First nosocomial outbreak of Pseudomonas aeruginosa producing an integron-borne metallo-beta-lactamase (VIM-2) in the United States

Draft Genome Sequence of VIM-2-Producing Multidrug-Resistant Pseudomonas aeruginosa ST175, an Epidemic High-Risk Clone

The VIM-2-producing multidrug-resistant high-risk clone Pseudomonas aeruginosa sequence type (ST) 175 was isolated in the setting of a large outbreak in Hospital Universitario 12 de Octubre (Spain) from 2007 to 2010. This strain was resistant to all β-lactams, fluoroquinolones, and aminoglycosides, with the exception of amikacin, and has become an endemic clone in our institution.

Complete sequence of broad-host-range plasmid pNOR-2000 harbouring the metallo-β-lactamase gene blaVIM-2 from Pseudomonas aeruginosa

OBJECTIVES

Metallo-β-lactamases (MBLs) are increasingly reported not only in Enterobacteriaceae but also in Pseudomonas spp. These enzymes hydrolyse all β-lactams, including carbapenems, and are not inhibited by β-lactamase inhibitors. The aim of this study was to fully characterize a plasmid bearing the blaVIM-2 MBL gene identified in a Pseudomonas aeruginosa isolate.

METHODS

This plasmid was fully sequenced by high-density pyrosequencing and annotated using the GenDB version 2.0 annotation tool. The evaluation of the broad-host-range replication of the pNOR-2000 replication initiation gene was assessed using electro-transformation and conjugation assays and the distribution of this replicase gene was evaluated using an international collection of VIM-producing Pseudomonas spp.

RESULTS

Analysis of the 21 880 bp sequence of pNOR-2000 revealed a truncated and non-functional transfer operon, in addition to novel genes encoding a serine protease and toxin/antitoxin addiction systems. This broad-host-range plasmid shares high gene synteny with part of the mobile genomic island pKLC102 identified in P. aeruginosa strain C.

CONCLUSIONS

We report here the complete nucleotide sequence of plasmid pNOR-2000 from a P. aeruginosa clinical isolate harbouring the integron-located MBL gene blaVIM-2.

VIM-2-producing multidrug-resistant Pseudomonas aeruginosa ST175 clone, Spain

This clone is a major public health problem because it limits antimicrobial drug therapy.

A total of 183 patients were colonized or infected with multidrug-resistant Pseudomonas aeruginosa isolates at a hospital in Spain during 2007–2010; prevalence increased over this period from 2.8% to 15.3%. To characterize these isolates, we performed molecular epidemiologic and drug resistance analysis. Genotyping showed that 104 (56.8%) isolates belonged to a single major clone (clone B), which was identified by multilocus sequence typing as sequence type (ST) 175. This clone was initially isolated from 5 patients in 2008, and then isolated from 23 patients in 2009 and 76 patients in 2010. PCR analysis of clone B isolates identified the bla_VIM-2 gene in all but 1 isolate, which harbored bla_IMP-22. ST175 isolates were susceptible to only amikacin (75%) and colistin (100%). Emergence of the ST175 clone represents a major health problem because it compromises therapy for treatment of P. aeruginosa nosocomial infections.

Mechanisms of resistance and clinical relevance of resistance to β-lactams, glycopeptides, and fluoroquinolones

The widespread use of antibiotics has resulted in a growing problem of antimicrobial resistance in the community and hospital settings. Antimicrobial classes for which resistance has become a major problem include the β-lactams, the glycopeptides, and the fluoroquinolones. In gram-positive bacteria, β-lactam resistance most commonly results from expression of intrinsic low-affinity penicillin-binding proteins. In gram-negative bacteria, expression of acquired β-lactamases presents a particular challenge owing to some natural spectra that include virtually all β-lactam classes. Glycopeptide resistance has been largely restricted to nosocomial Enterococcus faecium strains, the spread of which is promoted by ineffective infection control mechanisms for fecal organisms and the widespread use of colonization-promoting antimicrobials (especially cephalosporins and antianaerobic antibiotics). Fluoroquinolone resistance in community-associated strains of Escherichia coli, many of which also express β-lactamases that confer cephalosporin resistance, is increasingly prevalent. Economic and regulatory forces have served to discourage large pharmaceutical companies from developing new antibiotics, suggesting that the antibiotics currently on the market may be all that will be available for the coming decade. As such, it is critical that we devise, test, and implement antimicrobial stewardship strategies that are effective at constraining and, ideally, reducing resistance in human pathogenic bacteria.

Nosocomial outbreak of imipenem-resistant Pseudomonas aeruginosa producing VIM-2 metallo-β-lactamase in a kidney transplantation unit

Background

Twenty four non replicate imipenem resistant P. aeruginosa were isolated between January and November 2008, in the kidney transplantation unit of Charles Nicolle Hospital of Tunis (Tunisia). This study was conducted in order to establish epidemiological relationship among them and to identify the enzymatic mechanism involved in imipenem resistance.

Methods

Analysis included antimicrobial susceptibility profile, phenotypic (imipenem-EDTA synergy test) and genotypic detection of metallo-β-lactamase (MBL) (PCR), O-serotyping and pulsed-field gel electrophoresis.

Results

All strains showed a high level of resistance to all antimicrobials tested except to colistin. The presence of MBL showed concordance between phenotypic and genotypic methods. Sixteen isolates were identified as VIM-2 MBL-producers and 13 of them were serotype O4 and belonged to a single pulsotype (A).

Conclusions

This study describes an outbreak of VIM-2-producing P. aeruginosa in a kidney transplantation unit. Clinical spread of bla_VIM-2 gene is a matter of great concern for carbapenem resistance in Tunisia.

