Mechanisms and implications of glycopeptide resistance in enterococci

Molecular Dynamics Simulation of Atomic Interactions in the Vancomycin Binding Site

Vancomycin is a glycopeptide antibiotic produced by Amycolaptopsis orientalis used to treat serious infections by Gram-positive pathogens including methicillin-resistant Staphylococcus aureus. Vancomycin inhibits cell wall biosynthesis by targeting lipid II, which is the membrane-bound peptidoglycan precursor. The heptapeptide aglycon structure of vancomycin binds to the d-Ala-d-Ala of the pentapeptide stem structure in lipid II. The third residue of vancomycin aglycon is asparagine, which is not directly involved in the dipeptide binding. Nonetheless, asparagine plays a crucial role in substrate recognition, as the vancomycin analogue with asparagine substituted by aspartic acid (V_D) shows a reduction in antibacterial activities. To characterize the function of asparagine, binding of vancomycin and its aspartic-acid-substituted analogue V_D to l-Lys-d-Ala-d-Ala and l-Lys-d-Ala-d-Lac was investigated using molecular dynamic simulations. Binding interactions were analyzed using root-mean-square deviation (RMSD), two-dimensional (2D) contour plots, hydrogen bond analysis, and free energy calculations of the complexes. The analysis shows that the aspartate substitution introduced a negative charge to the binding cleft of V_D, which altered the aglycon conformation that minimized the repulsive lone pair interaction in the binding of a depsipeptide. Our findings provide new insight for the development of novel glycopeptide antibiotics against the emerging vancomycin-resistant pathogens by chemical modification at the third residue in vancomycin to improve its binding affinity to the d-Ala-d-Lac-terminated peptidoglycan in lipid II found in vancomycin-resistant enterococci and vancomycin-resistant S. aureus.

Quantification of the d-Ala-d-Lac-Terminated Peptidoglycan Structure in Vancomycin-Resistant Enterococcus faecalis Using a Combined Solid-State Nuclear Magnetic Resonance and Mass Spectrometry Analysis

Induction of vancomycin resistance in vancomycin-resistant enterococci (VRE) involves replacement of the d-Ala-d-Ala terminus of peptidoglycan (PG) stems with d-Ala-d-Lac, dramatically reducing the binding affinity of vancomycin for lipid II. Effects from vancomycin resistance induction in Enterococcus faecalis (ATCC 51299) were characterized using a combined solid-state nuclear magnetic resonance (NMR) and liquid chromatography-mass spectrometry (LC-MS) analysis. Solid-state NMR directly measured the total amounts of d-Lac and l,d-Ala metabolized from [2-13C]pyruvate, accumulated Park's nucleotide, and changes to the PG bridge-linking density during the early exponential growth phase (OD660 = 0.4) in intact whole cells of VRE. A high level of accumulation of depsipeptide-substituted Park's nucleotide consistent with the inhibition of the transglycosylation step of PG biosynthesis during the initial phase of vancomycin resistance was observed, while no changes to the PG bridge-linking density following the induction of vancomycin resistance were detected. This indicated that the attachment of the PG bridge to lipid II by the peptidyl transferases was not inhibited by the d-Ala-d-Lac-substituted PG stem structure in VRE. Compositions of mutanolysin-digested isolated cell walls of VRE grown with and without vancomycin resistance induction were determined by LC-MS. Muropeptides with PG stems terminating in d-Ala-d-Lac were found only in VRE grown in the presence of vancomycin. Percentages of muropeptides with a pentapeptide stem terminating in d-Ala-d-Lac for VRE grown in the presence of vancomycin were 26% for the midexponential phase (OD660 = 0.6) and 57% for the stationary growth phase (OD660 = 1.0). These high percentages indicate that d-Ala-d-Lac-substituted lipid II was efficiently utilized for PG biosynthesis in VRE.

Isolation of Tn1546-like elements in vancomycin-resistant Enterococcus faecium isolated from wood frogs: an emerging risk for zoonotic bacterial infections to humans

AIM

Isolation and characterization of vancomycin-resistant enterococci (VRE), mainly Enterococcus faecium, from the faecal pellet of wood frogs (Rana sylvatica).

