Plasmid-mediated vanB glycopeptide resistance in enterococci

Enterococci as a One Health indicator of antimicrobial resistance

The rapid increase of antimicrobial-resistant bacteria in humans and livestock is concerning. Antimicrobials are essential for the treatment of disease in modern day medicine, and their misuse in humans and food animals has contributed to an increase in the prevalence of antimicrobial-resistant bacteria. Globally, antimicrobial resistance is recognized as a One Health problem affecting humans, animals, and environment. Enterococcal species are Gram-positive bacteria that are widely distributed in nature. Their occurrence, prevalence, and persistence across the One Health continuum make them an ideal candidate to study antimicrobial resistance from a One Health perspective. The objective of this review was to summarize the role of enterococci as an indicator of antimicrobial resistance across One Health sectors. We also briefly address the prevalence of enterococci in human, animal, and environmental settings. In addition, a 16S RNA gene-based phylogenetic tree was constructed to visualize the evolutionary relationship among enterococcal species and whether they segregate based on host environment. We also review the genomic basis of antimicrobial resistance in enterococcal species across the One Health continuum.

Enterococcal Genetics

The study of the genetics of enterococci has focused heavily on mobile genetic elements present in these organisms, the complex regulatory circuits used to control their mobility, and the antibiotic resistance genes they frequently carry. Recently, more focus has been placed on the regulation of genes involved in the virulence of the opportunistic pathogenic species Enterococcus faecalis and Enterococcus faecium. Little information is available concerning fundamental aspects of DNA replication, partition, and division; this article begins with a brief overview of what little is known about these issues, primarily by comparison with better-studied model organisms. A variety of transcriptional and posttranscriptional mechanisms of regulation of gene expression are then discussed, including a section on the genetics and regulation of vancomycin resistance in enterococci. The article then provides extensive coverage of the pheromone-responsive conjugation plasmids, including sections on regulation of the pheromone response, the conjugative apparatus, and replication and stable inheritance. The article then focuses on conjugative transposons, now referred to as integrated, conjugative elements, or ICEs, and concludes with several smaller sections covering emerging areas of interest concerning the enterococcal mobilome, including nonpheromone plasmids of particular interest, toxin-antitoxin systems, pathogenicity islands, bacteriophages, and genome defense.

Evolutionary dynamics of Enterococcus faecium reveals complex genomic relationships between isolates with independent emergence of vancomycin resistance

Enterococcus faecium, a major cause of hospital-acquired infections, remains problematic because of its propensity to acquire resistance to vancomycin, which currently is considered first-line therapy. Here, we assess the evolution and resistance acquisition dynamics of E. faecium in a clinical context using a series of 132 bloodstream infection isolates from a single hospital. All isolates, of which 49 (37 %) were vancomycin-resistant, underwent whole-genome sequencing. E. faecium was found to be subject to high rates of recombination with little evidence of sequence importation from outside the local E. faecium population. Apart from disrupting phylogenetic reconstruction, recombination was frequent enough to invalidate MLST typing in the identification of clonal expansion and transmission events, suggesting that, where available, whole-genome sequencing should be used in tracing the epidemiology of E. faecium nosocomial infections and establishing routes of transmission. Several forms of the Tn1549-like element–vanB gene cluster, which was exclusively responsible for vancomycin resistance, appeared and spread within the hospital during the study period. Several transposon gains and losses and instances of in situ evolution were inferred and, although usually chromosomal, the resistance element was also observed on a plasmid background. There was qualitative evidence for clonal expansions of both vancomycin-resistant and vancomycin-susceptible E. faecium with evidence of hospital-specific subclonal expansion. Our data are consistent with continuing evolution of this established hospital pathogen and confirm hospital vancomycin-susceptible and vancomycin-resistant E. faecium patient transmission events, underlining the need for careful consideration before modifying current E. faecium infection control strategies.