Carbapenem-resistant Enterobacteriaceae: epidemiology and prevention

Over the past 10 years, dissemination of Klebsiella pneumoniae carbapenemase (KPC) has led to an increase in the prevalence of carbapenem-resistant Enterobacteriaceae (CRE) in the United States. Infections caused by CRE have limited treatment options and have been associated with high mortality rates. In the previous year, other carbapenemase subtypes, including New Delhi metallo-β-lactamase, have been identified among Enterobacteriaceae in the United States. Like KPC, these enzymes are frequently found on mobile genetic elements and have the potential to spread widely. As a result, preventing both CRE transmission and CRE infections have become important public health objectives. This review describes the current epidemiology of CRE in the United States and highlights important prevention strategies.

IMP-producing carbapenem-resistant Klebsiella pneumoniae in the United States

The emergence and spread of carbapenem-resistant Enterobacteriaceae (CRE) producing acquired carbapenemases have created a global public health crisis. In the United States, CRE producing the Klebsiella pneumoniae carbapenemase (KPC) are increasingly common and are endemic in some regions. Metallo-β-lactamase (MBL)-producing CRE have recently been reported in the United States among patients who received medical care in countries where such organisms are common. Here, we describe three carbapenem-resistant K. pneumoniae isolates recovered from pediatric patients at a single U.S. health care facility, none of whom had a history of international travel. The isolates were resistant to carbapenems but susceptible to aztreonam, trimethoprim-sulfamethoxazole, and fluoroquinolones. The three isolates were closely related to each other by pulsed-field gel electrophoresis and contained a common plasmid. PCR and sequence analysis confirmed that these isolates produce IMP-4, an MBL carbapenemase not previously published as present among Enterobacteriaceae in the United States.

Metallo-β-lactamases: a last frontier for β-lactams?

Metallo-β-lactamases are resistance determinants of increasing clinical relevance in Gram-negative bacteria. Because of their broad range, potent carbapenemase activity and resistance to inhibitors, these enzymes can confer resistance to almost all β-lactams. Since the 1990s, several metallo-β-lactamases encoded by mobile DNA have emerged in important Gram-negative pathogens (ie, in Enterobacteriaceae, Pseudomonas aeruginosa, and Acinetobacter baumannii). Some of these enzymes (eg, VIM-1 and NDM-1) have been involved in the recent crisis resulting from the international dissemination of carbapenem-resistant Klebsiella pneumoniae and other enterobacteria. Although substantial knowledge about the molecular biology and genetics of metallo-β-lactamases is available, epidemiological data are inconsistent and clinical experience is still lacking; therefore, several unsolved or debatable issues remain about the management of infections caused by producers of metallo-β-lactamase. The spread of metallo-β-lactamases presents a major challenge both for treatment of individual patients and for policies of infection control, exposing the substantial unpreparedness of public health structures in facing up to this emergency.

Comparative diffusion assay to assess efficacy of topical antimicrobial agents against Pseudomonas aeruginosa in burns care

Background

Severely burned patients may develop life-threatening nosocomial infections due to Pseudomonas aeruginosa, which can exhibit a high-level of resistance to antimicrobial drugs and has a propensity to cause nosocomial outbreaks. Antiseptic and topical antimicrobial compounds constitute major resources for burns care but in vitro testing of their activity is not performed in practice.

Results

In our burn unit, a P. aeruginosa clone multiresistant to antibiotics colonized or infected 26 patients over a 2-year period. This resident clone was characterized by PCR based on ERIC sequences. We investigated the susceptibility of the resident clone to silver sulphadiazine and to the main topical antimicrobial agents currently used in the burn unit. We proposed an optimized diffusion assay used for comparative analysis of P. aeruginosa strains. The resident clone displayed lower susceptibility to silver sulphadiazine and cerium silver sulphadiazine than strains unrelated to the resident clone in the unit or unrelated to the burn unit.

Conclusions

The diffusion assay developed herein detects differences in behaviour against antimicrobials between tested strains and a reference population. The method could be proposed for use in semi-routine practice of medical microbiology.

Multiresistant Gram-negative infections: a global perspective

PURPOSE OF REVIEW

Multiresistant Gram-negative infections are an increasing problem in hospitals and healthcare facilities worldwide. While much attention has been paid to Gram-positive pathogens such as methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus lately, the importance of Gram-negative nosocomial infections has also been recognized globally.

RECENT FINDINGS

Recent reports have described the spread of carbapenemase-producing Klebsiella pneumoniae across North America. In addition, many strains of Pseudomonas and Acinetobacter in Asia are resistant to all known antibiotics. The global epidemiology of multiresistant Gram-negative pathogens seems to vary by continent. There are very few existing agents which can be used for these pathogens and there are limited options on the horizon. This limited therapeutic armamentarium has been an impetus for novel approaches including combination therapies and increased attention to infection control and prevention efforts.

SUMMARY

Clinicians need to be aware of the rising problem of resistance in nosocomial and community-acquired Gram-negative pathogens. Novel agents are urgently needed to combat these infections and innovative infection control strategies need to be devised to protect our vulnerable patients.

Epidemiology and genetics of VIM-type metallo-β-lactamases in Gram-negative bacilli

Metallo-β-lactamases (MBLs) are a rapidly evolving group of β-lactamases, which hydrolyze most β-lactams including the carbapenems. Of the known MBLs, VIMs are one of the most common families, with 27 variants detected in at least 23 species of Gram-negative bacilli from more than 40 countries/regions. The amino acid similarities of VIM variants range from 72.9 to 99.6% with 1–72 different residues. Most of the bla VIMs are harbored by a class 1 integron, a genetic platform able to acquire and express gene cassettes. The integrons are usually embedded in transposons and, in turn, accommodated on plasmids, making them highly mobile. Integrons display considerable diversity, with at least 110 different structures associated with the gain and spread of the bla VIMs. In most instances, the bla VIMs co-exist with one or more other resistance genes. The processes for the identification of bacteria harboring bla VIMs are also discussed in this article.

[Multi-drug resistant Pseudomonas aeruginosa: towards a therapeutic dead end?]

