Emergence of linezolid resistance in Enterococcus faecium not dependent on linezolid treatment

First study on detection of cryptic resistance to linezolid among clinical isolates of methicillin resistant Staphylococcus aureus from India

PURPOSE

Linezolid is an oral antibiotic which is widely used for serious infections caused by Methicillin Resistant Staphylococcus aureus (MRSA). With emergence of vancomycin MIC creep among clinical strains of MRSA, it is essential to know the possible emergence of subclinical resistance against linezolid as well. With this background, we aimed to detect evident (phenotypic) and cryptic (hidden or genotypic) linezolid resistance among MRSA isolates.

METHODS

250 clinical isolates of MRSA were collected and their susceptibility patterns were determined. Every third MRSA isolate was subjected to PCR for domain V of the 23S rRNA for the mutation hotspot in the 746bp segment which harbors the classical mutation for linezolid resistance. Restriction Fragment Length Polymorphism was done to confirm presence of the G2576U mutation.

RESULTS

Six isolates (2.4%) were phenotypically resistant to linezolid. Among these six LRSA isolates, 5 demonstrated the G2576U mutation by PCR - RFLP. Cryptic resistance to Linezolid was identified in two isolates among linezolid susceptible isolates.

CONCLUSIONS

In the present study, hidden resistance to linezolid was observed in linezolid susceptible clinical isolates. Emergence of resistance against over-the-counter drugs like linezolid is major challenge. Identification of cryptic resistance among patients implies impending resistance to linezolid. Judicious use of antimicrobials, application of strict infection control practices and prescription audit needs to be made mandatory to preserve such drugs.

Genomics of vancomycin-resistant Enterococcus faecium

Vancomycin-resistant Enterococcus faecium (VREfm) is a globally significant public health threat and was listed on the World Health Organization's 2017 list of high-priority pathogens for which new treatments are urgently needed. Treatment options for invasive VREfm infections are very limited, and outcomes are often poor. Whole-genome sequencing is providing important new insights into VREfm evolution, drug resistance and hospital adaptation, and is increasingly being used to track VREfm transmission within hospitals to detect outbreaks and inform infection control practices. This mini-review provides an overview of recent data on the use of genomics to understand and respond to the global problem of VREfm.

The emerging problem of linezolid-resistant enterococci

Enterococcus is a significant pathogen in numerous infections, particularly in nosocomial infections, and is thus a great challenge to clinicians. Linezolid (LNZ), an oxazolidinone antibiotic, is an important therapeutic option for infections caused by Gram-positive bacterial pathogens, especially vancomycin-resistant enterococci. A systematic review was performed of the available literature on LNZ-resistant enterococci (LRE) to characterise these infections with respect to epidemiological, microbiological and clinical features. The results validated the potency of LNZ against enterococcal infections, with a sustained susceptibility rate of 99.8% in ZAAPS and 99.2% in LEADER surveillance programmes. Patients with LRE had been predominantly exposed to LNZ prior to isolation of LRE, with a mean treatment duration of 29.8±48.8days for Enterococcus faecalis and 23.1±21.4days for Enterococcus faecium. Paradoxically, LRE could also develop in patients without prior LNZ exposure. LNZ resistance was attributed to 23S rRNA (G2576T) mutations (51.2% of E. faecalis and 80.5% of E. faecium) as well as presence of the cfr gene (4.7% and 4.8%, respectively), which could transfer horizontally among the strains. In addition to the cfr gene, 32 cases of optrA-positive LRE were identified. Further study is required to determine the prevalence of novel resistance genes. The emergence of LRE thus hampers the treatment of such infections, which warrants worldwide surveillance.

