Treatment Of Serious Enterococcal Infections Research Articles

A Clinician’s Guide to the Treatment of Vancomycin Resistant Enterococci Bacteremia and Endocarditis

Treatment of serious enterococcal infections is often difficult due to the inherent high level of antimicrobial resistance seen among some species including a rising level of high level aminoglycoside resistance (HLAR), vancomycin resistance (VRE), and multidrug resistance. Prior to the turn of this century, therapeutic options were limited and relied primarily on the use of ampicillin, aminoglycosides, and vancomycin. Over the past two decades, treatment has changed with the introduction of new antimicrobials with activity against VRE and an increasing use of aminoglycoside sparing regimens including the use of ampicillin and ceftriaxone. For the treatment of ampicillin susceptible infections with or without the presence of HLAR, the combination of ampicillin and ceftriaxone is now the preferred regimen due to the lower risk of adverse reactions including nephrotoxicity. Among the two antibiotics recommended for VRE severe infections in the 2015 AHA/IDSA infective endocarditis guidelines, daptomycin and linezolid, there is ongoing debate about the merits of both due to concerns about perceived mortality differences, adverse drug reactions, and proper dosing. Given the bactericidal activity of daptomycin, we currently prefer this agent for the initial treatment of serious VRE infections with proper dosing of at least 8 mg/kg and up to 10–12 mg/kg.

Characterization and antimicrobial susceptibility pattern of clinical isolates of Enterococci at a tertiary care hospital in Mumbai, India

Context: Enterococcus is considered an important nosocomial pathogen because of its intrinsic as well as acquired antibiotic resistance. It also has the potential of transferring vancomycin resistance to other organisms such as Listeria monocytogens and Staphylococcus aureus. Aims: The objective of the present study was to determine antibiotic-resistance pattern of Enterococcus with special reference to vancomycin. Settings and Design: A total of 54 clinical isolates of enterococci were collected during the study period of 1 year at a tertiary care center in Mumbai. Material and Methods: Speciation and antibiotic sensitivity testing were done by standard procedures. Minimum inhibitory concentration (MIC) to vancomycin was carried out by agar dilution method. Results: Speciation and antibiotic sensitivity testing were done by standard procedures. The MIC to vancomycin was done by agar dilution method. Conclusions: Vancomycin, Linezolid, and Teicoplanin can be safely used for the treatment of serious enterococcal infections.

Intrinsic resistance to aminoglycosides in Enterococcus faecium is conferred by the 16S rRNA m5C1404-specific methyltransferase EfmM

Aminoglycosides are ribosome-targeting antibiotics and a major drug group of choice in the treatment of serious enterococcal infections. Here we show that aminoglycoside resistance in Enterococcus faecium strain CIP 54-32 is conferred by the chromosomal gene efmM, encoding the E. faecium methyltransferase, as well as by the previously characterized aac(6')-Ii that encodes a 6'-N-aminoglycoside acetyltransferase. Inactivation of efmM in E. faecium increases susceptibility to the aminoglycosides kanamycin and tobramycin, and, conversely, expression of a recombinant version of efmM in Escherichia coli confers resistance to these drugs. The EfmM protein shows significant sequence similarity to E. coli RsmF (previously called YebU), which is a 5-methylcytidine (m⁵C) methyltransferase modifying 16S rRNA nucleotide C1407. The target for EfmM is shown by mass spectrometry to be a neighboring 16S rRNA nucleotide at C1404. EfmM uses the methyl group donor S-adenosyl-L-methionine to catalyze formation of m⁵C1404 on the 30S ribosomal subunit, whereas naked 16S rRNA and the 70S ribosome are not substrates. Addition of the 5-methyl to C1404 sterically hinders aminoglycoside binding. Crystallographic structure determination of EfmM at 2.28 Å resolution reveals an N-terminal domain connected to a central methyltransferase domain that is linked by a flexible lysine-rich region to two C-terminal subdomains. Mutagenesis of the methyltransferase domain established that two cysteines at specific tertiary locations are required for catalysis. The tertiary structure of EfmM is highly similar to that of RsmF, consistent with m⁵C formation at adjacent sites on the 30S subunit, while distinctive structural features account for the enzymes' respective specificities for nucleotides C1404 and C1407.

