Molecular epidemiological study of nosocomial Enterobacter aerogenes isolates in a Belgian hospital

Coexistence of bla NDM-1 and rmtC on a Transferrable Plasmid of a Novel ST192 Klebsiella aerogenes Clinical Isolate

Introduction

The occurrence and development of antibiotic resistance are mainly caused by the spread of large plasmids carrying multiple antibiotic resistance genes. Recently, the association between 16S rRNA methyltransferase genes and β-lactamase genes carried by the same plasmid is of concern.

Methods

The Klebsiella aerogenes 1564 was isolated from the catheter tip of a patient in a tertiary hospital, Shanghai, China. The presence of the bla _NDM-1 and rmtC genes were assessed by PCR. Complete sequence of plasmid p1564 was determined. The K. aerogenes 1564 was characterized by antimicrobial susceptibility testing, Carbapenemase phenotype confirmation testing, conjugation experiment, S1-PFGE and multilocus sequence typing (MLST).

Results

Herein, we found that a New Delhi Metallo-β-lactamase-1 gene (bla _NDM-1) and a 16S rRNA methyltransferase gene (rmtC) coexisted on a transferrable plasmid of a carbapenem-resistant K. aerogenes clinical isolate. The K. aerogenes clinical isolate was found to belong to a novel sequence type 192 (ST192) determined by MLST. The sequencing results of the plasmid p1564 carrying bla _NDM-1 gene and rmtC gene showed that the size and guanine-cytosine content of the plasmid were 136, 902 bp and 51.8%, with 164 putative ORFs and two multidrug resistance gene islands. In addition to bla _NDM-1and rmtC, the plasmid contained bleomycin resistance gene (ble _MBL), CMY-6β-lactamase gene (bla _CMY-6), quaternary ammonium compound resistance gene (sugE), truncated quaternary ammonium compound resistance gene (qacEΔ1), aminoglycoside resistance gene (aacA4) and sulfonamide resistance gene (sul1). By comparison, p1564 has high homology with pHS36-NDM from Salmonella enterica subsp. enterica serovar Stanley reported in China, with similar size and both belonging to plasmid incompatibility group A/C.

Conclusion

The present study demonstrated for the first time the co-existence of rmtC and bla _NDM-1 in a novel ST192 K. aerogenes. The spread of plasmids harboring both bla _NDM-1 and rmtC may occur among Enterobacteriaceae in China.

Single-Tube Multiplex Polymerase Chain Reaction for the Detection of Genes Encoding Enterobacteriaceae Carbapenemase

A multiplex PCR assay in a single tube was developed for the detection of the carbapenemase genes of Enterobacteriaceae. Primers were designed to amplify the following six carbapenemase genes: bla_KPC, bla_IMP, bla_NDM, bla_VIM, bla_OXA-48-like, and bla_GES. Of 70 bla_IMP variants, 67 subtypes were simulated to be PCR-positive based on in silico simulation and the primer-design strategy. After determining the optimal PCR conditions and performing in vitro assays, the performance of the PCR assay was evaluated using 51 and 91 clinical isolates with and without carbapenemase genes, respectively. In conclusion, the combination of multiplex PCR primers and QIAGEN Multiplex PCR Plus Kit was used to determine the best performance for the rapid and efficient screening of carbapenemase genes in Enterobacteriaceae. The assay had an overall sensitivity and specificity of 100%. This PCR assay compensates for the limitations of phenotypic testing, such as antimicrobial susceptibility testing and the modified carbapenem inactivation method, in clinical and public health settings.

Enterobacter spp.: Update on Taxonomy, Clinical Aspects, and Emerging Antimicrobial Resistance

The genus Enterobacter is a member of the ESKAPE group, which contains the major resistant bacterial pathogens. First described in 1960, this group member has proven to be more complex as a result of the exponential evolution of phenotypic and genotypic methods. Today, 22 species belong to the Enterobacter genus. These species are described in the environment and have been reported as opportunistic pathogens in plants, animals, and humans. The pathogenicity/virulence of this bacterium remains rather unclear due to the limited amount of work performed to date in this field. In contrast, its resistance against antibacterial agents has been extensively studied. In the face of antibiotic treatment, it is able to manage different mechanisms of resistance via various local and global regulator genes and the modulation of the expression of different proteins, including enzymes (β-lactamases, etc.) or membrane transporters, such as porins and efflux pumps. During various hospital outbreaks, the Enterobacter aerogenes and E. cloacae complex exhibited a multidrug-resistant phenotype, which has stimulated questions about the role of cascade regulation in the emergence of these well-adapted clones.

Isolation, characterization and efficacy of phage MJ2 against biofilm forming multi-drug resistant Enterobacter cloacae

Biofilm is involved in a variety of infections, playing a critical role in the chronicity of infections. Enterobacter cloacae is a biofilm-forming and multi-drug-resistant (MDR) nosocomial pathogen leading to significant morbidity and mortality. This study aimed at isolation of a bacteriophage against MDR clinical strain of E. cloacae and its efficacy against bacterial planktonic cells and biofilm. A bacteriophage MJ2 was successfully isolated from wastewater and was characterized. The phage exhibited a wide range of thermal and pH stability and demonstrated considerable adsorption to host bacteria in the presence of CaCl₂ or MgCl₂. Transmission electron microscopy (TEM) showed MJ2 head as approximately 62 and 54 nm width and length, respectively. It had a short non-contractile tail and was characterized as a member of the family Podoviridae [order Caudovirales]. The phage MJ2 was found to possess 11 structural proteins (12-150 kDa) and a double-stranded DNA genome with an approximate size of 40 kb. The log-phase growth of E. cloacae both in biofilm and suspension was significantly reduced by the phage. The E. cloacae biofilm was formed under different conditions to evaluate the efficacy of MJ2 phage. Variable reduction pattern of E. cloacae biofilm was observed while treating it for 4 h with MJ2, i.e., biofilm under static conditions. The renewed media with intervals of 24, 72, and 120 h showed biomass decline of 2.8-, 3-, and 3.5-log, respectively. Whereas, the bacterial biofilm formed with dynamic conditions with refreshing media after 24, 72, and 120 h demonstrated decline in growth at 2.5-, 2.6-, and 3.3-log, respectively. It was, therefore, concluded that phage MJ2 possessed considerable inhibitory effects on MDR E. cloacae both in planktonic and biofilm forms.

