MRSA and VRE colonization in solid organ transplantation: a meta-analysis of published studies.

The burden of methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococcus (VRE) colonization among the increasing number of solid organ transplant patients has not been systematically explored. We searched PubMed and EMBASE for pertinent articles, performed a meta-analysis of prevalence across eligible studies and estimated the risk of ensuing MRSA or VRE infections relative to colonization status. We stratified effects in the pretransplant and posttransplant period. Twenty-three studies were considered eligible. Seventeen out of 23 (74%) referred to liver transplants. Before transplantation, the pooled prevalence estimate for MRSA and VRE was 8.5% (95% confidence interval [CI] 3.2­15.8) and 11.9% (95% CI 6.8­18.2), respectively. MRSA estimate was influenced by small studies and was lower (4.0%; 95% CI 0.4­10.2) across large studies (>200 patients). After transplantation, the prevalence estimates were 9.4% (95% CI 3.0­18.5) for MRSA and 16.2% (95% CI 10.7­22.6) for VRE. Pretransplant as well as posttransplant MRSA colonization significantly increased the risk for MRSA infections (pooled risk ratio [RR] 5.51; 95% CI 2.36­12.90 and RR 10.56; 95% CI 5.58­19.95, respectively). Pretransplant and posttransplant VRE colonization were also associated with significant risk of VRE infection (RR 6.65; 95% CI 2.54­17.41 and RR 7.93; 95% CI 2.36­26.67, respectively). Solid organ transplantation is a high-risk setting for MRSA and VRE colonization, and carrier state is associated with infection. Upgraded focus in prevention and eradication strategies is warranted.

Multidrug-resistant, extensively drug-resistant and pandrug-resistant bacteria: an international expert proposal for interim standard definitions for acquired resistance.

Many different definitions for multidrug-resistant (MDR), extensively drug-resistant (XDR) and pandrug-resistant (PDR) bacteria are being used in the medical literature to characterize the different patterns of resistance found in healthcare-associated, antimicrobial-resistant bacteria. A group of international experts came together through a joint initiative by the European Centre for Disease Prevention and Control (ECDC) and the Centers for Disease Control and Prevention (CDC), to create a standardized international terminology with which to describe acquired resistance profiles in Staphylococcus aureus, Enterococcus spp., Enterobacteriaceae (other than Salmonella and Shigella), Pseudomonas aeruginosa and Acinetobacter spp., all bacteria often responsible for healthcare-associated infections and prone to multidrug resistance. Epidemiologically significant antimicrobial categories were constructed for each bacterium. Lists of antimicrobial categories proposed for antimicrobial susceptibility testing were created using documents and breakpoints from the Clinical Laboratory Standards Institute (CLSI), the European Committee on Antimicrobial Susceptibility Testing (EUCAST) and the United States Food and Drug Administration (FDA). MDR was defined as acquired non-susceptibility to at least one agent in three or more antimicrobial categories, XDR was defined as non-susceptibility to at least one agent in all but two or fewer antimicrobial categories (i.e. bacterial isolates remain susceptible to only one or two categories) and PDR was defined as non-susceptibility to all agents in all antimicrobial categories. To ensure correct application of these definitions, bacterial isolates should be tested against all or nearly all of the antimicrobial agents within the antimicrobial categories and selective reporting and suppression of results should be avoided.

Impact of short-course quinolone therapy on susceptible and resistant populations of Staphylococcus aureus.

We previously demonstrated that therapy duration has a differential impact on susceptible and resistant subpopulations of Staphylococcus aureus. What our previous investigation did not address was what would transpire after stopping therapy and whether these events would be different in susceptible and resistant subpopulations. We used the regimen previously demonstrated to amplify resistant subpopulation at day 4-5 (area under the concentration-time curve/minimum inhibitory concentration ratio, 100). Therapy was started in our hollow-fiber infection model on day 0; garenoxacin was administered in 4, 5, or 6 doses (days 3-5). The system was observed until day 13. Four drug doses kept the susceptible population dominant, but with the resistant subpopulation amplifying. Five and 6 doses caused the resistant population to exceed the susceptible population at the end of therapy and for a variable time thereafter. Ultimately, the susceptible population became dominant by day 13. Modeling demonstrated that the resistant isolates grew more slowly and had a higher natural death rate.

Antibiotics and gastrointestinal colonization by vancomycin-resistant enterococci.