Conjugative transposition of the vancomycin resistance carrying Tn1549: enzymatic requirements and target site preferences

Rapid spread of resistance to vancomycin has generated difficult to treat bacterial pathogens worldwide. Though vancomycin resistance is often conferred by the conjugative transposon Tn1549, it is yet unclear whether Tn1549 moves actively between bacteria. Here we demonstrate, through development of an in vivo assay system, that a mini-Tn1549 can transpose in E. coli away from its natural Gram-positive host. We find the transposon-encoded INT enzyme and its catalytic tyrosine Y380 to be essential for transposition. A second Tn1549 protein, XIS is important for efficient and accurate transposition. We further show that DNA flanking the left transposon end is critical for excision, with changes to nucleotides 7 and 9 impairing movement. These mutations could be partially compensated for by changing the final nucleotide of the right transposon end, implying concerted excision of the two ends. With changes in these essential DNA sequences, or without XIS, a large amount of flanking DNA transposes with Tn1549. This rescues mobility and allows the transposon to capture and transfer flanking genomic DNA. We further identify the transposon integration target sites as TTTT-N6-AAAA. Overall, our results provide molecular insights into conjugative transposition and the adaptability of Tn1549 for efficient antibiotic resistance transfer.

Occurrence of vancomycin-resistant enterococci in turkey flocks

In the present study, the prevalence of vancomycin-resistant enterococci (VRE) in turkeys in the southwest of Germany was investigated. For this purpose, 200 cloacal swab samples and 5 environmental dust samples (tested as a pooled sample) of each of the 20 flocks (10 female and 10 male flocks) included in this study were examined. The VRE could be isolated by means of a procedure combining bacterial cultivation in an enrichment broth and on a selective solid media. Enterococci were identified biochemically and subsequently tested on the presence of the vancomycin resistance genes vanA, vanB (B1/B2/B3), and vanC (C1/C2/C3) using real-time PCR assays. In 54 (27%) turkeys originating from 11 (55%) flocks and in 14 (70%) of the dust samples, exclusively vanA and vanC1 genes could be detected. Of the turkeys examined, 46 were colonized with VRE bearing the resistance gene vanC1 and 8 vanA, originating from 9 and 2 flocks, respectively. None of the birds carried vanB, vanC2, or vanC3 positive VRE. The results obtained from the birds are largely confirmed by the dust samples originating from 4 vanA and 10 vanC1 positive flocks. However, one flock housing animals colonized with vanC1 positive VRE could not be confirmed by the dust samples that revealed vanA bearing VRE. However, in one case vanA and in 3 cases vanC1 carrying VRE could be detected in dust samples of the turkey houses, but not in the turkeys of the associated flock. In 5 flocks the turkeys as well as the dust samples were free of VRE.

Mobile genetic elements and their contribution to the emergence of antimicrobial resistant Enterococcus faecalis and Enterococcus faecium

Mobile genetic elements (MGEs) including plasmids and transposons are pivotal in the dissemination and persistence of antimicrobial resistance in Enterococcus faecalis and Enterococcus faecium. Enterococcal MGEs have also been shown to be able to transfer resistance determinants to more pathogenic bacteria such as Staphylococcus aureus. Despite their importance, we have a limited knowledge about the prevalence, distribution and genetic content of specific MGEs in enterococcal populations. Molecular epidemiological studies of enterococcal MGEs have been hampered by the lack of standardized molecular typing methods and relevant genome information. This review focuses on recent developments in the detection of MGEs and their contribution to the spread of antimicrobial resistance in clinically relevant enterococci.

Transfer of plasmid and chromosomal glycopeptide resistance determinants occurs more readily in the digestive tract of mice than in vitro and exconjugants can persist stably in vivo in the absence of glycopeptide selection

OBJECTIVES AND METHODS

The transferability of vanA and vanB glycopeptide resistance determinants with a defined plasmid (n = 9) or chromosomal (n = 4) location between Enterococcus faecium strains of human and animal origins was compared using filter mating (in vitro) and germ-free mice (in vivo) as experimental models. Moreover, the stability of exconjugants in vivo in the absence of antibiotic selection was examined.