Establishment of a Molecular Serotyping Scheme and a Multiplexed Luminex-Based Array for Enterobacter aerogenes

Serotyping based on surface polysaccharide antigens is important for the clinical detection and epidemiological surveillance of pathogens. Polysaccharide gene clusters (PSgcs) are typically responsible for the diversity of bacterial surface polysaccharides. Through whole-genome sequencing and analysis, eight putative PSgc types were identified in 23 Enterobacter aerogenes strains from several geographic areas, allowing us to present the first molecular serotyping system for E. aerogenes. A conventional antigenic scheme was also established and correlated well with the molecular serotyping system that was based on PSgc genetic variation, indicating that PSgc-based molecular typing and immunological serology provide equally valid results. Further, a multiplex Luminex-based array was developed, and a double-blind test was conducted with 97 clinical specimens from Shanghai, China, to validate our array. The results of these analyses indicated that strains containing PSgc4 and PSgc7 comprised the predominant groups. We then examined 86 publicly available E. aerogenes strain genomes and identified an additional seven novel PSgc types, with PSgc10 being the most abundant type. In total, our study identified 15 PSgc types in E. aerogenes, providing the basis for a molecular serotyping scheme. From these results, differing epidemic patterns were identified between strains that were predominant in different regions. Our study highlights the feasibility and reliability of a serotyping system based on PSgc diversity, and for the first time, presents a molecular serotyping system, as well as an antigenic scheme for E. aerogenes, providing the basis for molecular diagnostics and epidemiological surveillance of this important emerging pathogen.

Enterobacter aerogenes and Enterobacter cloacae; versatile bacterial pathogens confronting antibiotic treatment

Enterobacter aerogenes and E. cloacae have been reported as important opportunistic and multiresistant bacterial pathogens for humans during the last three decades in hospital wards. These Gram-negative bacteria have been largely described during several outbreaks of hospital-acquired infections in Europe and particularly in France. The dissemination of Enterobacter sp. is associated with the presence of redundant regulatory cascades that efficiently control the membrane permeability ensuring the bacterial protection and the expression of detoxifying enzymes involved in antibiotic degradation/inactivation. In addition, these bacterial species are able to acquire numerous genetic mobile elements that strongly contribute to antibiotic resistance. Moreover, this particular fitness help them to colonize several environments and hosts and rapidly and efficiently adapt their metabolism and physiology to external conditions and environmental stresses. Enterobacter is a versatile bacterium able to promptly respond to the antibiotic treatment in the colonized patient. The balance of the prevalence, E. aerogenes versus E. cloacae, in the reported hospital infections during the last period, questions about the horizontal transmission of mobile elements containing antibiotic resistance genes, e.g., the efficacy of the exchange of resistance genes Klebsiella pneumoniae to Enterobacter sp. It is also important to mention the possible role of antibiotic use in the treatment of bacterial infectious diseases in this E. aerogenes/E. cloacae evolution.

NDM-1 encoded by a pNDM-BJ01-like plasmid p3SP-NDM in clinical Enterobacter aerogenes

A carbapenem-nonsusceptible Enterobacter aerogenes strain named 3-SP was isolated from a human case of pneumonia in a Chinese teaching hospital. NDM-1 carbapenemase is produced by a pNDM-BJ01-like conjugative plasmid designated p3SP-NDM to account for carbapenem resistance of 3-SP. p3SP-NDM was fully sequenced and compared with all publically available pNDM-BJ01-like plasmids. The genetic differences between p3SP-NDM and pNDM-BJ01 include only 18 single nucleotide polymorphisms, a 1 bp deletion and a 706 bp deletion. p3SP-NDM and pNDM-BJ01 harbor an identical Tn125 element organized as ISAba125, bla_NDM−1, ble_MBL, ΔtrpF, dsbC, cutA, ΔgroES, groEL, ISCR27, and ISAba125. The bla_NDM−1 surrounding regions in these pNDM-BJ01-like plasmids have a conserved linear organization ISAba14-aphA6-Tn125-unknown IS, with considerable genetic differences identified within or immediately downstream of Tn125. All reported pNDM-BJ01-like plasmids are exclusively found in Acinetobacter, whereas this is the first report of identification of a pNDM-BJ01-like plasmid in Enterobacteriaceae.

Risk factor analysis for extended-spectrum β-lactamase-producing Enterobacter cloacae bloodstream infections in central Taiwan

BACKGROUND

Enterobacter cloacae (E.cloacae) bloodstream infection (EcBSI) is an important cause of morbidity and mortality, with an increasing incidence in our hospital. We wanted to elucidate the risk factors of mortality among patients with ESBL-positive EcBSI in central Taiwan.

METHODS

We ordered the clinical and microbiological data of cases with diagnosis of EcBSI, and analyzed the isolates by using antibiotyping, detection of ESBL, detection of class 1 integron and genomic fingerprinting by pulsed-field gel electrophoresis (PFGE).