Although several classes of antimicrobial agents have been associated with colonization or infection with glycopeptide-resistant enterococci (GRE) in individual clinical studies, the agents most commonly implicated are extended-spectrum cephalosporins and compounds with potent activity against anaerobic bacteria, including ticarcillin-clavulanic acid. In some clinical studies, formulary alterations designed to minimize the use of extended-spectrum cephalosporins or ticarcillin-clavulanic acid have resulted in significant decreases in colonization and infection by GRE. Experimental data using a mouse model of GRE gastrointestinal colonization indicate that persistence of high-level GRE colonization of the mouse gastrointestinal tract is promoted by exposure to agents with potent activity against anaerobic bacteria, suggesting that reduction of competing flora is the major factor leading to persistence of high-level colonization. One study performed in humans is consistent with this model and suggests that high levels of colonization may promote spread of resistant organisms in the nosocomial setting. Establishing colonization with GRE in uncolonized mice correlates with exposure to agents that are (a) secreted into the bile in significant concentrations and (b) have negligible activity against the colonizing enterococcal strain. Differences between piperacillin-tazobactam and ceftriaxone in the establishment model can be attributed directly to differences in their anti-enterococcal activity. Modification of antimicrobial prescribing practices may play an important role in facilitating successful infection control efforts to limit GRE in the nosocomial setting.

Gene dosage and linezolid resistance in Enterococcus faecium and Enterococcus faecalis.

Resistance to linezolid has been associated with a G2576U mutation in domain V of the 23S rRNA. We analyzed nine clinical isolates of linezolid-resistant enterococci and showed a clear association between the number of 23S rRNA genes containing this mutation and the level of linezolid resistance expressed.

Association of different mobile elements to generate novel integrative elements.

Among the more important problems in modern hospitals is the prevalence of bacterial pathogens expressing resistance to multiple antimicrobial agents. The frequency of multiresistance suggests mechanisms by which bacterial species can concentrate and efficiently exchange a variety of resistance determinants. Mechanisms by which this occurs include insertion of transposons within transposons, coalescence through the activity of insertion sequences and the employment of integrons. In some instances, more than one of these mechanisms is involved in creating large multiresistance genetic elements. The association of the elements with transferable elements or transposons may promote rapid dissemination among clinical strains, and create further opportunities for inclusion of additional resistance determinants.

Inhibition of vancomycin-resistant enterococci by an in vitro continuous-flow competitive exclusion culture containing human stool flora.

An in vitro anaerobic continuous-flow competitive exclusion (CFCE) culture model was used to study the ability of human stool flora to inhibit the growth of vancomycin-resistant (VR) enterococci (VRE). The CFCE culture was established from a stool sample obtained from a healthy adult. When 10(3) or 10(6) cfu/mL of VR Enterococcus faecium were added to the CFCE culture, the VRE were eliminated within 6 or 9 days, respectively. When 10(7) cfu/mL of the CFCE culture was added to a continuous-flow culture that contained 10(7) cfu/mL of VRE, the density of VRE was reduced but not eliminated. These data support the hypothesis that the indigenous intestinal flora inhibit growth of VRE and suggest that CFCE cultures may be a useful means to study interactions between the indigenous flora and VRE.

Outcome of cephalosporin treatment for serious infections due to apparently susceptible organisms producing extended-spectrum beta-lactamases: implications for the clinical microbiology laboratory.

Although extended-spectrum beta-lactamases (ESBLs) hydrolyze cephalosporin antibiotics, some ESBL-producing organisms are not resistant to all cephalosporins when tested in vitro. Some authors have suggested that screening klebsiellae or Escherichia coli for ESBL production is not clinically necessary, and when most recently surveyed the majority of American clinical microbiology laboratories did not make efforts to detect ESBLs. We performed a prospective, multinational study of Klebsiella pneumoniae bacteremia and identified 10 patients who were treated for ESBL-producing K. pneumoniae bacteremia with cephalosporins and whose infecting organisms were not resistant in vitro to the utilized cephalosporin. In addition, we reviewed 26 similar cases of severe infections which had previously been reported. Of these 36 patients, 4 had to be excluded from analysis. Of the remaining 32 patients, 100% (4 of 4) patients experienced clinical failure when MICs of the cephalosporin used for treatment were in the intermediate range and 54% (15 of 28) experienced failure when MICs of the cephalosporin used for treatment were in the susceptible range. Thus, it is clinically important to detect ESBL production by klebsiellae or E. coli even when cephalosporin MICs are in the susceptible range (

Emergence of vancomycin-resistant enterococci.

