isolates (MICs > 1,024 micro g/ml) from animals failed to generate a PCR product for any of the genes tested and likely contain a new unidentified aminoglycoside resistance gene. Pulsed-field gel electrophoresis (PFGE) analysis showed a diversity of strains. However, 1 human and 18 pork E. faecalis isolates from Michigan with the aac(6')-Ie-aph(2")-Ia gene had related PFGE patterns and 2 E. faecalis isolates from Oregon (1 human and 1 grocery store chicken isolate) had indistinguishable PFGE patterns. We found that when a gentamicin-resistant gene was present in resistant enterococci from animals, that gene was also present in enterococci isolated from food products of the same animal species. Although these data indicate much diversity among gentamicin-resistant enterococci, the data also suggest similarities in gentamicin resistance among enterococci isolated from humans, retail food, and farm animals from geographically diverse areas and provide evidence of the spread of gentamicin-resistant enterococci from animals to humans through the food supply.

Efficacy of ampicillin plus arbekacin in experimental rabbit endocarditis caused by an Enterococcus faecalis strain with high-level gentamicin resistance

Enterococcus faecalis LC40 is an ampicillin-susceptible clinical isolate with high-level gentamicin resistance due to the aac(6')-Ie-aph(2")-Ia aminoglycoside resistance gene. The combination of ampicillin plus arbekacin reduced mean bacterial vegetation counts significantly more than ampicillin alone or ampicillin plus gentamicin in a rabbit model of aortic-valve endocarditis caused by E. faecalis LC40.

In-vitro synergistic activity of the combination of ampicillin and arbekacin against vancomycin-and high-level gentamicin-resistant Enterococcus faecium with the aph(2")-Id gene

The combination of ampicillin plus arbekacin produced synergistic killing against 8 of 13 vancomycin-, ampicillin-, gentamicin-, and streptomycin-resistant Enterococcus faecium isolates that possess the high-level gentamicin resistance gene, aph(2")-Id. This combination may prove useful in treating infections caused by multiresistant enterococci.

Aminoglycoside resistance in enterococci

High-level aminoglycoside resistance in enterococci is mediated generally by aminoglycoside-modifying enzymes, which eliminate the synergistic bactericidal effect usually seen when a cell wall-active agent is combined with an aminoglycoside. Clinical microbiology laboratories currently screen for aminoglycoside resistance in enterococci by testing gentamicin and streptomycin susceptibility. If the recently detected aminoglycoside resistance genes, aph(2")-Ib, aph(2")-Ic, and aph(2")-Id, become more prevalent among clinical isolates, the approach for detecting susceptibility to aminoglycoside synergism in enterococci will require modification. More potent aminoglycosides need to be developed that will be resistant to modification by a broad spectrum of aminoglycoside-modifying enzymes present in enterococci.

In-vitro activity of arbekacin alone and in combination with vancomycin against gentamicin- and methicillin-resistant Staphylococcus aureus

In-vitro susceptibility studies were performed on 99 clinical Staphylococcus aureus isolates. A total of 68 of 73 methicillin-resistant S. aureus and 2 of 26 methicillin-susceptible S. aureus were gentamicin-resistant (gentamicin MIC range 16 to 1,024 microg/mL). All 70 gentamicin-resistant isolates contained the aac(6')-Ie-aph(2'')-Ia aminoglycoside resistance gene, and none possessed the aph(2'')-Ic or aph(2'')-Id aminoglycoside resistance genes. The arbekacin MIC for the 70 gentamicin-resistant isolates ranged from 0.25 to 4 microg/mL. The combination of arbekacin plus vancomycin produced synergistic killing against 12 of 13 gentamicin-resistant MRSA isolates. The combination of gentamicin plus vancomycin produced synergistic killing against 7 of the same 13 isolates. Arbekacin may prove useful when used in combination with vancomycin in treating infections caused by gentamicin-resistant MRSA.