RESULTS

Seventy episodes of EcBSI from 70 patients (56 hospital-acquired infections) were enrolled. Significant differences were found between ESBL-positive and ESBL-negative isolates with regard to risk factors, including the diseases severity (p = 0.03), category of health care-associated infection (p = 0.04), prior use of antibiotics (p = 0.023), and prior use of a ventilator (p = 0.037). A significant difference in mortality between two groups (p = 0.004) was determined using the chi-square test, and a trend in mortality between two groups (p = 0.006, OR = 4.750, 95% C.I.=1.573-14.344) was determined using univariate logistic regression analysis. The predominant clone in ESBL-positive strains was associated with a higher mortality rate but not with the presence of the integron.

CONCLUSIONS

The study disclosed four types of clinical characteristics to obtain ESBL-positive EcBSI, and there was a trend in mortality too. We suggested the need to review antibiotic prescription practices, and the possible need to consider ESBL-positive strains in empirical treatment of bloodstream infection.

Occurrence of endemic plasmids causing β-lactam resistance inEnterobacteriaceae in children’s university hospital in Munich

Susceptibility of 62 clinical isolates of Enterobacteriaceae to 15 aminoglycosides, beta-lactams and fluoroquinolones was determined. The isolates originating from 3 intensive care units (neonatal, pediatric, and surgical) and the Department of Infant Internal Medicine of the Children's University Hospital City Center in Munich (Germany) were collected in August 1999, and March and October 2000. Transferability of antibiotic resistance from donors to their E. coli transconjugants was also demonstrated. The majority of isolates were resistant to ampicillin, cefoxitin, ceftriaxone, cefotaxime, ceftazidime and azthreonam but they were susceptible to cefepime, meropenem, aminoglycosides and fluoroquinolones. The occurrence of beta-lactamases and extended-spectrum beta-lactamases (ESBL) was also shown. In August 1999 75% of isolates produced beta-lactamases and 15% ESBL, in March 2000 95% of isolates produced beta-lactamases and 9% ESBL; in October 2000 all isolates produced beta-lactamases and only 5% produced ESBL. Plasmid DNA analysis in randomly chosen isolates and their transconjugants revealed the presence of plasmids ranging from 19 to 136 kb; in the majority of isolates a 120-kb plasmid was observed. Further analysis using restriction endonuclease suggested a dissemination and persistence of an endemic plasmid at all 4 wards of the large pediatric hospital in the City Center of Munich which may be responsible for resistance to beta-lactams among Enterobacteriaceae isolates.

Successive emergence of extended-spectrum beta-lactamase-producing and carbapenemase-producing Enterobacter aerogenes isolates in a university hospital

Sixty-two clinical isolates of Enterobacter aerogenes resistant to expanded-spectrum cephalosporins were collected between July 2003 and May 2005. Among these isolates, 23 (37.1%) were imipenem (IPM) susceptible, and 39 (62.9%) were IPM insusceptible, of which 89.7% (35/39) were resistant and 10.3% (4/39) were intermediate. Isolate genotypes were compared by pulsed-field gel electrophoresis. Of 62 isolates, 48 belonged to epidemic pulsotype A (77.4%). This pulsotype included 37.5% and 58.4% of beta-lactam phenotypes b and a, respectively. Nine isolates (14.5%) belonged to pulsotype E, which included 22.3% and 77.7% of phenotypes b and a, respectively. The beta-lactamases with pIs of 5.4, 6.5, 8.2, and 8.2 corresponded to extended-spectrum beta-lactamases (ESBLs) TEM-20, TEM-24, SHV-5, and SHV-12, respectively. Of 39 IPM-insusceptible E. aerogenes isolates, 26 (66.6%) were determined to be metallo-beta-lactamase producers, by using a phenotypic method. Of these isolates, 24 harbored a bla(IMP-1) gene encoding a protein with a pI of >9.5, and two carried the bla(VIM-2) gene encoding a protein with a pI of 5.3, corresponding to beta-lactamases IMP-1 and VIM-2, respectively. The remaining 13 (33.4%) isolates were negative for the bla(IMP-1) and bla(VIM-2) genes but showed an alteration of their outer membrane proteins (OMPs). Ten of these isolates produced the two possible OMPs (32 and 42 kDa), with IPM MICs between 8 and 32 microg/ml, and three others produced only a 32-kDa OMP with IPM MICs >32 microg/ml. This work demonstrates that, in addition to resistance to expanded-spectrum cephalosporins, IPM resistance can occur in ESBL-producing E. aerogenes isolates by carbapenemase production or by the loss of porin in the outer membrane.

Phenotypic and genetic analysis of Enterobacter spp. from a Brazilian oligotrophic freshwater lake

We characterized a population of Enterobacter spp. of the Enterobacter cloacae complex isolated from an oligotrophic lake; most isolates were identified as E. cloacae. Fingerprinting polymerase chain reaction (PCR), along with morphological, biochemical, physiological, and plasmid profiles analyses, including antimicrobial susceptibility testing, were performed on 22 environmental isolates. Misidentification occurred when using the API 20E identification system. Analysis of 16S rDNA sequences confirmed the close relatedness between species of the E. cloacae complex. The tDNA PCR allowed the differentiation and identification of the E. cloacae isolates. Evaluation of genetic diversity by 16S rDNA sequence, tDNA, internal transcribed spacers, and enterobacterial repetitive intergenic concensus profiles revealed nearly identical isolates, although they exhibited different physiological and antimicrobial resistance profiles. Among the Enterobacter isolates, 96% were resistant to at least one antimicrobial; multiple resistance was also found at a high frequency (86%). The antimicrobials against which resistance was found most frequently were beta-lactams, chloramphenicol, and streptomycin. Plasmids were found in 21 of the 22 Enterobacter isolates. This confirms the conception that antibiotic resistance can occur in oligotrophic freshwater lake bacteria, which has important implications for public health.