Vancomycin and ampicillin resistance in clinical Enterococcus faecium strains has developed in the past decade. Failure to adhere to strict infection control to prevent the spread of these pathogens has been well established. New data implicate the use of specific classes of antimicrobial agents in the spread of vancomycin-resistant enterococci (VRE). Extended-spectrum cephalosporins and drugs with potent activity against anaerobic bacteria may promote infection and colonization with VRE and may exert different effects on the initial establishment and persistence of high-density colonization. Control of VRE will require better understanding of the mechanisms by which different classes of drugs promote gastrointestinal colonization.

Penicillin-binding protein 5 and expression of ampicillin resistance in Enterococcus faecium.

We report a structural and transcriptional analysis of the pbp5 region of Enterococcus faecium C68. pbp5 exists within a larger operon that includes upstream open reading frames (ORFs) corresponding to previously reported psr (penicillin-binding protein synthesis repressor) and ftsW (whose product is a transmembrane protein that interacts with PBP3 in Escherichia coli septum formation) genes. Hybridization of mRNA from C68, CV133, and four ampicillin-resistant CV133 mutants revealed four distinct transcripts from this region, consisting of (i) E. faecium ftsW (ftsW(Efm)) alone; (ii) psr and pbp5; (iii) pbp5 alone; and (iv) ftsW(Efm), psr, and pbp5. Quantities of the different transcripts varied between strains and did not always correlate with quantities of PBP5 or levels of ampicillin resistance. Since the psr of C68 is presumably nonfunctional due to an insertion of an extra nucleotide in the codon for the 44th amino acid, the region extending from the ftsW(Efm) promoter through the pbp5 gene of C68 was cloned in E. coli to facilitate mutagenesis. The psr ORF was regenerated using site-directed mutagenesis and introduced into E. faecium D344-SRF on conjugative shuttle vector pTCV-lac (pCWR558 [psr ORF interrupted]; pCWR583 [psr ORF intact]). Ampicillin MICs for both D344-SRF(pCWR558) and D344-SRF(pCWR583) were 64 microg/ml. Quantities of pbp5 transcript and protein were similar in strains containing either construct regardless of whether they were grown in the presence or absence of ampicillin, arguing against a role for PSR as a repressor of pbp5 transcription. However, quantities of psr transcript were increased in D344-SRF(pCWR583) compared to D344-SRF(pCWR558), especially after growth in ampicillin; suggesting that PSR acts in some manner to activate its own transcription.

Antimicrobial pressure and the emergence and spread of multiresistant enterococci.

Multiresistant enterococci persist as important pathogens, particularly in US tertiary care hospitals. The emergence of vancomycin and ampicillin resistance in clinical Enterococcus faecium strains and the ascension of this species as an important nosocomial pathogen are among the more remarkable developments in the past decade. This emergence and spread is due to both the failure to adhere to well-established infection control measures and the persistent selective antibiotic pressure exerted in tertiary care institutions. Accumulating data specifically implicate the use of extended-spectrum cephalosporins and drugs with potent activity against anaerobic bacteria in the spread of vancomycin-resistant enterococci (VRE). These antibiotics appear to exert different effects on the initial establishment or the persistence of high levels of VRE colonization. Successful strategies to control the spread of multiresistant enterococci will require a more detailed understanding of the specific impacts of antimicrobial pressures as well as renewed commitment to adherence to well-established infection control measures.

Rapid method of extraction and analysis of extended-spectrum beta-lactamases from clinical strains of Klebsiella pneumoniae.

The extraction of periplasmic beta-lactamases from Gram-negative bacilli is a necessary preliminary step to analytical isoelectric focusing. Previously described methods are time-consuming and require large amounts of broth. We describe a lysozyme-based method which needs just 5 mL broth and requires less than 24 h to perform. The method was reproducible in extracting beta-lactamases from reference strains containing known beta-lactamases. We applied the method to a collection of more than 70 extended-spectrum beta-lactamase-producing isolates from a multinational study of bacteremic isolates of Klebsiella pneumoniae. Further studies are being undertaken to assess the method's applicability to other bacterial species.