RESULTS

Higher transfer rates were observed in vivo for four of six vanA and five of six vanB donor strains. For plasmid-encoded resistance, several log higher transfer frequencies were observed in vivo for some strains. Moreover, the in vivo model supported transfer of plasmid-encoded vanB (1 x 10(-7) exconjugants/donor) when repeated in vitro experiments were negative (estimated < 1 x 10(-9) exconjugants/donor). Readily detectable transfer of plasmid-located vanA and vanB as well as large chromosomal (>200 kb) vanB elements was observed after 24 h. The number of plasmid-mediated vanA exconjugants generally decreased markedly after 3 days. However, exconjugants containing a plasmid harbouring the vanA transposon Tn1546 linked to the post-segregational killing system omega-epsilon-zeta persisted stably in vivo in the absence of glycopeptides for more than 20 days.

CONCLUSIONS

The overall results support the notion that the in vitro model underestimates the transfer potential. Rapid transfer of vanA plasmids from poultry- and pig-derived strains to human faecal E. faecium shows that even transiently colonizing strains may provide a significant reservoir for transfer of resistance genes to the permanent commensal flora. Newly acquired resistance genes may be stabilized and persist in new populations in the absence of antibiotic selection.

Enterococcus gallinarum carrying the vanA gene cluster: first report in Brazil

In 2000, Enterococcus faecalis resistant to vancomycin was first reported at a tertiary hospital in Porto Alegre, southern Brazil. The resistance spread to other hospitals and surveillance programs were established by hospital infection committees to prevent the spread of vancomycin-resistant enterococci. In February 2002, an isolate initially identified at the genus level as Enterococcus was obtained by surveillance culture (rectal swab) from a patient admitted to a hospital for treatment of septic arthritis in the shoulder. The isolate proved to be resistant to vancomycin by the disc diffusion method and confirmed by an E-test resulting in a minimal inhibitory concentration of > or = 256 microg/ml. This isolate was sent to a reference laboratory (Laboratorio Especial de Bacteriologia e Epidemiologia Molecular, Faculdade de Ciencias Farmaceuticas de Ribeirao Preto, USP) for further study and proved to be an E. gallinarum by the polymerase chain reaction (PCR) using specific primers for the species. Due to the phenotype of unusually high vancomycin resistance, the isolate presumably had the resistance genes (vanA and vanB) and this was confirmed by PCR, which indicated the presence of the vanA gene. A 10.8-kb Tn1546-related transposon was also identified by long-PCR. Interspecies transfer of the vancomycin-resistance gene from the donor E. gallinarum was performed in a successful conjugation experiment in vitro, using E. faecium GE-1 and E. faecalis JH22 as receptors. This is the first report of the detection of a vanA determinant naturally acquired by E. gallinarum in Brazil, indicating the importance of characterizing VRE by both phenotype and genotype methods.

Transferable vanB2 Tn5382-containing elements in fecal streptococcal strains from veal calves

Three vancomycin-resistant veal calf fecal streptococci, identified as Streptococcus gallolyticus (n = 2) and Streptococcus lutetiensis, were shown to harbor vanB2 Tn5382-like elements earlier described in enterococci. One S. gallolyticus strain had a 1,495-bp IS256-related element inserted in vanS(B). The vanB2 Tn5382 element present in the plasmid-free S. lutetiensis strain was transferable to Enterococcus faecium BM4105-RF, Enterococcus faecalis JH2-2, and its recombination-deficient derivative, UV202. The transfer frequencies were comparable between recipient strains (from 1 x 10(-7) to 7 x 10(-6)). All transconjugants acquired a vanB-containing chromosomal insert of approximately 100 kb, apparently by site-specific integration. Secondary transconjugants were not observed in intraspecies retransfer experiments. These observations are consistent with a conjugative, selftransmissible, integrative element that might be involved in the interspecies spread of vanB2 resistance determinants. Two JH2-2-derived transconjugants had also gained additional copies of large vanB-containing chromosomal fragments, a process that involves unexplained mechanisms that seems to require functional host cell-dependent recombination mechanisms.