Aminoglycoside 2″-Phosphotransferase Type IIIa from Enterococcus

Aminoglycoside 2''-phosphotransferases mediate high level resistance to aminoglycoside antibiotics in Gram-positive microorganisms, thus posing a serious threat to the treatment of serious enterococcal infections. This work reports on cloning, purification, and detailed mechanistic characterization of aminoglycoside 2''-phosphotransferase, known as type Ic enzyme. In an unexpected finding, the enzyme exhibits strong preference for guanosine triphosphate over adenosine triphosphate as the phosphate donor, a unique observation among all characterized aminoglycoside phosphotransferases. The enzyme phosphorylates only certain 4,6-disubstituted aminoglycosides exclusively at the 2''-hydroxyl with k(cat) values of 0.5-1.0 s(-1) and K(m) values in the nanomolar range for all substrates but kanamycin A. Based on this unique substrate profile, the enzyme is renamed aminoglycoside 2''-phosphotransferase type IIIa. Product and dead-end inhibition patterns indicated a random sequential Bi Bi mechanism. Both the solvent viscosity effect and determination of the rate constant for dissociation of guanosine triphosphate indicated that at pH 7.5 the release of guanosine triphosphate is rate-limiting. A computational model for the enzyme is presented that sheds light on the structural aspects of interest in this family of enzymes.

Influence of adjunctive interferon-γ on treatment of gentamicin- and vancomycin-resistant Enterococcus faecalis infection in mice

Increasing antibiotic resistance and the development of multidrug-resistance in the enterococci has complicated the treatment of serious enterococcal infections. It has been demonstrated in vitro that interferon-gamma (IFN-γ) significantly augments the activities of gentamicin and vancomycin against Enterococcus faecalis resistant to these antibiotics. The present study was aimed at determining whether this beneficial effect of IFN-γ on antienterococcal antibiotic activity can be validated in vivo. Following intraperitoneal inoculation in mice with a gentamicin- and vancomycin-resistant E. faecalis clinical isolate, the animals received IFN-γ, antibiotic or a combination of both agents, subcutaneously, at determined dosing regimens. Treatment with IFN-γ alone significantly improved survival of infected animals in a dose-dependent manner. High dose IFN-γ was not beneficial and the level of enterococcal infectious burden influenced the effectiveness of the cytokine. The addition of IFN-γ to therapy with gentamicin or vancomycin, or a combination of both antibiotics was associated with a marked increase in survival of infected non-neutropenic mice compared to treatments with the agents alone. However, the same treatments made in infected neutropenic mice did not show an enhancement effect by IFN-γ after a combination therapy with antibiotics. In a study to examine pharmacokinetic interactions, concurrent administration with IFN-γ significantly modified the disposition of gentamicin but not that of vancomycin. The results of this study suggest that the use of IFN-γ in combination with vancomycin or gentamicin is a new treatment option that might improve the outcome of therapy of multidrug-resistant E. faecalis infections.

Comparison of the in vitro activity of levofloxacin and other antimicrobial agents against vancomycin-susceptible and vancomycin resistant Enterococcus species

The in vitro activities of levofloxacin and other antibiotics against 133 clinical isolates of vancomycin-resistant and vancomycin-susceptible enterococci were evaluated. Only 14 (39%) of the vancomycin-susceptible isolates and 11 (11%) of the vancomycin-resistant isolates were susceptible to levofloxacin. Levofloxacin exhibited a marked inoculum effect for all enterococci tested. These results suggest that levofloxacin may be of limited use in the treatment of serious enterococcal infections.

