Donald Morrison

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.

Comparison of PCR with Phenotypic Methods for the Speciation of Enterococci

In recent years, enterococci have emerged as an important cause of hospital infections, particularly in patients with serious underlying disease. There are currently 17 recognized species in the genus Enterococcus, although E. faecalis and E. faecium usually account for over 90% of clinical isolates. The LHI receives approximately 800 enterococci annually for speciation, which has been undertaken using biochemical and other phenotypic tests (eg. motility and pigment production). We have recently evaluated a multiplex PCR assay designed to allow the identification of E. faecalis, E. faecium, E. gallinarum and E. casseliflavus/E. flavescens 1. This assay is based upon amplification of regions of the species-specific genes encoding production either of D-alanyl-D-alanine ligase (ddl E. faecalis or ddl E. faecium ) or the alternative ligase responsible for intrinsic vancomycin resistance (vanC-1 in E. gallinarum and vanC-2/3 in E. casseliflavus/E. flavescens).

PCR Typing of Enterococcus faecium

In the last two decades enterococci, especially E. faecium, have emerged as a major cause of nosocomial infection. A knowledge of their epidemiology within the hospital environment is crucial for the implementation of effective infection control measures. Hence, in recent years much effort has been invested in evaluating various typing methods for use in these studies. These include serotyping, phage typing, enterococcine typing, biotyping, analysis of whole or digested plasmid DNA, digestion of amplified fragments of glycopeptide resistance genes, restriction endonuclease analysis of chromosomal DNA, ribotyping, IS6770 probing and various pulsed-field gel electrophoresis techniques (PFGE). Of these methods PFGE is becoming the method of choice and is regarded by some as the gold standard against which other methods should be assessed. However, the down side of PFGE is the expensive equipment required and the length of time (usually about five days) before results are available. For these reasons attention has turned to PCR typing methods, where equipment is relatively cheaper, the method is simple to perform and results are obtained within two days. Two PCR based methods have been applied to type E. faecium with varying success. PCR-ribotyping, although initially promising (4), is now not considered a useful option (1, 5). More recently Issack (3) applied one out of 26 short (10bp) primers screened to study the epidemiology of an outbreak of VRE at a teaching hospital. The method appeared useful and reasonably good reproducibility was achieved. However, no comparison with other typing methods was undertaken as in this study, where we have evaluated longer primers (15–23bp) and compared our results with those obtained by ribotyping and PFGE.

Dynamics of Enterococcus faecalis Colonization of Bone Marrow Transplant Patients

Enterococci have become increasingly important as a cause of hospital acquired infections. They are reported to be the third commonest cause of hospital acquired infections, responsible for approximately 10.7% of such infections6. Furthermore, enterococci have developed resistance to the antibiotics currently in use4. Up to now, few studies have investigated the dynamics of enterococcal colonization and this would appear to be a relevant step toward a better understanding of antibiotic-resistant enterococcal (ARE) colonization and infection. Many of the difficulties encountered in the study of enterococcal epidemiology have been due to the lack of a good typing method. But this has been aided in recent years by the introduction of DNA-based typing methods2, 7, 8, 9. In order to further our knowledge of enterococcal epidemiology we have studied the dynamics of E. faecalis colonization in bone marrow transplant patients using four typing methods.