Mobilization of transposons : rationale and techniques for detection.

The ability to share genetic information with other bacteria represents one of the most important adaptive mechanisms available to bacteria pathogenic for humans. The exchange of many different types of genetic information appears to occur frequently and exchange of determinants responsible for antimicrobial resistance is the best studied, since the movements of resistance determinants are easy to follow and the clinical importance of resistance dissemination is so great. The most common vehicles by which bacteria exchange resistance determinants are plasmids and transposons.

Bacterial monopolists: the bundling and dissemination of antimicrobial resistance genes in gram-positive bacteria.

Antibiotic resistance is the unavoidable result of our placing selective pressure on the microbial community. Advances in molecular biology techniques in the past 2 decades have allowed us to greatly improve our understanding of the mechanisms by which resistance emerges and disseminates among human pathogenic bacteria. Gram-positive bacteria employ a diverse array of elements, including plasmids, transposons, insertion sequences, and bacteriophages, to disseminate resistance. An understanding of these mechanisms and their prevalence can improve our ability to treat clinical infections in hospitalized patients, as well as to predict and control the spread of resistant bacteria in the nosocomial environment.

Hydrocephalus in coccidioidal meningitis: case report and review of the literature.

OBJECTIVE AND IMPORTANCE: Coccidioidomycosis was once confined to the southwest United States and northern Mexico. It has become a larger concern because of the concentration of military bases in these areas, the increasing mobility of populations, and the rising population of immunocompromised persons. Outside endemic areas, the diagnosis is rarely considered. Patients with coccidioidomycosis may develop occult basilar meningitis progressing to communicating hydrocephalus and death. CLINICAL PRESENTATION: A 60-year-old white man presented with a 1-month history of vertigo, falls, and vomiting. Computed tomography of the head revealed mild hydrocephalus. Lumbar puncture results were remarkable for 1065 mg/dl protein; acid-fast bacillus stain, Gram's stain, and culture results were negative. Postgadolinium magnetic resonance imaging demonstrated enhancement of basilar and cervical meninges, suggesting inflammation, and communicating hydrocephalus. For 48 hours, the patient's level of consciousness decreased progressively. INTERVENTION: A ventriculoperitoneal shunt was placed, and antifungal agents were initiated on an emergent basis. CONCLUSION: Coccidioidomycosis should be considered in the differential diagnosis of occult basilar meningitis. The diagnosis is established by the discovery of a high (>1:2) titer of complement-fixing antibody in the cerebrospinal fluid. Communicating hydrocephalus is a common complication of untreated coccidioidal meningitis, and it may develop during appropriate treatment (oral fluconazole, 200-400 mg/d, continued indefinitely). Patients with hydrocephalus and evidence of increased intracranial pressure require a shunt.

Effect of parenteral antibiotic administration on the establishment of colonization with vancomycin-resistant Enterococcus faecium in the mouse gastrointestinal tract.

A mouse model of intestinal colonization with vancomycin-resistant enterococci (VRE) was used to study the effect of different beta-lactam antibiotics on establishment of VRE colonization. A clinical VanB VRE isolate, Enterococcus faecium C68 (102 or 104 cfu), was inoculated by gastric gavage in conjunction with subcutaneous administration of antibiotics. The MIC of ceftriaxone and ticarcillin against VRE strain C68 is >10,000 microg/mL, and the MIC of piperacillin is 1250 microg/mL. Ceftriaxone and ticarcillin-clavulanate treatment groups developed persistently high levels of stool VRE compared with both the saline and the piperacillin-tazobactam (Pip-Taz) groups (P<.008). The level of stool VRE in the Pip-Taz group did not differ from that for the saline group. Thus, in this mouse model, beta-lactam antibiotics with minimal anti-enterococcal activity promoted establishment of high-level VRE colonization, but Pip-Taz (a beta-lactam antibiotic with more potent activity against VRE) did not.

Geographic distribution of a large mobile element that transfers ampicillin and vancomycin resistance between Enterococcus faecium strains.

In several clonally unrelated VanB-type vancomycin-resistant Enterococcus faecium strains, we demonstrated a common physical relationship between pbp5 and Tn5382 as well as common mutations within pbp5. The majority of these strains transferred vancomycin and ampicillin resistance to E. faecium in vitro, suggesting the dissemination of similar transferable pbp5-vanB-containing mobile elements throughout the United States.