Pseudomonas aeruginosa is a major hospital-associated pathogen that can cause severe infections, most notably in patients with cystic fibrosis or those hospitalized in intensive care units. In this context, the current increase in incidence of multi-drug resistant (MDR) isolates of P. aeruginosa (MDRPA) raises serious concerns. MDR in P. aeruginosa is defined as the resistance to 3 or 4 of the following antibiotic classes: penicillins/cephalosporins/monobactams, carbapenems, aminoglycosides, and fluoroquinolones. These strains constantly cumulate several resistance mechanisms as a consequence of multiple genetic events, i.e., chromosomal mutations or horizontal transfers of resistance genes. Involved mechanisms may include active efflux, impermeability resulting from porins loss, plasmid-encoded b-lactamases/carbapenemases or aminoglycosides-modifying enzymes, and enzymatic or mutation-associated changes in antibiotics targets. Antibiotic selection pressure represents the leading risk factor for MDRPA acquisition. Colistin (polymyxin E) remains active on virtually all MDRPA isolates, and increasingly appears as the last available option to treat infections caused by these strains. However, the emergence of colistin resistance has been reported in P. aeruginosa, which may announce the spread of pan-resistant strains in a close future.

The accessory genome of Pseudomonas aeruginosa

Pseudomonas aeruginosa strains exhibit significant variability in pathogenicity and ecological flexibility. Such interstrain differences reflect the dynamic nature of the P. aeruginosa genome, which is composed of a relatively invariable "core genome" and a highly variable "accessory genome." Here we review the major classes of genetic elements comprising the P. aeruginosa accessory genome and highlight emerging themes in the acquisition and functional importance of these elements. Although the precise phenotypes endowed by the majority of the P. aeruginosa accessory genome have yet to be determined, rapid progress is being made, and a clearer understanding of the role of the P. aeruginosa accessory genome in ecology and infection is emerging.

Prevalence, resistance mechanisms, and susceptibility of multidrug-resistant bloodstream isolates of Pseudomonas aeruginosa

Pseudomonas aeruginosa is an important pathogen commonly implicated in nosocomial infections. The occurrence of multidrug-resistant (MDR) P. aeruginosa strains is increasing worldwide and limiting our therapeutic options. The MDR phenotype can be mediated by a variety of resistance mechanisms, and the corresponding relative biofitness is not well established. We examined the prevalence, resistance mechanisms, and susceptibility of MDR P. aeruginosa isolates (resistant to > or =3 classes of antipseudomonal agents [penicillins/cephalosporins, carbapenems, quinolones, and aminoglycosides]) obtained from a large, university-affiliated hospital. Among 235 nonrepeat bloodstream isolates screened between 2005 and 2007, 33 isolates (from 20 unique patients) were found to be MDR (crude prevalence rate, 14%). All isolates were resistant to carbapenems and quinolones, 91% were resistant to penicillins/cephalosporins, and 21% were resistant to the aminoglycosides. By using the first available isolate for each bacteremia episode (n = 18), 13 distinct clones were revealed by repetitive-element-based PCR. Western blotting revealed eight isolates (44%) to have MexB overexpression. Production of a carbapenemase (VIM-2) was found in one isolate, and mutations in gyrA (T83I) and parC (S87L) were commonly found. Growth rates of most MDR isolates were similar to that of the wild type, and two isolates (11%) were found to be hypermutable. All available isolates were susceptible to polymyxin B, and only one isolate was nonsusceptible to colistin (MIC, 3 mg/liter), but all isolates were nonsusceptible to doripenem (MIC, >2 mg/liter). Understanding and continuous monitoring of the prevalence and resistance mechanisms of MDR P. aeruginosa would enable us to formulate rational treatment strategies to combat nosocomial infections.

Relation between the antimicrobial susceptibility of clinical isolates of Pseudomonas aeruginosa from respiratory specimens and antimicrobial use density (AUD) from 2005 through 2008

OBJECTIVE

To examine the relation between annual trends in the antimicrobial susceptibility of Pseudomonas aeruginosa and drug usage, we compared annual changes in the susceptibility rates of P. aeruginosa clinical isolates during a 4-year period and annual trends in the overall usage of antimicrobials during the same period.

METHODS

We studied annual trends in MIC(90)/MIC(50), antimicrobial use density (AUD), and antimicrobial susceptibility rates based on clinical breakpoints for 150 strains of P. aeruginosa isolated from respiratory specimens at Dokkyo Medical University Hospital from 2005 through 2008.

RESULTS

The MIC(90)/MIC(50) of antimicrobials effective against P. aeruginosa in years 2005, 2006, 2007, and 2008 were as follows: imipenem, 32/2, 32/1, 8/2, and 16/1 microg/mL; meropenem, 8/1, 8/1, 4/0.5, and 4/0.5 microg/mL; and biapenem, 16/1, 32/0.5, 4/0.5, and 8/0.5 microg/mL, indicating that susceptibility to carbapenems increased slightly. The MIC(90)/MIC(50) was 4/0.25, 2/0.125, 1/0.125, and 2/0.25 microg/mL for ciprofloxacin, 8/4, 8/4, 4/4, and 8/4 microg/mL for amikacin, 64/16, 64/16, 64/16, and 64/16 microg/mL for sulbactam/cefoperazone, 8/2, 16/2, 32/2, and 8/2 microg/mL for ceftazidime, indicating little change. The AUDs of fourth-generation cephalosporins increased from 2005 to 2008 (16.2, 18.4, 28.0, and 23.0), while the AUDs of carbapenems decreased (25.7, 23.7, 10.9, and 12.5).

CONCLUSION

The decrease in the AUDs of carbapenems was associated with increased susceptibility rates of P. aeruginosa to carbapenem derivatives. A continuous understanding of trends in the resistance of P. aeruginosa and various other pathogens is essential for designing countermeasures against nosocomial infections, including the proper and effective use of antimicrobials.