Epidemiology of the genetic elements responsible for acquired glycopeptide resistance in enterococci

Five genotypes of acquired glycopeptide resistance have been documented in enterococci, with vanA and vanB being the most globally widespread and prevalent. Resistance results from the production of peptidoglycan precursors with reduced binding affinity for glycopeptides and is encoded by complex clusters of van genes. The prototype VanA element is Tn1546, a 10.8-kb transposon that carries the vanRSHAXYZ genes. Diverse VanA elements exist, but all share the vanRSHAX cluster and are believed to be derived from a Tn1546 progenitor. The sequences of these genes are remarkably conserved, with only a few point mutations identified. VanA elements do however vary by the presence of deletions and insertion sequences (IS) in nonessential genes (orf1, orf2, vanY, and vanZ) and intergenic regions. IS transposition probably plays a key role in VanA element evolution. By contrast, vanB gene clusters show greater sequence divergence. Three vanB alleles have been reported, of which vanB2 appears to be the most widespread, generally as part of Tn5382 and related elements. To date, only four Enterococcus faecium strains with VanD resistance have been reported, and each contained a distinct vanD allele. The VanE and VanG types have each been identified in single strains of Enterococcus faecalis. The existence of distinct genotypes, together with the allelic nature of vanB and vanD, suggests that van clusters have transferred to enterococci on multiple occasions from undefined donor species, with subsequent horizontal dissemination, particularly of VanA and VanB elements, among enterococci. Characterization of glycopeptide resistance elements yields information on their evolution and broadens our insights into the epidemiology of resistant enterococci.

Comparison of agar-based media for primary isolation of glycopeptide-resistant enterococci

OBJECTIVE: To compare four vancomycin-containing agar media for the isolation of glycopeptide-resistant enterococci (GRE) from clinical fecal specimens: kanamycin---aesculin---azide (KAA) agar; bile---aesculin---polymixin (BAP) agar; aztreonam---amphotericin blood (CBAA) agar; and neomycin blood (CBN) agar. METHODS: Fecal specimens from 125 patients were inoculated onto each medium. Media were examined for enterococci after incubation for up to 48 h. Enterococci were identified to species level, and glycopeptide phenotypes were determined by measuring minimum inhibitory concentrations of vancomycin and teicoplanin. RESULTS: GRE were isolated from 44/125 samples. Enterococcus faecalis and Enterococcus faecium isolates, expressing glycopeptide resistance of the VanA or VanB phenotypes, were recovered from 27/33 (82%) specimens on BAP medium, 26/33 (79%) on KAA medium, and 21/33 (64%) on CBN and CBAA media. Enterococcus gallinarum and Enterococcus casseliflavus isolates expressing low-level glycopeptide resistance (VanC phenotype) were recovered from 14/15 (93%) specimens on CBAA medium, 7/15 (47%) on KAA and CBN media, and 6/15 (40%) on BAP medium. CONCLUSIONS: The media tested in this study, with the exception of CBN medium, detected at least 75% of patients colonized by GRE. Further development of BAP, CBAA and KAA media is warranted to improve growth and selectivity.

[Risk factors and prevention of the acquisition and transmission of glycopeptide resistant enterococci]

We reviewed the literature concerning the role of antibiotic use as a risk factor for glycopeptide-resistant enterococci (VRE) infection/colonisation, to enable us to develop measures for preventing the acquisition and transmission of VRE. We found that the length of stay, the number of stays in hospital and the transfer of patients between hospitals and units were all risk factors for acquiring VRE infection. However, analysis of group and individual data showed that there was also a clear link between vancomycin and third-generation cephalosporin use and the prevalence of VRE colonisation/infection. Evidence for this link was provided by the consistent association and dose-effect relationship observed, and from the frequently consistent variations observed over time. However, it is difficult to give precise recommendations because very few studies have investigated both intrinsic bacterial factors making specific strains more epidemic and the precise characteristics of the conditions determining antibiotic selection pressure. In the absence of this information, and maintaining the prevention measures against cross-contamination which remain a priority, these results suggest that programs aimed at improving the prescription of antibiotics should be initiated in hospitals.

Characterization of a divergent vanD-type resistance element from the first glycopeptide-resistant strain of Enterococcus faecium isolated in Brazil

Enterococcus faecium 10/96A from Brazil was resistant to vancomycin (MIC, 256 microg/ml) but gave no amplification products with primers specific for known van genotypes. A 2,368-bp fragment of a van cluster contained one open reading frame encoding a peptide with 83% amino acid identity to VanH(D), and a second encoding a D-alanine-D-lactate ligase with 83 to 85% identity to VanD. The divergent glycopeptide resistance phenotype was designated VanD4.