Detecting the form of selection in the outer membrane protein C of Enterobacter aerogenes strains and Salmonella species

The types of selective pressure operating on the outer membrane protein C (ompC) of Enterobacter aerogenes strains, the causative agent for nosocomial infections, and Salmonella sp., the hazardous pathogen are investigated using the maximum likelihood-based codon substitution models. Although the rate of amino acid replacement to the silent substitution (omega) across the entire codon sites of ompC of E. aerogenes (omega=0.3194) and Salmonella sp. (omega=0.2047) indicate that the gene is subjected to purifying selection (i.e. omega<1), approximately 3.7% of ompC codon sites in E. aerogenes (omega=21.52) are under the influence of positive Darwinian selection (i.e. omega>1). Such contrast in the intensity of selective pressures in both pathogens could be associated with the differential response to the adverse environmental changes. In E. aerogenes, majority of the positively selected sites are located in the hypervariable cell-surface-exposed domains whereas the trans-membrane domains are functionally highly constrained.

Prevalence of extended-spectrum β-lactamases in isolates of the Enterobacter cloacae complex from German hospitals

In 2002, 119 isolates of the Enterobacter cloacae complex were collected randomly from 11 German laboratories nationwide. Antibiotic susceptibilities were tested by disk-diffusion tests according to CLSI guidelines, and MICs were determined using Etests. PCRs were performed to amplify all TEM and SHV, and most CTX-M and OXA beta-lactamase genes. PCR products were sequenced to identify the precise extended spectrum beta-lactamase (ESBL) types. Isoelectric focusing (IEF) and PM/PML Etests were used to confirm production of the respective ESBLs. According to susceptibility tests and CLSI criteria, 49 (40%) isolates were resistant to extended-spectrum cephalosporins. Seven (5.8%) isolates were positive in at least one of the PCR assays. Sequencing identified production of TEM-1 beta-lactamase genes by three (2.9%) isolates, and ESBL genes of the CTX-M and SHV beta-lactamase families by five (4.2%) isolates. IEF confirmed the production of beta-lactamases in the expected pI ranges of the respective ESBLs, and four of the five ESBL-producers were detected using the PM/PML Etest. All ESBL-producing isolates showed co-resistance to sulphonamides.

Use of flow cytometry for analysis of phage-mediated killing of Enterobacter aerogenes

In this study, the use of flow cytometry to analyze phage-mediated killing of Enterobacter aerogenes under varying conditions of temperature and nutrient availability was assessed. Bacteriophage UZ1, specific for an E. aerogenes strain, was applied at a multiplicity of infection (MOI) of 1 and 1000 to a Teflon surface, artificially infected with its host at a level of 4.5 log cells. After incubation for 20 h, bacteriophages were quantified using the soft agar layer method. For the quantification of bacterial cells, plate counting and flow cytometric analysis of live/dead stained cells were performed in parallel. At an MOI of 1, phage treatment was successful only after incubation under nutrient-rich conditions at 37 degrees C: E. aerogenes cells were not detected and a tenfold increase in phage UZ1 was observed. At a MOI of 1000, no E. aerogenes cells could be cultured after incubation at 37 and 4 degrees C. However, flow cytometric analysis revealed that lysis did not occur at 4 degrees C but was achieved during subsequent plate culture. In conclusion, the use of flow cytometry enabled identification of culture-based bias during plate culture. The flow cytometric assay used in this study proved to be rapid, as this culture-independent method does not require lengthy incubation periods post-sampling. The bacteriophage-mediated killing of E. aerogenes cells on Teflon surfaces indicated that disinfection of E. aerogenes with bacteriophage UZ1 can be successful when high MOIs are achieved, while at low multiplicities of infection conditions favorable for phage replication are required.

Plasmid-mediated quinolone resistance in isolates obtained in german intensive care units

Screening of 703 isolates of Enterobacteriaceae, obtained from 34 German intensive care units (ICUs), revealed qnr-positive, integron-containing isolates of Enterobacter sp. and Citrobacter freundii from four patients in 2 German ICUs. This is one of the first reports of qnr-positive strains obtained from patients in Europe.

Molecular epidemiology of Enterobacteriaceae isolates producing extended-spectrum beta-lactamases in a French hospital

In 2002, 80 isolates of Enterobacteriaceae producing extended-spectrum beta-lactamases (ESBLs) were collected from infected patients in our hospital. Enterobacter aerogenes was the most common bacterium isolated from all specimens (36.5%). The ESBLs were predominantly (90%) TEM derivatives (TEM-24, TEM-3). Pulsed-field gel electrophoresis highlighted that E. aerogenes, Klebsiella pneumoniae, and Citrobacter koseri had a clonal propagation.

Integrons as tools for epidemiological studies

The integron content of Gram-negative strains implicated in three distinct episodes of suspected cross-infection among inpatients was investigated and compared with ribotyping. In the first episode, ribotyping identified a strain of Acinetobacter, isolated over a 3-month period, responsible for an outbreak associated with the use of mechanical ventilation in the intensive care unit (ICU). The second episode concerned simultaneous isolations of Pseudomonas aeruginosa and Serratia marcescens from 13 bronchoscopy patients. In these two episodes, results obtained by analysis of integron content and ribotyping were in agreement and correctly identified the epidemiologically related strains. In the third episode, isolates of Enterobacter cloacae were collected from patients in the neonatal ICU over a 3-month period. Although several isolates belonged to the same ribotype, cross-infection could not always be confirmed when the integron content was analysed. Integron detection can be considered a useful tool for studying molecular epidemiology in hospital environments, facilitating the quick detection of possible cross-infection cases, especially in critical wards such as the ICU.