Dispersal of carbapenemase blaVIM-1 gene associated with different Tn402 variants, mercury transposons, and conjugative plasmids in Enterobacteriaceae and Pseudomonas aeruginosa

The emergence of bla(VIM-1) within four different genetic platforms from distinct Enterobacteriaceae and Pseudomonas aeruginosa isolates in an area with a low prevalence of metallo-beta-lactamase producers is reported. Forty-three VIM-1-producing isolates (including 19 Enterobacter cloacae, 2 Escherichia coli, and 2 P. aeruginosa isolates, 18 Klebsiella pneumoniae isolate, and 2 Klebsiella oxytoca isolate) recovered from 2005 to 2007 and corresponding to 15 pulsed-field gel electrophoresis types were studied. The Enterobacteriaceae isolates corresponded to a hospital outbreak, and the P. aeruginosa isolates were sporadically recovered. The genetic context of the integrons carrying bla(VIM-1) (arbitrarily designated types A, B, C, and D) was characterized by PCR mapping based on known Tn402 and mercury transposons and further sequencing. Among Enterobacteriaceae isolates, bla(VIM-1) was part of integrons located either in an In2-Tn402 element linked to Tn21 (type A; In110-bla(VIM-1)-aacA4-aadA1) or in a Tn402 transposon lacking the whole tni module [type B; In113-bla(VIM-1)-aacA4-dhfrII (also called dfrB1)-aadA1-catB2] and the transposon was associated with an IncHI2 or IncI1 plasmid, respectively. Among P. aeruginosa isolates, bla(VIM-1) was part of a new gene cassette array located in a defective Tn402 transposon carrying either tniBDelta3 and tniA (type C; bla(VIM-1)-aadA1) or tniC and DeltatniQ (type D; bla(VIM-1)-aadB), and both Tn402 variants were associated with conjugative plasmids of 30 kb. The dissemination of bla(VIM-1) was associated with different genetic structures and bacterial hosts, depicting a complex emergence and evolutionary network scenario in our facility, Ramón y Cajal University Hospital, Madrid, Spain. Knowledge of the complex epidemiology of bla(VIM-1) is necessary to control this emerging threat.

Hydrolysis and inhibition profiles of beta-lactamases from molecular classes A to D with doripenem, imipenem, and meropenem

The stability of doripenem to hydrolysis by beta-lactamases from molecular classes A to D was compared to the stability for imipenem and meropenem. Doripenem was stable to hydrolysis by extended-spectrum beta-lactamases and AmpC type beta-lactamases and demonstrated high affinity for the AmpC enzymes. For the serine carbapenemases SME-3 and KPC-2 and metallo-beta-lactamases IMP-1 and VIM-2, doripenem hydrolysis was generally 2- to 150-fold slower than imipenem hydrolysis. SPM-1 hydrolyzed meropenem and doripenem fourfold faster than imipenem.

Pseudomonas aeruginosa - a phenomenon of bacterial resistance

Pseudomonas aeruginosa is one of the leading nosocomial pathogens worldwide. Nosocomial infections caused by this organism are often hard to treat because of both the intrinsic resistance of the species (it has constitutive expression of AmpC beta-lactamase and efflux pumps, combined with a low permeability of the outer membrane), and its remarkable ability to acquire further resistance mechanisms to multiple groups of antimicrobial agents, including beta-lactams, aminoglycosides and fluoroquinolones. P. aeruginosa represents a phenomenon of bacterial resistance, since practically all known mechanisms of antimicrobial resistance can be seen in it: derepression of chromosomal AmpC cephalosporinase; production of plasmid or integron-mediated beta-lactamases from different molecular classes (carbenicillinases and extended-spectrum beta-lactamases belonging to class A, class D oxacillinases and class B carbapenem-hydrolysing enzymes); diminished outer membrane permeability (loss of OprD proteins); overexpression of active efflux systems with wide substrate profiles; synthesis of aminoglycoside-modifying enzymes (phosphoryltransferases, acetyltransferases and adenylyltransferases); and structural alterations of topoisomerases II and IV determining quinolone resistance. Worryingly, these mechanisms are often present simultaneously, thereby conferring multiresistant phenotypes. This review describes the known resistance mechanisms in P. aeruginosa to the most frequently administrated antipseudomonal antibiotics: beta-lactams, aminoglycosides and fluoroquinolones.

Detection of Pseudomonas aeruginosa carried a new array of gene cassettes within class 1 integron isolated from a teaching hospital in Nanjing, China

We report here novel array of gene cassettes found in single variable region of class 1 integron disseminated in Pseudomonas aeruginosa isolated from a teaching hospital in Nanjing, Jiangsu Province, China. 29 of 47 (61%) P. aeruginosa strains were confirmed haboured class 1 integron, and all the positive strains have the same variable region confirmed by PCR and RFLP methods. The variable region contained an unreported order of four gene cassettes aac(6')-II-aadA13-cmlA8-oxa-10. Of those, cmlA8 gene was a variant of cmlA5 encoding non-enzymatic protein which putatively confer resistance to chloramphenicol. Susceptibility testing revealed multidrug-resistant mechanisms were involved in the class 1 integron positive clinical isolates. And the class 1 integron located on an about 15 kb transferable plasmid was certified by conjugation experiment and plasmid DNA analysis. The macro restriction profile indicated those clinical strains were clonally related.

Molecular dynamic simulations of the metallo-beta-lactamase from Bacteroides fragilis in the presence and absence of a tight-binding inhibitor

The beta-lactam-based antibiotics are among the most prescribed and effective antibacterial agents. Widespread use of these antibiotics, however, has created tremendous pressure for the emergence of resistance mechanisms in bacteria. The most common cause of antibiotic resistance is bacterial production of actamases that efficiently degrade antibiotics. The metallo-beta-lactamases are of particular clinical concern due to their transference between bacterial strains. We used molecular dynamics (MD) simulations to further study the conformational changes that occur due to binding of an inhibitor to the dicanzinc metallo-beta-lactamase from Bacteroides fragilis. Our studies confirm previous findings that the major flap is a major source of plasticity within the active site, therefore its dynamic response should be considered in drug development. However, our results also suggest the need for care in using MD simulations in evaluating loop mobility, both due to relaxation times and to the need to accurately model the zinc active site. Our study also reveals two new robust responses to ligand binding. First, there are specific localized changes in the zinc active site--a local loop flip--due to ligand intercalation that may be critical to the function of this enzyme. Second, inhibitor binding perturbs the dynamics throughout the protein, without otherwise perturbing the enzyme structure. These dynamic perturbations radiate outward from the active site and their existence suggests that long-range communication and dynamics may be important in the activity of this enzyme.

