Linezolid-resistant Enterococcus faecium isolated from a patient without prior exposure to an oxazolidinone: report from the SENTRY Antimicrobial Surveillance Program

Revision of linezolid disk diffusion quality control guidelines for testing Staphylococcus aureus ATCC 25923: an independent seven-laboratory trial

The accuracy of the published disk diffusion quality control (QC) range for linezolid for Staphylococcus aureus ATCC 25923 has been reported to be problematic, prompting a multicenter study to determine if revision was necessary. Five out of seven laboratories reported values outside the current QC range. Three participants had significant (13.3-60.0%) 'unacceptable' results with zones smaller than the established range. Overall, a range of 21-32 mm was shown (82.4% within the NCCLS published range). A revision (25-31 mm; 95.7% of results) of the QC range is necessary when testing S. aureus ATCC 25923 against linezolid.

Validation of commercial dry-form broth microdilution panels for susceptibility testing of AZD2563, a new long-acting oxazolidinone

The MIC results using a dry-form broth microdilution panel (TREK Diagnostic/Sensititre, Westlake, OH, USA) were validated for AZD2563, a novel oxazolidinone compound. In comparision studies against reference frozen-form panels, the commercial MIC results were the same as the reference calue for 82.7% of organisms and all results were within +/- one log2 dilution. Using 462 organisms, most from three genus groups (enterococci, staphylococci, streptococci), test results indicate that Sensititre MIC values were comparable to the reference test and can be utilized in clinical trials of for routine laboratory use when testing AZD2563 and linezolid, the drug class comparator.

Linezolid resistance since 2001: SENTRY Antimicrobial Surveillance Program

BACKGROUND

The oxazolidinone class of antimicrobials has demonstrated remarkable activity against gram-positive cocci. Linezolid has proven to be a first-line therapeutic option for vancomycin-resistant strains. Linezolid clinical trial results and subsequent published case reports cite rare resistance emerging in patients receiving prolonged therapy.

OBJECTIVE

To report the initial linezolid-resistant organisms from cases obtained through the SENTRY Antimicrobial Surveillance Program, after screening >40,000 gram-positive cocci without resistance between 1998 and 2000.

METHODS

During 2001-2002, 8 resistant strains (from 8 different patients) located in 6 states from 7 different participating SENTRY institutions in the US were identified among bloodstream, respiratory, skin and soft tissue, and urinary tract infection isolates of Enterococcus faecalis, Enterococcus faecium, Staphylococcus epidermidis, and Streptococcus oralis. Resistance was detected by reference broth microdilution methods and confirmed by identical results using Etest (AB BIODISK, Solna, Sweden) and the standardized disk diffusion method.

RESULTS

Minimum inhibitory concentration (MIC) and disk diffusion tests showed elevated MICs (> or =8 microg/mL) and small inhibitory zone diameters (< or =15 mm) for all strains to both linezolid and the investigational oxazolidinone AZD2563. Vancomycin resistance was detected in 2 of the 8 linezolid-resistant strains. All enterococci and the viridans-group streptococcus (S. oralis) strain showed resistance to erythromycin. E. faecium strains were resistant to penicillins, but susceptible to quinupristin/dalfopristin. Only 3 of the patients had previously received the drug.

CONCLUSIONS

Linezolid resistance remains rare, with only 8 isolates among 9833 (0.08%) monitored isolates identified between January 1, 2001, and June 30, 2002. Resistance, however, was no longer limited to enterococci. Clinical laboratories should test linezolid more widely to detect emerging resistance, especially for patients receiving oxazolidinone therapy. Longitudinal surveillance programs are warranted to detect a trend in the development of resistance, determine the molecular mechanism of resistance, and recommend alternative therapies or epidemiologic interventions.

Linezolid-resistant, vancomycin-resistant Enterococcus faecium infection in patients without prior exposure to linezolid

We describe 2 patients without prior exposure to linezolid who were infected with closely related strains of linezolid- and vancomycin-resistant Enterococcus faecium (LRVREF) that may have been hospital acquired. Polymerase chain reaction amplification of the domain V region of the 23S ribosomal RNA gene demonstrated the presence of the G2576U mutation previously reported to be associated with linezolid resistance. Nosocomial transmission of LRVREF is an ominous sign and underscores the importance of meticulous infection-control measures.