Stability and activity of an Enterobacter aerogenes-specific bacteriophage under simulated gastro-intestinal conditions

A bacteriophage, designated UZ1 and showing lytic activity against a clinically important strain (BE1) of Enterobacter aerogenes was isolated from hospital sewage. The stability and lytic activity against this strain under simulated gastro-intestinal conditions was evaluated. After addition of bacteriophage UZ1 to a liquid feed at gastric pH 2, the phage was immediately inactivated and could not be recovered. However, by use of an antacid to neutralize stomach acidity, no significant changes in phage titer were observed after 2 h incubation at 37 degrees C. After supplementing pancreatic juice and further incubation for 4 h, the phage titer remained stable. The persistence of UZ1 in a mixed microbial ecosystem that was representative for the large intestine was monitored using an in vitro simulation of the human intestinal microbial ecosystem. A pulse administration of bacteriophage UZ1 at a concentration of 10(5) plaque-forming units (PFU)/ml to reactor 3 (which simulates the ascending colon) showed that, in the absence of the host, bacteriophage UZ1 persisted for 13 days in the simulated colon, while the theoretical washout was calculated at 16 days. To assess its lytic activity in an intestinal microbial ecosystem, a green fluorescent protein (gfp)-labeled E. aerogenes BE1 strain was constructed and gfp-specific primers were designed in order to quantify the host strain using real-time PCR. It was observed that bacteriophage UZ1 was able to replicate and showed lytic activity against E. aerogenes BE1/ gfp in an intestinal microbial ecosystem. Indeed, after 17 h a 2 log unit reduction of E. aerogenes BE1/ gfp was measured as compared with the assay without bacteriophage UZ1, while the phage titer increased by 2 log units at an initial multiplicity of infection of 0.07 PFU/colony-forming unit. This is the first report of an in vitro model to study bacteriophage activity in the complex intestinal microbial community.

Imipenem and expression of multidrug efflux pump in Enterobacter aerogenes

Imipenem is often used to treat intensive care unit patients infected by Enterobacter aerogenes, but it is leading to an increasing number of antibiotic resistant strains. Clinical isolates and imipenem resistant variants presented a high level of resistance to beta-lactam antibiotic group and to chemically unrelated drugs. We report here that imipenem selects strains which contain active efflux pumps ejecting various unrelated antibiotics including quinolones, tetracycline, and chloramphenicol. An increase of AcrA, an efflux pump component, was observed in the imipenem resistant variants. The overexpression of marA, involved in the genetic control of membrane permeability via porin and efflux pump expression, indicated the activation of the resistance genetic cascade in imipenem resistant variants.

Carbapenem resistance in a clinical isolate of Enterobacter aerogenes is associated with decreased expression of OmpF and OmpC porin analogs

We investigated the mechanism of imipenem resistance in Enterobacter aerogenes strain 810, a clinical isolate from the United States for which the imipenem MIC was 16 micro g/ml and the meropenem MIC was 8 micro g/ml. An imipenem-susceptible revertant, strain 810-REV, was obtained after multiple passages of the strain on nonselective media. For the revertant, the imipenem MIC was /=128 micro g/ml), cefoxitin (>/=32 micro g/ml), and cefotaxime (>/=64 micro g/ml) remained the same. The beta-lactamase and porin profiles of the parent, the revertant, and carbapenem-susceptible type strain E. aerogenes ATCC 13048 were determined. Strains 810 and 810-REV each produced two beta-lactamases with pIs of 8.2 and 5.4. The beta-lactamase activities of the parent and revertant were similar, even after induction with subinhibitory concentrations of imipenem. While 810-REV produced two major outer membrane proteins of 42 and 39 kDa that corresponded to Escherichia coli porins OmpC and OmpF, respectively, the parent strain appeared to produce similar quantities of the 39-kDa protein (OmpF) but decreased amounts of the 42-kDa protein (OmpC). When the parent strain was grown in the presence of imipenem, the 42-kDa protein was not detectable by gel electrophoresis. However, Western blot analysis of the outer membrane proteins of the parent and revertant with polyclonal antisera raised to the OmpC and OmpF analogs of Klebsiella pneumoniae (anti-OmpK36 and anti-OmpK35, respectively) showed that strain 810 expressed only the 42-kDa OmpC analog in the absence of imipenem (the 39-kDa protein was not recognized by the anti-OmpF antisera) and neither the OmpC nor the OmpF analog in the presence of imipenem. The OmpC analog is apparently down-regulated in the presence of imipenem; however, 810-REV expressed both OmpC and OmpF analogs. These data suggest that imipenem resistance in E. aerogenes 810 is primarily associated with the lack of expression of the analogs of the OmpC (42-kDa) and OmpF (39-kDa) outer membrane proteins, which also results in decreased susceptibility to meropenem and cefepime.

New model of ventilator-associated pneumonia in immunocompetent rabbits

OBJECTIVE

Despite the high rate of therapeutic failures in ventilator-associated pneumonia, up to now there has been no animal model specifically designed for antimicrobial evaluation. A rabbit model of ventilator-associated pneumonia is described for the first time in this study. DESIGN Prospective, randomized experimental study.

SETTING

An animal research laboratory.

SUBJECTS

Male New Zealand healthy rabbits (n = 44).