Acinetobacter baumannii: an emerging multidrug-resistant threat

Amid the recent attention focused on the growing impact of methicillin-resistant Staphylococcus aureus and multidrug-resistant Pseudomonas aeruginosa infections, the pathogen Acinetobacter baumannii has been stealthily gaining ground as an agent of serious nosocomial and community-acquired infection. Historically, Acinetobacter spp. have been associated with opportunistic infections that were rare and of modest severity; the last two decades have seen an increase in both the incidence and seriousness of A. baumannii infection, with the main targets being patients in intensive-care units. Although this organism appears to have a predilection for the most vulnerable patients, community-acquired A. baumannii infection is an increasing cause for concern. The increase in A. baumannii infections has paralleled the alarming development of resistance it has demonstrated. The persistence of this organism in healthcare facilities, its inherent hardiness and its resistance to antibiotics results in it being a formidable emerging pathogen. This review aims to put into perspective the threat posed by this organism in hospital and community settings, describes new information that is changing our view of Acinetobacter virulence and resistance, and calls for greater understanding of how this multifaceted organism came to be a major pathogen.

The current state of multidrug-resistant gram-negative bacilli in North America

Although much of today's media focuses on multidrug-resistant gram-positive bacteria such as methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus, resistance within gram-negative bacilli continues to rise, occasionally creating situations in which few or no antibiotics that retain activity are available. Extended-spectrum beta-lactamase (ESBL)-producing Escherichia coli and Klebsiella sp are emerging threats nationally. Although carbapenems are considered the antibiotic class of choice to treat ESBL-producing Enterobacteriaceae, the ability of these organisms to produce carbapenemases has now become apparent in some regions throughout the United States. Although still rare, Klebsiella sp that produce KPC-2 retain susceptibility only to tigecycline, polymyxins, and occasionally aminoglycosides. Multidrug resistance among Pseudomonas aeruginosa and Acinetobacter sp has always been apparent across many hospitals in the United States. Recent surveillance indicates increasing resistance to all currently available antibiotics, including carbapenems, cephalosporins, penicillins, fluoroquinolones, and aminoglycosides. Against many strains, only polymyxins retain activity; however, resistance has also been reported to these agents. Fortunately, resistance mechanisms such as metallo-beta-lactamases are still rare in the United States. As no new antibiotics with novel mechanisms against many of these gram-negative bacilli are expected to be developed in the foreseeable future, careful and conservative use of agents combined with good infection control practices is required.

The role of carbapenems in the treatment of severe nosocomial respiratory tract infections

The prevalence of antibiotic-resistant bacteria continues to increase, particularly in patients in the intensive care unit with nosocomial pneumonia. The intention of this review is to provide an overview of severe nosocomial pneumonia, carbapenems and the problem of bacterial resistance to antimicrobial agents. Attention was focused on the efficacy, safety and pharmacodynamics of imipenem, meropenem, ertapenem and doripenem. Issues on the impact of appropriate empiric antibiotic therapy for nosocomial pneumonia patients considered at risk for resistant pathogens are discussed. Critical decision making regarding the use of carbapenems for treating severe nosocomial pneumonia requires careful consideration of the four Ds of optimal antimicrobial therapy: right Drug, right Dose, De-escalated to pathogen-directed therapy and right Duration of therapy.

Molecular typing indicates an important role for two international clonal complexes in dissemination of VIM-producing Pseudomonas aeruginosa clinical isolates in Hungary

VIM metallo-beta-lactamase-producing serotype O11 or O12 Pseudomonas aeruginosa isolates infecting or colonising 19 patients from seven hospitals were reported in Hungary between 2003 and 2005. In this study we characterised VIM-producing Pseudomonas spp. clinical isolates from two novel locations in Hungary; we identified three new bla(VIM) carrying integron types and the presence of the bla(VIM-2) allele in Hungary. By applying various typing techniques, including multilocus sequence typing, we revealed an important role of two international clonal complexes, CC4 and CC11, in the dissemination of bla(VIM)-positive P. aeruginosa in hospitals in Hungary. Isolate P12-Q, a representative strain from France of the major European multiresistant P12 clone, displayed ST111 which, according to eBURST analysis, is the presently calculated founder sequence type of CC4. This is in accordance with the wide geographic distribution of the P12 clone. Our data indicate that, although the CC4 clonal complex includes serotype O1 and O6 isolates as well, it also contains the P12 clone. We characterised a P. aeruginosa nosocomial clone with a singleton sequence type (ST313), that may have acquired bla(VIM-2) and bla(VIM-4) gene cassettes from a yet unidentified local gene pool in Hungary.

Current concepts in antibiotic-resistant gram-negative bacteria

Gram-negative bacteria are the dominant killers among bacterial pathogens in the intensive care unit. Antibiotic resistance has become a threat in hospital settings and efforts are being made to understand the underlying mechanisms. This review describes current data on the most important mechanisms of resistance in prevalent gram-negative pathogens as well as newer therapeutic options.