Geographic variations and trends in antimicrobial resistance among Enterococcus faecalis and Enterococcus faecium in the SENTRY Antimicrobial Surveillance Program (1997-2000)

In the 1990s, the Enterococcus emerged as an important pathogen because of increasing prevalence and acquired resistances to glycopeptides and other agents. The seriousness of this problem can vary markedly worldwide and within nations; the SENTRY Antimicrobial Surveillance Program documents these differences. Over 8,000 enterococci were processed in the program (1997-2000) and the Enterococcus faecalis (EF; 4,034 strains) and Enterococcus faecium (EFM; 1,123 strains) isolates are tabulated. All strains were processed by three regional monitors using reference dilution methods. Identification to species was performed by participants and confirmed by the central laboratories. EF occurrence was greater than EFM by ratios of 3:1 to 5:1 in the Asia-Pacific (APAC), European (EU), and North American (NA) regions; the ratio was 17:1 in Latin America (LA). EF and EFM represented approximately 80-90% of all isolated enterococci. Glycopeptide-resistant enterococci (GRE) rates varied from nil for EF in APAC and LA to 43 to 54% in EFM in NA. A slight increase in GRE was noted in NA (EFM only). Van A phenotypes predominated all regions. The most recent (2000) rank order of % GRE by region was: for NA (13%) > LA (4%) > EU (3%) > APAC (1%). In NA potential therapeutic agents were (% S): ampicillin (81%), chloramphenicol (87%), quinupristin/dalfopristin (20%), ciprofloxacin (39%), gatifloxacin (51%), nitrofurantoin (83%) and linezolid (>99%). Resistances in enterococci continue to be documented worldwide, but rates within endemic areas like NA appears to be stabilizing. Van A resistance patterns predominate and therapeutic options continue to present dilemmas, although some of the older agents remain usable as primary therapy or as alternatives to the newer agents such as the oxazolidinones.

Quinupristin-dalfopristin and linezolid: evidence and opinion

Quinupristin-dalfopristin and linezolid demonstrate in vitro activity against a wide range of gram-positive bacteria, including many isolates resistant to earlier antimicrobials. Quinupristin-dalfopristin is inactive against Enterococcus faecalis but has been effective for treatment of infections due to vancomycin-resistant Enterococcus faecium associated with bacteremia. In comparative trials, linezolid proved to be equivalent to comparator agents, resulting in its approval for several clinical indications. The almost-complete bioavailability of linezolid permits oral administration. Each agent can cause adverse effects that may limit use in individual patients. Resistance to these drugs has been encountered infrequently among vancomycin-resistant E. faecium. Resistance to quinupristin-dalfopristin is rare among staphylococci in the United States, and resistance to linezolid is very rare. Whether there is any benefit to use of these agents in combination regimens, and whether there are circumstances in which they might be alternatives to cell-wall active antibiotics for treatment of bone or endovascular infections, are questions that deserve further study.

Ribosomal and non-ribosomal resistance to oxazolidinones: species-specific idiosyncrasy of ribosomal alterations

A derivative of Mycobacterium smegmatis, which carries only one functional rRNA (rrn) operon, was used to isolate mutants resistant to the ribosome-targeted antibiotic linezolid. Isolation and characterization of linezolid-resistant clones revealed two classes of mutants. Ribosomes from class I mutants are resistant to oxazolidinones in an in vitro peptidyl transferase assay, indicating that resistance maps to the ribosome component. In contrast, ribosomes from class II mutants show wild-type susceptibility to a linezolid derivative in vitro, pointing to a non-ribosomal mechanism of resistance. Introduction of a wild-type ribosomal RNA operon into linezolid-resistant strains restored linezolid sensitivity in class I mutants, indicating that resistance (i) maps to the rRNA and (ii) is recessive. Sequencing of the entire rrn operon identified a single nucleotide alteration in 23S rRNA of class I mutant strains, 2447G --> T (Escherichia coli numbering). Introduction of mutant rrl2447T into M. smegmatis rrn- resulted in a linezolid-resistant phenotype, demonstrating a cause-effect relationship of the 2447G --> T alteration. The 2447G --> T mutation, which renders M. smegmatis linezolid resistant, confers lethality in E. coli. This finding is strong evidence of structural and pos-sibly functional differences between the ribosomes of Gram-positive and Gram-negative bacteria. In agreement with the results of the in vitro assay, class II mutants show a wild-type sequence of the complete rRNA operon. The lack of cross-resistance of the class II mutants to other antibiotics suggests a resistance mechanism other than activation of a broad-spectrum multidrug transporter.