Comparison of ampicillin-sulbactam with vancomycin for treatment of experimental endocarditis due to a beta-lactamase-producing, highly gentamicin-resistant isolate of Enterococcus faecalis

Increasing antibiotic resistance in the enterococci, including the capacity for beta-lactamase production and the development of high-level aminoglycoside resistance, has complicated the treatment of serious enterococcal infections, which often require synergistic antibiotic combinations for cure. We utilized the rabbit model of aortic valve endocarditis to investigate the effects of various antibiotics, alone and in combination, against a multiply antibiotic-resistant isolate of Enterococcus faecalis. Female New Zealand White rabbits were infected with either a beta-lactamase-producing, gentamicin-resistant isolate of E. faecalis or a non-beta-lactamase-producing, aminoglycoside-susceptible isolate, and the mean log10 CFU per gram of vegetation were determined. The most active agents were low-dose ampicillin-sulbactam (200 mg/kg of body weight per day), high-dose ampicillin-sulbactam (400 mg/kg of body weight per day), and vancomycin (150 mg/kg of body weight per day), which reduced the titers of bacteria by 2.27, 2.76, and 2.85 log10 (CFU/g, respectively, compared with controls. While ampicillin-sulbactam and vancomycin were equally efficacious in reducing titers of bacteria in vegetations, no animals were cured (defined as < 2 log10 CFU/g of vegetation) by either agent, whether treatment was continued for 3 or 7 days. The addition of gentamicin was not associated with increased killing in rabbits infected with the aminoglycoside-resistant isolate. Both high-dose ampicillin-sulbactam and vancomycin regimens demonstrated significant, continued reduction in bacterial titers with the longer periods of treatment (P < or = 0.05); 7-day treatment with high-dose ampicillin-sulbactam produced a greater reduction in bacterial titers in vegetation than 7-day treatment with vancomycin (P < or = 0.05). We conclude that ampicillin-sulbactam and vancomycin are equally effective in the treatment of experimental endocarditis due to beta-lactamase-producing, highly gentamicin-resistant E. faecalis. The optimum therapy for such infections in humans is not known.

Susceptibility of enterococci to trimethoprim and trimethoprim-sulfamethoxazole

The in vitro activities of trimethoprim (TMP), alone and in combination with sulfamethoxazole (SMX), against 131 clinical isolates of enterococci, 126 Streptococcus faecalis isolates, and 5 Streptococcus faecium isolates were determined by a broth microdilution method with Mueller-Hinton broth that was substantially free of inhibitory substances. The geometric mean MIC of TMP for strains of S. faecalis was 0.164 micrograms/ml (range, 0.03 to 8 micrograms/ml), with a geometric mean MBC of 0.298 micrograms/ml (range, 0.063 to 8 micrograms/ml). Although all strains were resistant to the sulfonamide alone, the inhibitory and bactericidal activities of TMP against strains of S. faecalis were markedly potentiated when TMP was combined in a fixed ratio of 1:19 with SMX; the geometric mean MIC of TMP was reduced to 0.016 micrograms/ml (range, 0.002 to 0.25 micrograms/ml), with a geometric mean MBC of 0.031 micrograms/ml (range, 0.004 to 0.25 micrograms/ml). The combination had no synergistic effect against strains of S. faecium; the geometric mean MICs and MBCs of both agents were ca. 0.06 micrograms/ml. The MBC/MIC ratios for TMP and TMP-SMX were less than or equal to 16 for all 131 strains. MICs and MBCs for TMP-SMX were unchanged, and for TMP they decreased when performed in broth supplemented with 50% heat-inactivated pooled human serum. For TMP and TMP-SMX, the susceptibilities of isolates with high-level resistance to gentamicin or streptomycin were the same as those of isolates susceptible to less than or equal to 2,000 micrograms of aminoglycoside per ml. These results suggest that TMP-SMX and TMP alone could prove useful in the treatment of serious enterococcal infections, including infections by strains with high-level resistance to aminoglycosides.