Transferable vancomycin and teicoplanin resistance in Enterococcus faecium

The life and times of the Enterococcus

Enterococci are important human pathogens that are increasingly resistant to antimicrobial agents. These organisms were previously considered part of the genus Streptococcus but have recently been reclassified into their own genus, called Enterococcus. To date, 12 species pathogenic for humans have been described, including the most common human isolates, Enterococcus faecalis and E. faecium. Enterococci cause between 5 and 15% of cases of endocarditis, which is best treated by the combination of a cell wall-active agent (such as penicillin or vancomycin, neither of which alone is usually bactericidal) and an aminoglycoside to which the organism is not highly resistant; this characteristically results in a synergistic bactericidal effect. High-level resistance (MIC, greater than or equal to 2,000 micrograms/ml) to the aminoglycoside eliminates the expected bactericidal effect, and such resistance has now been described for all aminoglycosides. Enterococci can also cause urinary tract infections; intraabdominal, pelvic, and wound infections; superinfections (particularly in patients receiving expanded-spectrum cephalosporins); and bacteremias (often together with other organisms). They are now the third most common organism seen in nosocomial infections. For most of these infections, single-drug therapy, most often with penicillin, ampicillin, or vancomycin, is adequate. Enterococci have a large number of both inherent and acquired resistance traits, including resistance to cephalosporins, clindamycin, tetracycline, and penicillinase-resistant penicillins such as oxacillin, among others. The most recent resistance traits reported are penicillinase resistance (apparently acquired from staphylococci) and vancomycin resistance, both of which can be transferred to other enterococci. It appears likely that we will soon be faced with increasing numbers of enterococci for which there is no adequate therapy.

Antibacterial and plasmid curing activity of lomefloxacin in vitro

The in vitro activity of lomefloxacin in comparison to other recently developed quinolones was evaluated by determination of MICs for 89 test strains. Organisms tested included clinical isolates of gram-positive and gram-negative bacteria susceptible or resistant to quinolones, resistant mutants of Enterobacteriaceae, Pseudomonas aeruginosa, Staphylococcus aureus and Enterococcus faecalis obtained in vitro, and the wild type strain Escherichia coli K12 with its mutants na1A, nalB, nalC and nalB. The activity of lomefloxacin was similar to that of pefloxacin against gram-positive cocci and similar to that of norfloxacin against gram-negative bacteria. There was cross-resistance to all quinolones tested in pefloxacin resistant clinical isolates and in laboratory mutants. The activity of lomefloxacin was decreased against the Escherichia coli K12 mutants nalA and nalD but not nalB, whereas the nalC mutant showed increased susceptibility. Susceptible and mutant strains (n = 115) were used to establish the least-squares lines of regression with 5-micrograms and 10-micrograms lomefloxacin disks. Tests of the in vitro plasmid curing activity of lomefloxacin compared to other agents showed no statistically significant plasmid loss after treatment with lomefloxacin.

Susceptibility of enterococci and epidemiology of enterococcal infection in the 1980s

Enterococcisensu strictoform part of the normal gut flora (1) and may be found in the mouth, vagina and anterior urethra (2). They are opportunist pathogens which can cause serious infection including endocarditis. Nosocomial enterococcal infection appears to be increasing both in the UK (Public Health Laboratory Service [PHLS] Communicable Disease Surveillance Centre [CDSC], unpublished) and the USA (3) and to correspond to usage of broad spectrum β-lactam antimicrobial agents (4−7) and invasive surgical devices (8, 9). At the same time, the incidence of enterococci resistant or tolerant to previously commonly employed antimicrobial agents or their synergistic combinations is increasing and is compromising therapy of serious enterococcal infection. Strains of enterococci with high-level resistance to streptomycin and kanamycin (minimal inhibitory concentrations [MICs] > 2000 mg/L) were first reported in 1970 (10, 11) and rapidly became widespread (8, 12−14).