Genetic linkage of the vanB2 gene cluster to Tn5382 in vancomycin-resistant enterococci and characterization of two novel insertion sequences

VanB-type vancomycin resistance is encoded by the vanB gene cluster, which disseminates by horizontal gene transfer and clonal spread of vancomycin-resistant enterococci (VRE). Genetic linkage of the vanB gene cluster to transposon Tn5382 and the insertion sequences IS16 and IS256-like has previously been shown. In this study linkage of defined vanB gene cluster subtypes to these elements was examined. All the vanB2 subtype strains studied (n=14) revealed co-hybridization of vanB and Tn5382, whereas the strains of vanB1 (n=8) and vanB3 (n=1) subtypes were Tn5382 negative. Conjugative cotransfer of the vanB2 gene cluster and Tn5382 was demonstrated for two strains. DNA sequencing of the vanX(B)-ORFC region in vanB2 strains confirmed that the vanB2 gene cluster is an integral part of Tn5382. No general pattern of linkage was observed with regard to IS16 and IS256-like. Two novel insertion sequences were identified in specific vanB2 subtype strains. (i) A 1611 bp element (ISEnfa110) was detected in the left flank of Tn5382. Its insertion site, lack of terminal inverted and direct repeats, and two conserved motifs in its putative transposase all conform to the conventions of the IS110 family. (ii) A 787 bp element (ISEnfa200) was detected in the vanS(B)-vanY(B) intergenic region. Its ORF encoded a putative protein with 60-70% identity to transposases of the IS200 family. No further copies of ISEnfa110 were found by colony hybridization of 181 enterococcal isolates, whereas ISEnfa200 was found in four additional vanB2 strains from the USA. The five strains had identical ISEnfa200 element insertion sites, and Tn5382 was located downstream from a pbp5 gene conferring high-level ampicillin resistance. These isolates showed related PFGE patterns, suggesting possible clonal spread of a VRE strain harbouring a Tn5382-vanB2-ISEnfa200 element linked to a pbp5 gene conferring ampicillin resistance.

Characterization of transposon Tn1549, conferring VanB-type resistance in Enterococcus spp

Transfer of VanB-type resistance to glycopeptides among enterococci has been reported to be associated with the movement of large chromosomal genetic elements or of plasmids. The authors report the characterization of the 34 kb transposon Tn1549 borne by a plasmid related to pAD1 and conferring vancomycin resistance in clinical isolates of Enterococcus spp. Tn1549 contained 30 ORFs and appeared to be organized like the Tn916 family of conjugative transposons into three functional regions: (i) the right end, implicated in the excision-integration process; (ii) the central part, in which the vanB2 operon replaces the tet(M) gene; and (iii) the left extremity, in which eight of the 18 ORFs could be implicated in the conjugative transfer.

Effects of the movement of insertion sequences on the structure of VanA glycopeptide resistance elements in Enterococcus faecium

A Tn1546-related element with IS1216V at position 8839 underwent a structural change after storage of the host strain of Enterococcus faecium at 4 degrees C. The element acquired IS1542 at position 3932, nucleotides 8732 to 8831 were deleted, and the first 3417 nucleotides were lost and replaced by an inverted copy of the IS1216V-vanY-vanZ-inverted-repeat block from the 3' end. Insertion sequence movement is likely to play a key role in the evolution of VanA resistance elements.

Role of transposon Tn5482 in the epidemiology of vancomycin-resistant Enterococcus faecium in the pediatric oncology unit of a New York City Hospital