INTERVENTIONS

After oral intubation and an hour of mechanical ventilation, animals in the ventilator-associated pneumonia group (n = 22) were infected intrabronchially with a calibrated inoculum of. The nonventilated pneumonia group (n = 22) was composed of animals that received the same inoculum in the absence of mechanical ventilation. Rabbits from both groups were randomly killed 3, 6, 12, 24, or 48 hrs after inoculation. Pneumonia evaluation was based on histologic (macroscopic and microscopic score) and bacteriologic (bacterial count) findings.

MAIN RESULTS

Infected animals undergoing mechanical ventilation rapidly developed a progressive bilateral and multifocal pneumonia. Lung bacterial mean (sd) concentration was 6.48 (0.71) log10 colony-forming units (cfu) per gram of tissue at the 48th hour, whereas bacteremia occurred in most cases. In the nonventilated pneumonia group, pneumonia was less severe in terms of bacterial count (3.18 [1.86] log10 cfu/g; p <.05), and spleen cultures remained negative. In addition, microscopic examination revealed noninfectious lung injury in the ventilator-associated pneumonia group, especially hyaline membrane filling alveolar spaces. Of note, these features were never observed in the nonventilated pneumonia group.

CONCLUSIONS

An animal model of ventilator-associated pneumonia was obtained in immunocompetent rabbits. Histopathologic and bacteriologic features were similar to those found in humans. Obviously, pneumonia was more severe when animals underwent mechanical ventilation, especially in terms of systemic spread. Noninfectious lung injury corresponding to ventilation-induced lung injury may explain the difference. This model emphasizes the strong impact of both mechanical ventilation and infection on lung because they seem to act synergistically when causing alveolar damage. Moreover, it seems well suited to testing antimicrobial effectiveness.

Risk factors for infection and molecular typing in patients in the intensive care unit colonized with nosocomial Enterobacter aerogenes

OBJECTIVES

To determine the frequency of colonization by Enterobacter aerogenes in patients in the intensive care unit (ICU) for more than 48 hours and to evaluate the risk factors for infection in patients colonized by this bacteria.

DESIGN

An 8-month prospective study.

SETTING

A 12-bed medical-surgical ICU in a 450-bed, university-affiliated, tertiary-care hospital in Belgium.

METHOD

Pulsed-field gel electrophoresis was used to determine the genotypes of E. aerogenes isolates.

RESULTS

We observed two major clones of E. aerogenes in the ICU. Interestingly, 87.5% of infected patients had the same genomic profile for colonization and infection. Risk factors for infection in this particular population included younger age, prolonged hospital stay, mechanical ventilation, and bronchoscopy.

CONCLUSIONS

Colonization is a major prerequisite for infection. The identification of risk factors for infection in colonized patients can optimize the quality of treatment in the ICU.

Outbreak of infections caused by Enterobacter cloacae producing the integron-associated beta-lactamase IBC-1 in a neonatal intensive care unit of a Greek hospital

Nineteen of 27 ceftazidime-resistant Enterobacter cloacae isolates from a neonatal intensive care unit in Thessaloniki, Greece, had genes coding for the novel extended-spectrum beta-lactamase IBC-1; 18 of those 19 harbored similar conjugative plasmids and belonged to two distinct genetic lineages. A synergy test with ceftazidime and imipenem enabled us to identify five unrelated bla(IBC-1)-carrying E. cloacae isolates from other wards of the hospital. It seems that this integron-associated gene is capable of dispersing both by clonal spread and by gene dissemination.

Cloning and sequencing of Enterobacter aerogenes OmpC-type osmoporin linked to carbapenem resistance

Using outbreak-related strains of Enterobacter aerogenes, we cloned and sequenced ompK39, the structural gene coding for outer membrane protein OmpK39. Its lack of expression was closely associated with a phenotype exhibiting low-level carbapenem resistance. Detailed alignment of the predicted amino acid sequence revealed that OmpK39 is a member of the OmpC subclass of enterobacterial porins, with the highest degree of homology to Klebsiella pneumoniae OmpK36. Based on a computerized alignment including Escherichia coli PhoE and OmpF, the 3D structures of which are known from X-ray studies, OmpK39 can be assumed to form the typical beta-barrel structure which is common to all enterobacterial porins. Since no inhibitory DNA sequences could be detected in ompk39 in the resistant strains, porin deficiency leading to carbapenem resistance seems to involve alterations in key regulatory genes and/or the promotor sequence rather than a direct mutation in the structural gene.

Molecular epidemiological typing of Enterobacter cloacae isolates from a neonatal intensive care unit: three-year prospective study

Since 1992, there has been an increase in the incidence of Enterobacter sepsis in the neonatal intensive care unit (NICU) of the authors' hospital. From 1995 to 1997, a prospective molecular epidemiological survey of the colonizing and infecting strains isolated from neonates was conducted. Enterobacter cloacae was the most frequent cause of neonatal sepsis, accounting for 19.2% of all neonatal infections, reaching a peak incidence of 2.2/1000 during 1996. Fifty isolates from the NICU and four epidemiologically unrelated strains were characterized by pulse-field gel electrophoresis (PFGE), ribotyping, enterobacterial repetitive intergenic consensus (ERIC)-PCR and plasmid profiling. PFGE was the most discriminatory technique and identified 13 types (two of them classified into two and three subtypes) compared with ERIC-PCR, plasmid profiling and ribotyping that identified 11, 11 and seven types, respectively. A good correlation was found between all techniques. Five different clones caused 15 cases of sepsis. Clones A and B were prevalent in 1995 and 1996, but they were not isolated in 1997. An outbreak caused by clone G in 1997 was controlled by cohort nursing and hygienic measures, without changing the antibiotic policy. Strains were characterized by their antibiotic resistance pattern and divided into three groups. Group I correlated with PFGE types A, B1 and B2, which hyperproduced Bush type 1 chromosomal beta-lactamase and expressed extended-spectrum ?-lactamases (ESBLs). Group II only hyperproduced Bush type 1 chromosomal beta-lactamase and correlated with PFGE-types D1, D2, D3 and I. Finally, Group III, with inducible beta-lactamases, correlated with the rest of PFGE types. The sudden disappearance of E. cloacae after reinforcement of hygienic measures confirms the importance of patient-to-patient transmission.