Increased frequency of linezolid resistance among clinical Enterococcus faecium isolates from German hospital patients

Linezolid is an antibiotic of last resort for the treatment of infections with vancomycin-resistant enterococci (VRE). Here we report the increasing prevalence of linezolid resistance among clinical Enterococcus faecium strains from German hospital patients. Linezolid minimum inhibitory concentrations (MICs) were determined for 4461 clinical E. faecium strains isolated between 2008 and 2014. Isolates originated from the network of diagnostic laboratories collaborating with the National Reference Centre (NRC) for Staphylococci and Enterococci covering all German federal states. All linezolid-resistant isolates were determined by broth microdilution and confirmed by Etest as well as by analysing the 23S rDNA for putative mutations. Marker genes were determined by PCR. Genotyping was performed by SmaI macrorestriction analysis in pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST) for selected isolates. An increase in linezolid resistance was observed, from <1% in 2008 to >9% in 2014. Occasionally, outbreaks with linezolid-resistant VRE (ST117) were observed. In total, 232 (92.4%) of 251 linezolid-resistant E. faecium isolates (including 61 vanA and 29 vanB) contained the G2576T 23S rDNA mutation and showed a varying mixture of wild-type and mutated alleles per genome sufficient to confer linezolid resistance. In vitro growth experiments revealed a stable linezolid MIC. Of the 251 linezolid-resistant isolates, 5 were cfr-positive. In conclusion, these NRC data identified a country-wide ongoing trend of increasing linezolid resistance among clinical E. faecium isolates within the last 5 years.

Therapeutic options for infections due to vancomycin-resistant enterococci

BACKGROUND

Vancomycin-resistant enterococci (VRE) are an important cause of nosocomial infection occurring in critical care or immunocompromised patients.

OBJECTIVES

To provide updated information about therapeutic options for VRE infection.

METHODS

MEDLINE, EMBASE, and the Cochrane Library were searched to identify in vitro susceptibility data of VRE isolates, randomized and non-randomized controlled trials, case series, and cohort studies of VRE therapy published before 31 July 2008.

RESULTS/CONCLUSION

The updated in vitro susceptibility data for VRE show high resistance to ampicillin and aminoglycosides. Quinupristin-dalfopristin is limited by its lack of activity against vancomycin-resistant Enterococcus faealis and its musculoskeletal side effects. Emerging linezolid resistance has been reported to cause hospital spread and may be related to prolonged linezolid use. Quinupristin-dalfopristin resistance is usually linked to agricultural use of streptogramin. Nitrofurantoin and fosfomycin are alternatives in uncomplicated VRE urinary tract infection. Daptomycin and tigecycline have shown excellent potential for treating VRE infection.

Colonization of the tip of a thoracic catheter by Enterococcus faecalis resistant to vancomycin and linezolid

We report the isolation ofEnterococcus faecalisresistant to vancomycin and linezolid from the tip of a thoracic drainage catheter in an elderly patient. He was treated with vancomycin for a pleural empyema due to a meticillin-resistantStaphylococcus aureusbut never received linezolid. A surveillance rectal swab yielded both linezolid-susceptible and -resistant strains, and the two isolates were not genotypically related. Careful monitoring for linezolid-resistance is critical to avoid potential therapy failure and transmission of resistantE. faecalis.

Response to emerging infection leading to outbreak of linezolid-resistant enterococci

These bacteria have emerged as a hospital problem that appears to be caused by both linezolid exposure and patient-to-patient transmission.

Linezolid was approved in 2000 for treatment of gram-positive coccal infections. We performed a case-control study during a hospital outbreak of linezolid-resistant enterococci (LRE) infections, comparing cases of LRE infection (cases) with linezolid-sensitive enterococci infections (controls). Nasal and perirectal swab samples were obtained from all patients in a 1-day point-prevalence survey. We examined antimicrobial drug use and calculated the defined daily dose of linezolid per 1,000 patient-days. Fifteen LRE cases were identified (13 Enterococcus faecalis and 2 E. faecium); 7 were vancomycin-resistant. Compared with controls, case-patients had increased in-hospital mortality rates and lengths of stay. Multivariate analysis identified independent predictors of LRE infection: prior cultures positive for methicillin-resistant Staphylococcus aureus (adjusted odds ratio [AOR] 27), hospitalization duration before index culture (AOR 1.1 per day), and duration of preceding linezolid therapy (AOR 1.1 per day). Linezolid exposure and patient-to-patient transmission appear to be responsible for LRE infections, an important emerging hospital problem.