During a 36-month period between 1993 and 1995 in the Pediatric Oncology Unit of Memorial Sloan Kettering Cancer Center, 74 patients experienced episodes of infection or colonization caused by vancomycin-resistant enterococci (VRE). Characterization of the 74 bacterial isolates by microbiological and molecular techniques (pulsed-field gel electrophoresis and hybridization with DNA probes specific for the vanA and vanB genes and for IS1251) identified 73 Enterococcusfaecium and one Enterococcusfaecalis (vanB) among the primary VRE isolates. Most (69/73) of the E. faecium isolates carried vanA and four isolates, the vanB gene complex. The overwhelming majority (67/69) of the vanA -positive isolates also gave hybridization signal for IS1251, indicating the presence of the newly described conjugative transposon Tn5482. No hybridization with IS1251 was obtained with the four vanB-carrying isolates. About 30% of the vanA-positive strains (23/69) were represented by PFGE subtype variants of a single clone, most isolates of which were recovered during a 4-month period between April to June of 1994. The larger portion of the vanA-carrying VRE represented by close to 70% of the isolates (46/69) belonged to as many as 37 different clonal types, indicating tremendous genetic diversity. Among 67 of the 69 vanA-carrying isolates, the localization of the Tn5482-associated vanA gene complex could be unequivocally identified either on the chromosome (40/69) or in plasmids (27/69). Transconjugants recovered from filter mating experiments using either a chromosomally located or plasmid-borne vanA donor strain and a single vancomycin-susceptible strain of either E. faecium or E. faecalis were analyzed by molecular typing techniques. Seven out of 10 independent transconjugants recovered from the same cross showed extensive differences in PFGE pattern and also in the localization of the vanA hybridizing DNA fragment transferred from the common VRE donor with chromosomally located vanA. The observations suggest that the extensive genetic diversity observed among the clinical isolates of VRE may be generated during conjugation between vancomycin-resistant and -susceptible enterococcal isolates. The observations also suggest that the epidemic spread of VRE in the United States may be linked to the frequent presence of Tn5482 among the American isolates.

vanA and vanB incorporate into an endemic ampicillin-resistant vancomycin-sensitive Enterococcus faecium strain: effect on interpretation of clonality

Clonal spread and horizontal transfer in the spread of vancomycin resistance genes were investigated. Multiplex PCR, pulsed-field gel electrophoresis (PFGE), hybridization of enterococcal plasmids with the vanA and vanB probes, and sequencing of a fragment of vanB were used in the analysis. Before May 1996, 12 vancomycin-resistant Enterococcus faecium (VRE) isolates were found in Finland. Between May 1996 and October 1997, 156 VRE isolates were found in the Helsinki area. Between December 1997 and April 1998, fecal samples from 359 patients were cultured for VRE. One new case of colonization with VRE was found. During the outbreak period, 88% (137 of 155) of the VRE isolates belonged to two strains (VRE types I and II), as determined by PFGE. Each VRE type I isolate possessed vanB, and five isolates also had vanA. Of the 34 VRE type II isolates, 27 possessed vanA and 7 possessed vanB. Fifteen of 21 (71%) ampicillin-resistant, vancomycin-sensitive E. faecium (VSE) isolates found during and after the outbreak period in one ward were also of type II. Two VSE type II isolates were found in the hospital before the outbreak in 1995. By PFGE, the three groups (vanA, vanB, or no van gene) of type II shared the same band differences with the main type of VRE type II with vanA. None of the differences was specific to or determinative for any of the groups. Our material suggests that vanA and vanB incorporate into an endemic ampicillin-resistant VSE strain.

Detection of glycopeptide-resistant enterococci in routine diagnostic faeces specimens

Faeces received in a diagnostic laboratory were screened for glycopeptide-resistant enterococci (GRE) on modified Lewisham medium, with and without enrichment in Enterococcosel broth. Colonization by GRE was detected in 102/838 patients (12.2%). In 74 (73%) of colonized patients GRE were detected by both methods and in 28 (27%) they were detected only after enrichment. The carriage rate in hospitalized patients was 32% (93/289) compared with 2.3% (11/425) in the community (GP patients and food-handlers). Carriage of GRE increased with age. Clostridium difficile isolation was associated with GRE colonization, odds ratio 6.76 (P<0.001). Fifty-nine percent (60/102) of the GRE had the VanA phenotype and 41% (42/102) had the VanB phenotype. In the community VanA predominated (91%), whereas 64% (57/89) of the isolates from hospitalised patients were of the VanB phenotype.