Molecular epidemiology and mechanisms of carbapenem resistance in Pseudomonas aeruginosa isolates from Spanish hospitals

All (236) Pseudomonas aeruginosa isolates resistant to imipenem and/or meropenem collected during a multicenter (127-hospital) study in Spain were analyzed. Carbapenem-resistant isolates were found to be more frequently resistant to all beta-lactams and non-beta-lactam antibiotics than carbapenem-susceptible isolates (P < 0.001), and up to 46% of the carbapenem-resistant isolates met the criteria used to define multidrug resistance (MDR). Pulsed-field gel electrophoresis revealed remarkable clonal diversity (165 different clones were identified), and with few exceptions, the levels of intra- and interhospital dissemination of clones were found to be low. Carbapenem resistance was driven mainly by the mutational inactivation of OprD, accompanied or not by the hyperexpression of AmpC or MexAB-OprM. Class B carbapenemases (metallo-beta-lactamases [MBLs]) were detected in a single isolate, although interestingly, this isolate belonged to one of the few epidemic clones documented. The MBL-encoding gene (bla(VIM-2)), along with the aminoglycoside resistance determinants, was transferred to strain PAO1 by electroporation, demonstrating its plasmid location. The class 1 integron harboring bla(VIM-2) was characterized as well, and two interesting features were revealed: intI1 was found to be disrupted by a 1.1-kb insertion sequence, and a previously undescribed aminoglycoside acetyltransferase-encoding gene [designated aac(6')-32] preceded bla(VIM-2). AAC(6')-32 showed 80% identity to AAC(6')-Ib' and the recently described AAC(6')-31, and when aac(6')-32 was cloned into Escherichia coli, it conferred resistance to tobramycin and reduced susceptibility to gentamicin and amikacin. Despite the currently low prevalence of epidemic clones with MDR, active surveillance is needed to detect and prevent the dissemination of these clones, particularly those producing integron- and plasmid-encoded MBLs, given their additional capacity for the intra- and interspecies spread of MDR.

Nosocomial spread of Pseudomonas aeruginosa producing the metallo-β-lactamase VIM-2 in a Spanish hospital: clinical and epidemiological implications

Thirty-four isolates of pan-resistant Pseudomonas aeruginosa producing VIM-2 metallo-beta-lactamase (MBL) were detected at a university hospital in Spain (July 2004-September 2006). Eleven (32%) patients had clinically significant infections, and three (27%) of these patients died. A single clone of MBL-producing P. aeruginosa was identified by pulsed-field gel electrophoresis. A cluster of isolates associated with the vascular surgery ward involved ten patients and appeared as a series of low-grade, sustained and misdiagnosed endemic infections in the hospital. The identification of MBL-positive P. aeruginosa should be considered mandatory in the surveillance of pan-resistant P. aeruginosa and requires a high index of suspicion in the context of endemic infections with a low attack rate.

Carbapenemases: the versatile beta-lactamases

Carbapenemases are beta-lactamases with versatile hydrolytic capacities. They have the ability to hydrolyze penicillins, cephalosporins, monobactams, and carbapenems. Bacteria producing these beta-lactamases may cause serious infections in which the carbapenemase activity renders many beta-lactams ineffective. Carbapenemases are members of the molecular class A, B, and D beta-lactamases. Class A and D enzymes have a serine-based hydrolytic mechanism, while class B enzymes are metallo-beta-lactamases that contain zinc in the active site. The class A carbapenemase group includes members of the SME, IMI, NMC, GES, and KPC families. Of these, the KPC carbapenemases are the most prevalent, found mostly on plasmids in Klebsiella pneumoniae. The class D carbapenemases consist of OXA-type beta-lactamases frequently detected in Acinetobacter baumannii. The metallo-beta-lactamases belong to the IMP, VIM, SPM, GIM, and SIM families and have been detected primarily in Pseudomonas aeruginosa; however, there are increasing numbers of reports worldwide of this group of beta-lactamases in the Enterobacteriaceae. This review updates the characteristics, epidemiology, and detection of the carbapenemases found in pathogenic bacteria.

Prevalence, mechanisms, and risk factors of carbapenem resistance in bloodstream isolates of Pseudomonas aeruginosa

We examined the prevalence of various carbapenem resistance mechanisms in Pseudomonas aeruginosa bloodstream isolates from a university-affiliated hospital. Isolates obtained in 2003 and 2004 were screened for meropenem/imipenem resistance, and clonality was assessed by repetitive-element-based polymerase chain reaction. The presence of carbapenemase and AmpC overexpression was ascertained by spectrophotometric assays. Outer membrane protein profiles were examined by sodium dodecyl sulfate polyacrylamide gel electrophoresis, and efflux pump overexpression was confirmed by Western blotting. We examined 129 nonrepeat isolates; 21 isolates (from 13 distinct clones) were resistant to meropenem or imipenem (prevalence rate = 16.3%). Nineteen (90.5%) carbapenem-resistant isolates had reduced OprD expression, and 6 (28.6%) isolates had overexpression of MexB. Increased length of hospital stay was identified as a significant risk factor for bacteremia due to carbapenem-resistant P. aeruginosa. Understanding the prevalence and mechanism of carbapenem resistance in P. aeruginosa may guide empiric therapy for nosocomial infections in our hospital.

Interactions of ceftobiprole with beta-lactamases from molecular classes A to D

The interactions of ceftobiprole with purified beta-lactamases from molecular classes A, B, C, and D were determined and compared with those of benzylpenicillin, cephaloridine, cefepime, and ceftazidime. Enzymes were selected from functional groups 1, 2a, 2b, 2be, 2d, 2e, and 3 to represent beta-lactamases from organisms within the antibacterial spectrum of ceftobiprole. Ceftobiprole was refractory to hydrolysis by the common staphylococcal PC1 beta-lactamase, the class A TEM-1 beta-lactamase, and the class C AmpC beta-lactamase but was labile to hydrolysis by class B, class D, and class A extended-spectrum beta-lactamases. Cefepime and ceftazidime followed similar patterns. In most cases, the hydrolytic stability of a substrate correlated with the MIC for the producing organism. Ceftobiprole and cefepime generally had lower MICs than ceftazidime for AmpC-producing organisms, particularly AmpC-overexpressing Enterobacter cloacae organisms. However, all three cephalosporins were hydrolyzed very slowly by AmpC cephalosporinases, suggesting that factors other than beta-lactamase stability contribute to lower ceftobiprole and cefepime MICs against many members of the family Enterobacteriaceae.

blaVIM-2-harboring integrons isolated in India, Russia, and the United States arise from an ancestral class 1 integron predating the formation of the 3' conserved sequence

The metallo-beta-lactamase gene bla(VIM-2) was identified in a strain of Pseudomonas aeruginosa isolated in India. The integron encoding bla(VIM-2) was virtually identical to those recently found in the United States and Russia. These unusual structures are likely to have arisen from an ancestral integron predating the formation of the 3' conserved sequence.