Detection of oxazolidinone-resistant Enterococcus faecalis and Enterococcus faecium strains by real-time PCR and PCR-restriction fragment length polymorphism analysis

A real-time PCR assay identified linezolid-resistant Enterococcus faecalis and Enterococcus faecium isolates with a G2576U rRNA mutation. PCR-restriction fragment length polymorphism analysis of ribosomal DNA amplicons with NheI also detected this mutation. Both assays detected isolates heterozygous at this position. Recognition of isolates with what is presently the most frequent oxazolidinone resistance mutation may aid surveillance and individual case management.

DNA binding ligands with excellent antibiotic potency against drug-resistant gram-positive bacteria

An efficient synthesis of DNA binding molecules consisting of four heterocyclic carboxamide units and various substituents at both termini is described. The minor-groove binding ligands showed excellent activity against a broad range of Gram-positive bacteria; no cross-resistance to known antibacterial drugs was observed.

First characterization of a cluster of VanA-type glycopeptide-resistant Enterococcus faecium, Colombia

From August 1998 to October 1999, glycopeptide-resistant enterococci (GRE) were isolated from 23 infected patients at a teaching hospital in Medellín, Colombia. Identification at the species level and by multiplex polymerase chain reaction assay indicated that all isolates were Enterococcus faecium. The isolates were highly resistant to ampicillin, ciprofloxacin, gentamicin, penicillin, streptomycin, teicoplanin, and vancomycin; they were susceptible only to chloramphenicol, linezolid, and nitrofurantoin. Determination of glycopeptide genotype indicated the presence of the vanA gene in all isolates. Molecular typing by pulsed field gel electrophoresis showed that all isolates were closely related. This study is the first molecular characterization of GRE in Colombia.

Spectrum and potency evaluation of a new oxazolidinone, linezolid: report from the SENTRY Antimicrobial Surveillance Program, 1998-2000

Resistance (R) among Gram-positive cocci has escalated in the last two decades to levels necessitating the development and use in the newer drug classes, oxazolidinones (linezolid) and streptogramins (quinupristin/dalfopristin [Q/D]). The SENTRY Antimicrobial Surveillance Program has monitored these classes before, during and after their release by various regulatory agencies. Over 30,000 Gram-positive strains were tested against >30 drugs by reference broth microdilution methods between 1998-2000 in four geographic regions (Asia-Western Pacific [APAC], Europe [EU], Latin America [LA], North America [NA]). The tested strains were 23,188 staphylococci; 5,103 enterococci and 2,045 streptococci. Among staphylococci, linezolid was active against all isolates (MICs, < or =4 microg/ml) regardless of susceptibility patterns of other antimicrobial agents. Similar results were noted for vancomycin (includes one VISA from Hong Kong), teicoplanin, and Q/D (<1% R). Gatifloxacin had the widest spectrum among fluoroquinolones (FQ) against Staphylococcus aureus (1.5-9.2% R) and coagulase-negative staphylococci (0.8-4.0%). Linezolid was also active against all enterococci (MIC50 and (90,) 2 microg/ml). Q/D was active against only 75.3% of vancomycin-resistant enterococci (VRE). The VRE rate was highest in NA (12.4%) > EU (3.2%) > LA (1.6%) > APAC (1.3%). Among streptococci, linezolid was consistently active (MIC(90,) 1 microg/ml) as were the glycopeptides and Q/D. Variable penicillin-R (MIC, > or = 2 microg/ml) was observed among regions: EU (32.5%) > APAC (15.1%) > LA (13.8%) > NA (9.6%), and macrolide-R was higher in EU (40.3%). Ciprofloxacin-R at > or =4 microg/ml in streptococcal strains was noted world wide highest in viridans group streptococci (18.4-25.6%). Linezolid remained active (MIC, < or =4 microg/ml) against all Gram-positive species strains tested in the SENTRY Program (1998-2000). Q/D, glycopeptides and newer FQ compounds were generally less effective in vitro. It remains a prudent practice to continue surveillance programs to detect emerging resistance patterns and recognize significant regional variations in the oxazolidinone susceptibilities.