Vancomycin-resistant enterococci: recent advances in genetics, epidemiology and therapeutic options

Vancomycin-resistant enterococci (VRE) have gained much attention in the last decade. Currently, there are five known types of vancomycin resistance based on genes encoding ligase enzymes that the organisms use to produce their cell wall precursors, namely, VanA, VanB, VanC, VanD and VanE. An additional unclassified type was discovered in Australia. The basis of resistance among these phenotypes appears to be similar in that the resistant organisms produce peptidoglycan precursors that end in moieties other than D-alanyl-D-alanine, the usual target of vancomycin. The other dipeptide-like termini identified to date include D-alanyl-D-lactate and D-alanyl-D-serine, which have low affinity for glycopeptides. Recent evidence suggests that glycopeptide-producing organisms might be the remote origin of the vancomycin resistance genes. In European countries, avoparcin, a glycopeptide used in farm animals as a growth promoter, has been linked to the occurrence of VRE and occasional common strains have been identified in food products, farm animals, healthy subjects and hospitalized patients. There have been no such reports in the USA where heavy use of vancomycin and use of broad spectrum antibiotics such as cephalosporins have been identified as important risk factors for acquisition of VRE. Transmission within the same or between hospitals has been reported in many countries. Infection control measures and efforts to use antibiotics, particularly vancomycin, more appropriately have been implemented in a number of healthcare facilities with varying degrees of success. Many antibiotics, as a single agent or a combination of drugs, as well as various new antibiotics have been tested in vitro, in animal models, or used in anecdotal cases but clinical data from large comparative trials are not available to date. Because of the limited susceptibility of many VRE to other agents, efforts to control these organisms are particularly important. Copyright 1999 Harcourt Publishers LtdCopyright 1999 Harcourt Publishers Ltd.

Epidemiology and control of vancomycin-resistant enterococci (VRE) in a renal unit

This study reports an outbreak of infection and colonization caused by vancomycin-resistant enterococci (VRE) in the renal service of a large teaching hospital. The polymerase chain reaction and pulsed-field gel electrophoresis were used to study the epidemiology of 26/34 strains of vancomycin-resistant Enterococcus faecalis and Enterococcus faecium from the outbreak in comparison with five strains from other hospitals in Edinburgh and the Borders, and three from other wards in the Royal Infirmary. The study revealed a heterogeneous population of vancomycin-resistant E. faecalis. Over 60% of E. faecium isolates had matching pulsed-field gel electrophoresis patterns and all of these were of VanA phenotype. These results suggest that clonal spread of VanA phenotype E. faecium within and possibly between hospitals is the major vancomycin-resistant enterococcal problem in Edinburgh. Screening of patients and isolation of colonized and infected patients appear to have been successful in controlling the spread of VRE.

Strains of glycopeptide-resistant Enterococcus faecium can alter their van genotypes during an outbreak

Two isolates of Enterococcus faecium with VanA glycopeptide resistance were isolated during a hospital outbreak of E. faecium with plasmid-mediated VanB resistance. Both were found to be identical to the VanB outbreak strain by pulsed-field gel electrophoresis. The genotype of this strain changed from vanB to vanA through an intermediate isolate that contained both the vanA and vanB gene clusters on distinct plasmids.

Epidemiology of an outbreak due to glycopeptide-resistant Enterococcus faecium on a leukaemia unit

The clinical and molecular epidemiology of two clusters of colonization and infection of patients by glycopeptide-resistant enterococci (GRE) on a leukaemia and bone marrow transplantation unit was studied over a two-and-a half-year period. Thirty-five patients became colonized, of whom six developed clinical infections. Of the 53 isolates of GRE, 49 were Enterococcus faecium, multiply-resistant to vancomycin and ampicillin. DNA fingerprinting of 48 E. faecium isolates by pulsed-field gel electrophoresis identified six DNA types. One strain of VanB phenotype E. faecium predominated during the initial outbreak, and an unrelated strain of the VanA phenotype was present in a second cluster. Environmental and patient isolates of E. faecium were indistinguishable by DNA typing. The VanA phenotype enterococci probably arose by transfer from the renal ward at a nearby hospital, and a patient with persistent diarrhoea may have contributed to contamination and cross-infection. GRE may cause significant infections in immunocompromised patients, and are readily transmitted between them. GRE were controlled, but not eradicated on the unit; infection control measures included improved environmental cleaning and modification of antibiotic use. In order to control GRE, it is necessary to educate healthcare workers and implement the traditional, effective values of good personal hygiene and environmental cleanliness.