METHODS AND RESULTS

The frog VRE isolates were tested for their susceptibility to various antibiotics and were found resistant to ampicillin (Am), chloramphenicol (Cm), erythromycin (Em), gentamicin (Gm), tetracycline (Tc), teicoplanin (Tp) and vancomycin (Vn). The linkage of multiple antibiotic resistances to Em, Tc, Tp and Vn was observed in 84% of resistant Ent. faecium. Inducible antibiotic resistance (MIC ≥ 512 μg ml(-1) ) to Vn was also detected in these isolates. PCR analysis revealed the presence of vanA in all strains, and none of the strains were positive for vanB, indicating the existence of vanA phenotype. Furthermore, the PCR-RFLP analysis of the frog vanA amplicon with PstI, BamHI and SphI generated identical restriction patterns similar to Tn1546-like elements found in human VRE isolates. DNA homoduplex analysis also confirmed that vanA from the frog VRE has DNA sequence homology with the vanA of Tn1546-like elements of human and animal isolates. Blastx analysis of frog vanA sequence showed similarities with protein sequences generated from protein database of Vn-resistant Ent. faecium, Baccilus circulans, Paenibacillus apiarius and Oerskovia turbata isolates. Horizontal transfer of Vn resistance was not detected in frog isolates as revealed by filter mating conjugal experiment.

CONCLUSIONS

In summary, our results demonstrated that wood frogs carry Vn-resistant bacteria, and resistance genes (vanA) are located on Tn1546-like elements.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study highlights a previously less recognized role of amphibians as sentinels for multidrug-resistant bacteria and alerts the public health workers for an emerging risk of zoonotic bacterial infections to humans.

In vitro activity of ramoplanin and comparator drugs against anaerobic intestinal bacteria from the perspective of potential utility in pathology involving bowel flora

Susceptibility of intestinal bacteria to various antimicrobial agents in vitro, together with levels of those agents achieved in the gut, provides information on the likely impact of the agents on the intestinal flora. Orally administered drugs that are poorly absorbed may be useful for treatment of intestinal infections and for certain other situations in which intestinal bacteria may play a role. The antimicrobial activity of ramoplanin (MDL 62,198) against 928 strains of intestinal anaerobic bacteria was determined using the NCCLS-approved Wadsworth brucella laked-blood agar dilution method. The activity of ramoplanin was compared with that of ampicillin, bacitracin, metronidazole, trimethoprim/sulfamethoxazole (TMP/SMX), and vancomycin. The organisms tested included Bacteroides fragilis group (n=89), other Bacteroides species (n=16), other anaerobic Gram-negative rods (n=56) anaerobic cocci (n=114), Clostridium species (n=426), and non-sporeforming anaerobic Gram-positive rods (n=227). The overall MIC(90)s of ramoplanin, ampicillin, bacitracin, metronidazole, and vancomycin were 256, 32, 128, 16, and 128 mcg/ml, respectively. Ramoplanin was almost always highly active vs. Gram-positive organisms and relatively poor in activity against Gram-negative organisms, particularly Bacteroides, Bilophila, Prevotella, and Veillonella. Vancomycin was quite similar to ramoplanin in its activity. Ampicillin was relatively poor in activity vs. organisms that often produce beta-lactamase, including most of the Gram-negative rods as well as Clostridium bolteae and C. clostridioforme. Bacitracin was relatively poor in activity against most anaerobic Gram-negative rods, but better vs. most Gram-positive organisms. Metronidazole was very active against all groups other than bifidobacteria and some strains of other types of non-sporeforming Gram-positive bacilli. TMP/SMX was very poorly active, with an MIC(90) of >2048 mcg/ml.

In vitro activities of OPT-80 and comparator drugs against intestinal bacteria

The activities of OPT-80 against 453 intestinal bacteria were compared with those of seven other drugs. OPT-80 showed good activity against most clostridia, staphylococci, and enterococci, but streptococci, aerobic and facultative gram-negative rods, anaerobic gram-negative rods, and Clostridium ramosum were resistant. Poor activity against anaerobic gram-negative rods may maintain colonization resistance.

Diversity among multidrug-resistant enterococci

Enterococci are associated with both community- and hospital-acquired infections. Even though they do not cause severe systemic inflammatory responses, such as septic shock, enterococci present a therapeutic challenge because of their resistance to a vast array of antimicrobial drugs, including cell-wall active agents, all commercially available aminoglycosides, penicillin and ampicillin, and vancomycin. The combination of the latter two occurs disproportionately in strains resistant to many other antimicrobial drugs. The propensity of enterococci to acquire resistance may relate to their ability to participate in various forms of conjugation, which can result in the spread of genes as part of conjugative transposons, pheromone-responsive plasmids, or broad host-range plasmids. Enterococcal hardiness likely adds to resistance by facilitating survival in the environment (and thus enhancing potential spread from person to person) of a multidrug-resistant clone. The combination of these attributes within the genus Enterococcus suggests that these bacteria and their resistance to antimicrobial drugs will continue to pose a challenge.