Activity of glycopeptides against vancomycin-resistant gram-positive bacteria

Gram-positive bacteria resistant to vancomycin are rare; but they include members of the genera Leuconostoc, Lactobacillus, and Pediococcus, as well as recently emerging vancomycin-resistant strains of Enterococcus faecium and Enterococcus faecalis. Vancomycin, teicoplanin, and several vancomycin derivatives were tested for their activities against vancomycin-resistant gram-positive bacteria. Vancomycin-resistant E. faecium and E. faecalis were generally cross-resistant to other glycopeptides, but some N-substituted vancomycin derivatives were active against the resistant strains, with MICs of 2 to 32 micrograms/ml. These vancomycin derivatives also had significant levels of activity against intrinsically vancomycin-resistant organisms such as Leuconostoc sp. While vancomycin resistance in E. faecium and E. faecalis was inducible, resistance in members of the genera Leuconostoc, Lactobacillus, and Pediococcus appeared to be expressed constitutively. Antibody to a vancomycin-induced membrane protein found in membranes of resistant enterococci did not detect a cross-reacting protein in other vancomycin-resistant species.

In vitro susceptibility studies of vancomycin-resistant Enterococcus faecalis

Vancomycin resistance exhibited by Enterococcus faecalis isolates V583, V586, and V587 is described. The vancomycin MICs ranged from 32 to 64 micrograms/ml. Although resistant to vancomycin, the isolates were susceptible to teicoplanin (MIC, less than or equal to 0.5 micrograms/ml). Such a glycopeptide susceptibility profile has not been previously described for E. faecalis. Time kill studies showed that vancomycin resistance adversely affected the synergistic activity that vancomycin and aminoglycoside combinations usually demonstrate against enterococci. However, the ability to detect vancomycin resistance varied with the susceptibility testing method used. Whereas broth microdilution, broth macrodilution, and agar dilution methods detected resistance, disk-agar diffusion and the AutoMicrobic system Gram-Positive GPS-A susceptibility card (Vitek Systems Inc., Hazelwood, Mo.) did not. To detect vancomycin resistance reliably and establish the incidence of such E. faecalis isolates, adjustments in some susceptibility testing methods may be necessary.

Cloning of a DNA region of Actinoplanes teichomyceticus conferring teicoplanin resistance

Teicoplanin is a glycopeptide antibiotic, produced by Actinoplanes teichomyceticus, active against Gram positive bacteria and recently introduced into clinical practice. It blocks cell wall biosynthesis by inhibiting peptidoglycan polymerization. The mechanism(s) of resistance of the producer strains of this class of antibiotics have not yet been characterized. We have constructed a genomic bank of A. teichomyceticus in Streptomyces lividans. A clone from this bank, PTR168, was able to confer resistance to teicoplanin on its sensitive host. The restriction map of plasmid pTR168 and the hybridization pattern to A. teichomyceticus DNA were determined; we have also studied the mechanism of this resistance which seems correlated with a reduced binding of the antibiotic to the cell wall.

High-level vancomycin-resistant enterococci causing hospital infections

Nosocomial infection or colonization due to enterococci with high-level resistance to vancomycin (minimal inhibitory concentrations [MICs] between 64 and greater than 2000 mg/L) has occurred in 41 patients with renal disease. These vancomycin-resistant enterococci were cultured from many sources including blood. All but one strain contained one or more plasmids ranging in molecular weight from 1.0 to 40 Megadaltons (MDa). Vancomycin resistance was transferable by conjugation to a susceptible recipient strain of Enterococcus faecalis but this was not always associated with plasmid DNA. The emergence of transferable high-level vancomycin resistance in enterococci causing significant clinical infections is of particular importance since vancomycin is widely regarded as a reserve drug for the management of infections with multi-resistant Gram-positive organisms.

Characterization of vancomycin resistance in Enterococcus faecium and Enterococcus faecalis

Vancomycin resistance in Enterococcus faecium 180, a clinical isolate from England, was studied. Resistance to vancomycin was transferable by conjugation to other enterococci. Expression of resistance was inducible and coincided with the appearance of a new membrane protein.