The microbial genetics of antibiotic cycling.

Cycling of currently available antibiotics to reduce resistance is an attractive concept. For cycling strategies to be successful, their implementation must have a demonstrable impact on the prevalence of resistance determinants already dispersed throughout the hospital and associated healthcare facilities. While antibiotic use in hospitals clearly constitutes a stimulus for the emergence of resistance, it is by no means the only important factor. The incorporation of resistance determinants into potentially stable genetic structures, including bacteriophages, plasmids, transposons, and the more newly discovered movable elements termed integrons and gene cassettes, forces some degree of skepticism about the potential for such strategies in institutions where resistance determinants are already prevalent. In particular, the expanding role of integrons may pose an ultimate threat to formulary manipulations such as cycling. Despite these concerns, the crisis posed by antimicrobial resistance warrants investigation of any strategy with the potential for reducing the prevalence of resistance. Over the next decade, new studies with carefully designed outcomes should determine the utility of antibiotic cycling as one control measure for nosocomial resistance.

High-level expression of chromosomally encoded SHV-1 beta-lactamase and an outer membrane protein change confer resistance to ceftazidime and piperacillin-tazobactam in a clinical isolate of Klebsiella pneumoniae.

We describe Klebsiella pneumoniae 15571, a clinical isolate resistant to ceftazidime MIC = 32 microg/ml) and piperacillin-tazobactam (MICs = 1,024 and 128 microg/ml). K. pneumoniae 15571 expresses a single beta-lactamase with a pI of 7.6. However, when cloned in a high-copy-number vector in Escherichia coli, this bla(SHV-1) gene did not confer resistance to ceftazidime, a spectrum consistent with the nucleotide sequence, which was nearly identical to those of previously described bla(SHV-1) genes. Outer membrane protein (OMP) analysis of K. pneumoniae 15571 revealed a decrease in the quantity of a minor 45-kDa OMP in comparison to that in K. pneumoniae 44NR, a low-level ampicillin-resistant strain that also expresses a chromosomally determined bla(SHV-1). Crude beta-lactamase enzyme extracts from K. pneumoniae 15571 produced roughly 200-fold more beta-lactamase activity than K. pneumoniae 44NR. Northern hybridization analysis revealed that this difference was explainable by quantifiable differences in transcription of the bla(SHV-1) gene in the two strains. Primer extension analysis of bla(SHV-1) mRNA from K. pneumoniae 15571 and 44NR indicated that the transcriptional start sites were identical in the two strains. DNA sequencing of the promoter regions upstream of the of bla(SHV-1) open reading frames in the two K. pneumoniae strains revealed an A-->C change in the second position of the -10 region in K. pneumoniae 44NR compared to that in 15571. Site-directed mutagenesis of the cloned K. pneumoniae 15571 bla(SHV-1), in which the A in the second position of the 15571 -10 region was changed to a C, resulted in a substantial lowering of the MIC of ampicillin. When the levels of beta-lactamase enzyme expression in E. coli were compared, the bla(SHV-1) downstream of the altered -10 region produced 17-fold less beta-lactamase enzyme. These results indicate that elevated levels of ceftazidime resistance can result from a combination of increased enzyme production and minor OMP changes and that levels of chromosomally encoded SHV-1 beta-lactamase production can vary substantially with a single-base-pair change in promoter sequence.

Therapeutic considerations in the treatment of respiratory infections caused by ceftazidime-resistant Klebsiella pneumoniae.

Klebsiella pneumoniae are important human pathogens, particularly as causes of nosocomial respiratory tract infections. Intrinsically resistant to ampicillin, in recent years K. pneumoniae strains have acquired resistance to a broad variety of extended-spectrum cephalosporins. This resistance is most commonly mediated by the extended-spectrum beta-lactamases (ESBLs), plasmid-mediated enzymes that have evolved through point mutations in the genes encoding the more susceptible penicillinases TEM-1 and SHV-1. In a small minority of cases, K. pneumoniae have also been found to express extended-spectrum cephalosporin resistance by the elaboration of plasmid-mediated AmpC-type enzymes. These mutant enzymes confer resistance to extended-spectrum cephalosporins, penicillins, and in some cases cefamycins or beta-lactam-beta-lactamase inhibitor combinations. The most reliable and effective antimicrobial treatment of infections caused by these strains are the carbapenems imipenem and meropenem.