Recognition, management and prophylaxis of endocarditis

Infective endocarditis (IE) remains a disease with high morbidity and mortality. In recent years, a higher frequency of IE has been observed in the elderly, in intravenous drug users and in patients with prosthetic valves. The diverse manifestations of this disease demand a high degree of suspicion from the practitioner, in order to make an early diagnosis. Advances in and increasing use of echocardiography (especially transoesophageal) allow us to identify valvular changes earlier and more precisely. The use of the new Duke's diagnostic criteria, based on clinical manifestations and microbiological and echocardiographic findings, facilitates the diagnosis and categorisation of IE. An increase in staphylococci and other problem pathogens, such as penicillin-resistant streptococci, enterococci resistant to beta-lactams, aminoglycosides and methicillin-resistant staphylococci has been observed. Important changes have also taken place in the management of IE. There is a clear trend towards the use of shorter treatment courses, oral and once-daily regimens and outpatient programmes, all of which aim to reduce costs and provide patients with improved quality of life. Antibiotic prophylaxis for the prevention of IE is still controversial. In the past few years more rational regimens have been used, and indications are now more precise. In spite of all this, however, few cases are prevented and patient compliance to the prophylaxis regimens remains low.

Diagnosis and treatment of bacteremia and intravascular catheter infections

The past decade has witnessed an explosive rise in the rate of bacteremia and intravascular catheter infection. Although gram-negative organisms continue to account for up to one third of these infections, gram-positive organisms have become increasingly prevalent pathogens. Virulent antibiotic-resistant bacterial strains have emerged and present a formidable treatment challenge. Simultaneously, management of catheter infection has evolved. Although patients who develop fungemia, gram-negative bacteremia, or sepsis syndrome are best treated by catheter removal in addition to antimicrobial therapy, an increasing body of evidence suggests that many gram-positive bacterial catheter infections can be treated by use of antimicrobial agents without catheter removal. Advances in catheter design and immunotherapy for sepsis syndrome also hold promise. Despite these innovations, determining the initial need for catheter placement, adherence to meticulous sterile surgical technique during insertion, and subsequent fastidious catheter maintenance remain the mainstays of preventing these potentially disastrous infections.

Enteric eradication of vancomycin-resistant Enterococcus faecium with oral bacitracin

The emergence of vancomycin-resistant Enterococcus faecium (VREF) has produced a therapeutic dilemma. The colonization of the intestinal tract with VREF may predispose patients to infections by this organism and may contribute to its nosocomial spread. It is reasonable to attempt to eradicate VREF from colonized patients. The optimal regimen, however, is unknown and this study was designed to evaluate the efficacy of oral regimens of vancomycin and bacitracin for the elimination of VREF from the enteric tract. Enterococcal isolates were tested for susceptibilities to vancomycin, bacitracin, and ampicillin with median minimum inhibitory concentrations of > 512 micrograms/ml, 10 units/ml, and 128 micrograms/ml, respectively. All patients were given an initial trial of oral vancomycin 125 mg every 6 h for 10 days. Those who failed oral vancomycin were then given oral bacitracin 25,000 units every 6 h for 10 days due to its favorable in vitro activity. VREF was eradicated from the stools of 42% of patients (eight of 19) receiving oral vancomycin as compared with all eight patients receiving oral bacitracin (P < 0.01). The organism recurred in two bacitracin patients (25%) 8 and 20 days after completion of therapy. Whether prior vancomycin therapy predisposed patients to colonization by VREF was also examined. Ten (53%) of 19 patients had received prior vancomycin therapy before isolation of VREF from the stool. Our data suggest that oral bacitracin may be an effective alternative to commercially available oral vancomycin for the eradication of VREF from the enteric tract.

Comparison of ribotyping and pulsed-field gel electrophoresis for subspecies differentiation of strains of Enterococcus faecalis

Hybridization of EcoRI- and HindIII-digested chromosomal DNAs from 41 isolates of Enterococcus faecalis with probes for rRNA genes was performed (ribotyping). The ability of ribotyping to distinguish strains at the subspecies level was compared with results previously determined by pulsed-field gel electrophoresis (PFGE). With EcoRI, seven ribopatterns (usually differing by only one band) were found, while PFGE had previously shown 25 clearly different patterns plus six related variants. Digestion with HindIII generated a few additional patterns but still failed to differentiate some strains that had very different PFGE patterns. Ribotyping with BscI has also been reported to be inadequate for subspecies strain differentiation (L. M. Hall, B. Duke, M. Guiney, and R. Williams, J. Clin. Microbiol. 30:915-919, 1992). Although ribotyping with other restriction endonucleases may perform better in distinguishing different strains, at present PFGE appears to be superior for strain differentiation.