Occurrence and characterization of carbapenemase-producing Enterobacteriaceae: report from the SENTRY Antimicrobial Surveillance Program (2000-2004)

Emergence and dissemination of Enterobacteriaceae isolates harboring carbapenemases in various geographic regions represents a significant threat to the management of nosocomial infections. Enterobacteriaceae isolates from the SENTRY Antimicrobial Surveillance Program (2000-2004) demonstrating decreased susceptibility to imipenem and meropenem (minimum inhibitory concentration [MIC], > or =2 mg/L) were evaluated for the production of metallo-beta-lactamases and serine carbapenemases using disk approximation and polymerase chain reaction (PCR) tests. Carbapenemase-producing strains were epidemiologically typed by automated riboprinting and pulsed-field gel electrophoresis (PFGE) to establish clonality. Among 37,557 Enterobacteriaceae (5 genus groups) evaluated, 119 (0.32%) had increased carbapenem MIC values, and a carbapenemase was identified in 51 (42.9%) of these strains. KPC-2 and KPC-3 were the most frequently occurring carbapenemases (24 isolates, 20.2%) in the United States and were detected in Klebsiella spp, Citrobacter spp., Enterobacter spp., and Serratia marcescens strains isolated in New York, Arkansas, and Virginia. SME-2-producing S. marcescens were isolated in the New York City area, Texas, and Ohio, while NMC-A was found in one E. cloacae strain from New York. In contrast, metallo-beta-lactamases were prevalent in Europe. IMP-1-producing E. cloacae (11 isolates) were detected in Turkey, while VIM-1-producing strains were found in Italy (Enterobacter spp.) and Greece (Klebsiella pneumoniae). Clonal dissemination of carbapenemase-producing strains was observed in several medical centers on both continents. The occurrence of carbapenemases in various Enterobacteriaceae remains rare but appears to be spreading geographically (not in Latin America), mainly with metallo-beta-lactamases being found in Mediterranean Europe and KPC enzymes in the New York City area.

Differences in antimicrobial susceptibility breakpoints for Pseudomonas aeruginosa, isolated from blood cultures, set by the Clinical and Laboratory Standards Institute (CLSI) and the Japanese Society of Chemotherapy

A study was made of the antimicrobial susceptibility to and efficacy of various kinds of antimicrobial agents against 179 strains of Pseudomonas aeruginosa that were isolated from blood cultures at Kansai Medical University Hospital from 1990 through 2004. The annual detection rate was highest in 1994, at 22 strains (6.5%). There were 9 multidrug resistant strains of Pseudomonas aeruginosa (5.0%). Among 14 antimicrobial agents tested for measurements, ciprofloxacin (CPFX) showed the best minimum inhibitory concentration (MIC) 50 value, of 0.25 microg/ml, followed by pazufloxacin (PZFX) and biapenem (BIPM), each at 0.5 microg/ml. When the period of 15 years was divided into three stages, the MIC50 value for each antimicrobial agent was highest in the middle stage (1995 to 1999). Assuming that the percentage of sensitive strains according to the breakpoints set by the Clinical and Laboratory Standards Institute (CLSI) represents the antimicrobial susceptibility rate, amikacin (AMK) showed the best value, of 85.5%. According to the sepsis breakpoint set by the Japanese Society of Chemotherapy (JSC), the efficacy of CPFX showed the highest rate (77.1%) of all the antimicrobial agents tested. Among beta-lactams, BIPM showed the highest efficacy rate, of 67.0%. When the efficacy rates were compared with each other, the difference in efficacy rate between the breakpoint set by the CLSI and the sepsis breakpoint set by the JSC was large for beta-lactams. Comparisons made based on the CLSI criteria showed no difference in cross-resistance rates between CPFX, meropenem (MEPM), and BIPM. However, when comparisons were made using the JSC sepsis breakpoint, MEPM showed a cross-resistance rate of 87.8%, while the rate for BIPM was lower, at 56.1%, with the chi2 test showing a significant difference, at P = 0.0014. In accordance with the pharmacokinetics/pharmacodynamics theory that has been advocated, breakpoints which are more suitable for the clinical setting in Japan should be set so that more effective and more appropriate treatment can be carried out.

Tigecycline: a glycylcycline antimicrobial agent

BACKGROUND

Tigecycline, the first glycylcycline to be approved by the US Food and Drug Administration, is a structural analogue of minocycline that was designed to avoid tetracycline resistance mediated by ribosomal protection and drug efflux. It is indicated for the treatment of complicated skin and skin-structure infections and complicated intra-abdominal infections and is available for intravenous administration only.

OBJECTIVE

This article summarizes the in vitro and in vivo activities and pharmacologic and pharmacokinetic properties of tigecycline, and reviews its clinical efficacy and tolerability profile.

METHODS

Relevant information was identified through a search of MEDLINE (1966-April 2006), Iowa Drug Information Service (1966-April 2006), and International Pharmaceutical Abstracts (1970-April 2006) using the terms tigecycline, GAR-936, and glycylcycline. Also consulted were abstracts and posters from meetings of the Infectious Diseases Society of America and the Interscience Conference on Antimicrobial Agents and Chemotherapy (1999-2006) and documents provided for formulary consideration by the US manufacturer of tigecycline.