Clinical and microbiological aspects of linezolid resistance mediated by the cfr gene encoding a 23S rRNA methyltransferase

The cfr (chloramphenicol-florfenicol resistance) gene encodes a 23S rRNA methyltransferase that confers resistance to linezolid. Detection of linezolid resistance was evaluated in the first cfr-carrying human hospital isolate of linezolid and methicillin-resistant Staphylococcus aureus (designated MRSA CM-05) by dilution and diffusion methods (including Etest). The presence of cfr was investigated in isolates of staphylococci colonizing the patient's household contacts and clinical isolates recovered from patients in the same unit where MRSA CM-05 was isolated. Additionally, 68 chloramphenicol-resistant Colombian MRSA isolates recovered from hospitals between 2001 and 2004 were screened for the presence of the cfr gene. In addition to erm(B), the erm(A) gene was also detected in CM-05. The isolate belonged to sequence type 5 and carried staphylococcal chromosomal cassette mec type I. We were unable to detect the cfr gene in any of the human staphylococci screened (either clinical or colonizing isolates). Agar and broth dilution methods detected linezolid resistance in CM-05. However, the Etest and disk diffusion methods failed to detect resistance after 24 h of incubation. Oxazolidinone resistance mediated by the cfr gene is rare, and acquisition by a human isolate appears to be a recent event in Colombia. The detection of cfr-mediated linezolid resistance might be compromised by the use of the disk diffusion or Etest method.

Determination of risk factors associated with isolation of linezolid-resistant strains of vancomycin-resistant Enterococcus

OBJECTIVE

To identify independent risk factors associated with isolation of linezolid-resistant, vancomycin-resistant Enterococcus (VRE).

DESIGN

A retrospective, case-case-control study.

SETTING

A tertiary care, academic medical center.Methods. VRE isolates from clinical cultures were retrospectively analyzed for linezolid resistance during our 18-month study period. Clinical data were obtained from electronic patient records, and the risk factors associated with isolation of linezolid-resistant VRE were determined by comparison of 2 case groups with a control group.

RESULTS

A total of 20% of the VRE isolates analyzed during the study period were linezolid resistant, and resistant isolates were most commonly recovered from the urine (40% of resistant isolates). Risk factors found to be associated with isolation of linezolid-resistant VRE were peripheral vascular disease and/or the receipt of a solid organ transplant, total parenteral nutrition, piperacillin-tazobactam, and/or cefepime. Only 25% of patients from whom linezolid-resistant VRE was isolated had previous linezolid exposure, and in the multivariate model this was not found to be a risk factor associated with the isolation of linezolid-resistant VRE.

CONCLUSIONS

The results of this analysis suggest that there is horizontal transmission of linezolid-resistant VRE in our institution and highlight the need for improved infection control measures. Furthermore, the high incidence of linezolid-resistant VRE demands a reassessment of our empirical antibiotic selection for patients infected with VRE.

Emergence of increasing linezolid-resistance in enterococci in a post-outbreak situation with vancomycin-resistant Enterococcus faecium

During 2004 and at the start of 2005 a university hospital in Southwest Germany was affected by an extensive outbreak of vancomycin-resistant Enterococcus faecium (VRE). Although the outbreak was contained, linezolid-resistant enterococci emerged during and after the outbreak as the usage of linezolid became more common. Linezolid resistance was no longer limited to VRE. Nosocomial spread of linezolid-resistant but vancomycin-susceptible E. faecium was detected and these strains also emerged in patients without prior drug exposure. Linezolid should therefore be used with caution and the susceptibility of isolates monitored over time. Isolation precautions and screening of contacts should be considered to avoid spread of resistant isolates.

Detection of mutations conferring resistance to linezolid in Enterococcus spp. by fluorescence in situ hybridization

A fluorescence in situ hybridization (FISH) assay was established to detect linezolid resistance (conferred by the mutation 2576G>T in the gene coding for the 23 string of the ribosomal RNA) in enterococci. The assay was evaluated with 106 Enterococcus isolates; it showed a sensitivity of 100% for the detection of phenotypic resistance and was even able to identify a single mutated allele in phenotypically linezolid-susceptible isolates.

Nosocomial spread of Enterococcus faecium resistant to vancomycin and linezolid in a tertiary care medical center

In May 2004 our institution encountered its first clinical isolate of linezolid-resistant, vancomycin-resistant Enterococcus faecium (LRVRE). Between October 2004 and July 2005, 40 patients from whom LRVRE organisms were recovered in clinical specimens were characterized. Epidemiologic investigation and pulsed-field gel electrophoresis patterns indicated a clonal outbreak related to nosocomial spread.