Distribution and prevalence of antimicrobial resistance among gram-negative isolates in intensive care units (ICU) in Belgian hospitals between 1996 and 1999

OBJECTIVE

To assess the distribution and prevalence of resistance rates among Gram-negative isolates in Belgian intensive care units (ICUs) between 1996 and 1999.

METHODS

During 1996-1997 and 1998-1999, over a total period of 10 and 9 months respectively, members of the NPRS Belgian Study group collected, on clinical indications, 3029 consecutive initial isolates of Gram-negative bacteria from patients admitted to 26 Belgian hospitals and performed minimal inhibitory concentration (MIC) determinations by means of the E-test. Break-points were defined according to the criteria of the NCCLS.

RESULTS

The overall distribution of bacterial species was, in decreasing order of frequency: Pseudomonas aeruginosa > E. coli > E. aerogenes > K. pneumoniae > P. mirabilis > S. marcescens > E. cloacae > K. oxytoca > M. morganii > Stenotrophomonas maltophilia > Acinetobacter spp. All together these species and genera constituted about 90% of all isolates. The frequency of resistance for all the initial Gram-negative isolates in 1998-9 were: amoxicillin-clavulanic acid 60%, piperacillin 31%, piperacillin-tazobactam 20%, cefuroxime 58%, ceftriaxone 31%, ceftazidime 17%, aztreonam 23%, cefepime 10%, imipenem 13%, gentamicin 12%, amikacin 12% and ciprofloxacin 21%. Apart for an increase in multiple drug resistance among P. aeruginosa isolates, no significant trends were observed neither in species distribution nor in the overall prevalence of antimicrobial resistance among Gram-negative isolates from Belgian ICUs between 1996-7 and 1998-9.

CONCLUSIONS

Among Gram-negative isolates in Belgian ICUs, a very high frequency of resistance was seen to amoxicillin-clavulanic acid and cefuroxime, and rather high frequencies of resistance to piperacillin, ceftriaxone and aztreonam. Taking into account the species distribution and the prevalence of resistance, cefepime, imipenem, amikacin and gentamicin appeared generally suitable for empirical therapeutic use in severe ICU-acquired Gram-negative infections in Belgium. However, the therapeutic strategy should be adapted according to the local ecology of resistance.

Comparative in vitro activity of isepamicin and other antibiotics against gram-negative bacilli from intensive care units (ICU) in Belgium

The in vitro activity of isepamicin was compared to that of amikacin, gentamicin, cefepime, ciprofloxacin and meropenem against Gram-negative bacilli isolated from patients hospitalized in the intensive care unit (ICU) of 11 Belgian university or general hospitals between November 1998 and July 1999. The minimal inhibitory concentrations (MIC) for 1087 non-duplicate, consecutive aerobic Gram-negative isolates, including 798 Enterobacteriaceae and 289 non-fermenters, were determined by E-test for each antibiotic. Overall, isepamicin was active against 91% of all isolates and was found more active than ciprofloxacin (84% susceptibility), gentamicin (88% susceptibility), cefepime and amikacin (89% susceptibility each), but less active than meropenem (94% susceptibility). Enterobacter aerogenes isolates exhibited the highest resistance rate to ciprofloxacin (72%) while P. aeruginosa appeared the most resistant (frequently multi-resistant) pathogen. Compared to amikacin, MIC values for isepamicin were usually two- to fourfold lower for most inducible Enterobacteriacieae species and for Klebsiella spp., while they were identical for P. aeruginosa and other non-fermenters. Complete cross-susceptibility or cross-resistance between amikacin and isepamicin was observed in more than 95% of all tested isolates. On the other hand, 12% of all E. aerogenes isolates appeared resistant to amikacin and susceptible to isepamicin, while 6% of the P. aeruginosa were found to be resistant (or intermediate) to isepamicin and intermediate (or susceptible) to amikacin. No significant differences in pathogen distribution nor in resistance rates were observed between hospitals except for P. aeruginosa. Taking into account the species distribution and the prevalence of resistance to the different antibiotics tested, isepamicin appears as a suitable agent for empiric therapeutic use in severe ICU-acquired Gram-negative infections in Belgium.

Coexistence of SHV-4- and TEM-24-producing Enterobacter aerogenes strains before a large outbreak of TEM-24-producing strains in a French hospital

In 1996, a monitoring program was initiated at the teaching hospital of Amiens, France, and carried out for 3 years. All extended-spectrum beta-lactamase (ESBL)-producing Enterobacter aerogenes isolates recovered from clinical specimens were collected for investigation of their epidemiological relatedness by pulsed-field gel electrophoresis and enterobacterial repetitive intergenic consensus PCR (ERIC-PCR) and determination of the type of ESBL harbored by isoelectric focusing and DNA sequencing. Molecular typing revealed the endemic coexistence, during the first 2 years, of two clones expressing, respectively, SHV-4 and TEM-24 ESBLs, while an outbreak of the TEM-24-producing strain raged in the hospital during the third year, causing the infection or colonization of 165 patients. Furthermore, this strain was identified as the prevalent clone responsible for outbreaks in many French hospitals since 1996. This study shows that TEM-24-producing E. aerogenes is an epidemic clone that is well established in the hospital's ecology and able to spread throughout wards. The management of the outbreak at the teaching hospital of Amiens, which included the reinforcement of infection control measures, failed to obtain complete eradication of the clone, which has become an endemic pathogen.