RESULTS

Like the tetracyclines, tigecycline binds to the 30S subunit of bacterial ribosomes and inhibits protein synthesis by preventing the incorporation of amino acid residues into elongating peptide chains. In vitro, tigecycline exhibits activity against a wide range of clinically significant gram-positive and gram-negative bacteria, including multidrug-resistant strains (eg, oxacillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci, extended-spectrum beta-lactamase-producing Enterobacteriaceae), and anaerobes (eg, Bacteroides spp). In pharmacokinetic studies in human adults, tigecycline had a large Vd (7-9 L/kg), was moderately bound to plasma protein (71%-89%), had an elimination t(1/2) of 42.4 hours, and was eliminated primarily by biliary/fecal (59%) and renal (33%) excretion. Dose adjustment did not appear to be necessary based on age, sex, renal function, or mild to moderate hepatic impairment (Child-Pugh class A-B). In patients with severe hepatic impairment (Child-Pugh class C), the maintenance dose should be reduced by 50%. In 4 Phase III clinical trials in patients with complicated skin and skin-structure infections and complicated intra-abdominal infections, tigecycline was reported to be noninferior to its comparators (vancomycin + aztreonam in 2 studies and imipenem/cilastatin in 2 studies), with clinical cure rates among clinically evaluable patients of >80% (P < 0.001 for noninferiority). The most frequently reported (> or =5 %) adverse events with tigecycline were nausea (28.5%), vomiting (19.4%), diarrhea (11.6%), local IV-site reaction (8.2%), infection (6.7%), fever (6.3%), abdominal pain (6.0%), and headache (5.6%). The recommended dosage of tigecycline is 100 mg IV given as a loading dose, followed by 50 mg IV g12h for 5 to 14 days.

CONCLUSIONS

In clinical trials, tigecycline was effective for the treatment of complicated skin and skin-structure infections and complicated intra-abdominal infections. With the exception of gastrointestinal adverse events, tigecycline was generally well tolerated. With a broad spectrum of activity that includes multidrug-resistant gram-positive and gram-negative pathogens, tigecycline may be useful in the treatment of conditions caused by these pathogens.

Prevalence of newer beta-lactamases in gram-negative clinical isolates collected in the United States from 2001 to 2002

Newer beta-lactamases such as extended-spectrum beta-lactamases (ESBLs), transferable AmpC beta-lactamases, and carbapenemases are associated with laboratory testing problems of false susceptibility that can lead to inappropriate therapy for infected patients. Because there appears to be a lack of awareness of these enzymes, a study was conducted during 2001 to 2002 in which 6,421 consecutive, nonduplicate clinical isolates of aerobically growing gram-negative bacilli from patients at 42 intensive care unit (ICU) and 21 non-ICU sites across the United States were tested on-site for antibiotic susceptibility. From these isolates, 746 screen-positive isolates (11.6%) were referred to a research facility and investigated to determine the prevalence of ESBLs in all gram-negative isolates, transferable AmpC beta-lactamases in Klebsiella pneumoniae, and carbapenemases in Enterobacteriaceae. The investigations involved phenotypic tests, isoelectric focusing, beta-lactamase inhibitor studies, spectrophotometric assays, induction assays, and molecular analyses. ESBLs were detected only in Enterobacteriaceae (4.9% of all Enterobacteriaceae) and were found in species other than those currently recommended for ESBL testing by the CLSI (formerly NCCLS). These isolates occurred at 74% of the ICU sites and 43% of the non-ICU sites. Transferable AmpC beta-lactamases were detected in 3.3% of K. pneumoniae isolates and at 16 of the 63 sites (25%) with no difference between ICU and non-ICU sites. Three sites submitted isolates that produced class A carbapenemases. No class B or D carbapenemases were detected. In conclusion, organisms producing ESBLs and transferable AmpC beta-lactamases were widespread. Clinical laboratories must be able to detect important beta-lactamases to ensure optimal patient care and infection control.

Molecular evolution of metallo-beta-lactamase-producing Pseudomonas aeruginosa in a nosocomial setting of high-level endemicity

An outbreak of multidrug-resistant Pseudomonas aeruginosa strains producing VIM-type metallo-beta-lactamases (MBLs) has occurred in an Italian hospital since 2000 (C. Lagatolla, E. A. Tonin, C. Monti-Bragadin, L. Dolzani, F. Gombac, C. Bearzi, E. Edalucci, F. Gionechetti, and G. M. Rossolini, Emerg. Infect. Dis. 10:535-538, 2004). In this work, using molecular methods, we characterized 128 carbapenem-resistant isolates (including 98 VIM-positive isolates) collected from that hospital from 2000 to 2002 to investigate the dynamics of the dissemination of MBL producers in the clinical setting. Genotyping by random amplification of polymorphic DNA and pulsed-field gel electrophoresis showed that most VIM-positive isolates belonged to two different clonal lineages, producing either a VIM-1- or a VIM-2-like MBL, whose ancestors were detected for the first time in the hospital in 1999, suggesting that clonal expansion played a predominant role in the dissemination of these isolates. The 86 clonally related isolates carrying a blaVIM-1-like gene on an In70-like integron were clearly related to a VIM-1-positive P. aeruginosa clone circulating in various Italian hospitals since the late 1990s. VIM-negative P. aeruginosa strains related to the VIM-1-positive clone were detected during the same period, suggesting that the latter strain was derived from a clonal lineage already circulating in the hospital. In the VIM-2-like positive clone, the MBL gene was carried by an unusual class 1 integron, named In71, lacking the 3' conserved sequence region typical of sul1-associated integrons. A different class 1 integron with an original structure carrying a blaVIM-2 determinant, named In74, was detected in a sporadic isolate. A retrospective investigation did not reveal the presence of strains related to any of the VIM-producing isolates earlier than 1997.

New integron-associated gene cassette encoding a trimethoprim-resistant DfrB-type dihydrofolate reductase

A sixth gene cassette containing a dfrB-type gene, dfrB6, was found in a dfrB6-aadA1 cassette array in class 1 integrons. This array was isolated from several multiply antibiotic-resistant Salmonella enterica serovar Infantis strains that appear to be clonally related. The DfrB6 dihydrofolate reductase conferred resistance to trimethoprim.