National epidemiologic surveys of Enterobacter aerogenes in Belgian hospitals from 1996 to 1998

Two national surveys were conducted to describe the incidence and prevalence of Enterobacter aerogenes in 21 Belgian hospitals in 1996 and 1997 and to characterize the genotypic diversity and the antimicrobial resistance profiles of clinical strains of E. aerogenes isolated from hospitalized patients in Belgium in 1997 and 1998. Twenty-nine hospitals collected 10 isolates of E. aerogenes, which were typed by arbitrarily primed PCR (AP-PCR) using two primers and pulsed-field gel electrophoresis. MICs of 10 antimicrobial agents were determined by the agar dilution method. Beta-lactamases were detected by the double-disk diffusion test and characterized by isoelectric point. The median incidence of E. aerogenes colonization or infection increased from 3.3 per 1,000 admissions in 1996 to 4.2 per 1000 admissions in the first half of 1997 (P < 0.01). E. aerogenes strains (n = 260) clustered in 25 AP-PCR types. Two major types, BE1 and BE2, included 36 and 38% of strains and were found in 21 and 25 hospitals, respectively. The BE1 type was indistinguishable from a previously described epidemic strain in France. Half of the strains produced an extended-spectrum beta-lactamase, either TEM-24 (in 86% of the strains) or TEM-3 (in 14% of the strains). Over 75% of the isolates were resistant to ceftazidime, piperacillin-tazobactam, and ciprofloxacin. Over 90% of the strains were susceptible to cefepime, carbapenems, and aminoglycosides. In conclusion, these data suggest a nationwide dissemination of two epidemic multiresistant E. aerogenes strains in Belgian hospitals. TEM-24 beta-lactamase was frequently harbored by one of these epidemic strains, which appeared to be genotypically related to a TEM-24-producing epidemic strain from France, suggesting international dissemination.

TEM-24 extended-spectrum beta-lactamase-producing Enterobacter aerogenes: long-term clonal dissemination in French hospitals

OBJECTIVE

To investigate interstrain relatedness of TEM-24-producing Enterobacter aerogenes clinical strains isolated between 1993 and 1998 in 10 French hospitals from nine areas by pulsed-field gel electrophoresis (PFGE) and plasmid patterns.

METHODS

Fifteen TEM-24-producing strains and a set of 16 control strains having various other antibiotic resistance phenotypes were genotyped by PFGE. Plasmid DNA from TEM-24-producing strains and transconjugants was analyzed.

RESULTS

Analysis of XbaI macrorestriction patterns revealed only minor variations, and showed that all 15 TEM-24-producing strains were closely related. Some isolates originating from distant areas had indistinguishable patterns. According to their clustering correlation coefficients, they were also genomically distant from the control strains. Two plasmid patterns were observed in TEM-24-producing strains, one of them in 13 of the strains. Large plasmids of 85 kb encoding TEM-24 beta-lactamase were present in all isolates and, in all except one strain, could be transferred with high frequency by conjugation.

CONCLUSIONS

These results confirm that the spread of the TEM-24 extended-spectrum beta-lactamase in France was essentially due to the dissemination of a single clone.

Molecular epidemiology of a nosocomial outbreak due to Enterobacter cloacae and Enterobacter agglomerans in Campinas, São Paulo, Brazil

A total of 73 isolates (57 Enterobacter cloacae and 16 Enterobacter agglomerans), recovered during an outbreak of bacteremia in the Campinas area, São Paulo, Brazil, were studied. Of these isolates, 61 were from parenteral nutrition solutions, 9 from blood cultures, 2 from a sealed bottle of parenteral nutrition solution, and one was of unknown origin. Of the 57 E. cloacae isolates, 54 were biotype 26, two were biotype 66 and one was non-typable. Of 39 E. cloacae isolates submitted to ribotyping, 87.2% showed the same banding pattern after cleavage with EcoRI and BamHI. No important differences were observed in the antimicrobial susceptibility patterns among E. cloacae isolates exhibiting the same biotype, serotype and ribotype. All E. agglomerans isolates, irrespective of their origin, showed same patterns when cleaved with EcoRI and BamHI. The results of this investigation suggest an intrinsic contamination of parenteral nutrition solutions and incriminate these products as a vehicle of infection in this outbreak.

Emergence of Enterobacter aerogenes as a major antibiotic-resistant nosocomial pathogen in Belgian hospitals

OBJECTIVE

To study the epidemiology of Enterobacter aerogenes infections in Belgian hospitals and determine whether recent trends show an increase in incidence of E. aerogenes infections and antimicrobial resistance.

METHODS

Data from the bloodstream infection component of the National Surveillance of Hospital Infections (October 1992 to September 1996 data in 45 hospitals) and from a retrospective study on E. aerogenes clinical isolates (1994 and 1995 data in 41 hospitals) were analyzed.

RESULTS

E. aerogenes was recovered from clinical specimens with a mean incidence of 4.6 isolates per 10 000 patient-days and caused 0.20 bloodstream infections per 10 000 patient-days during the surveyed periods, respectively. Both rates increased significantly throughout the years. The proportion of E aerogenes within the Enterobacter genus was 35.4% in clinical isolates and 41.2% in bloodstream infections. Both proportions significantly increased over time. Incidence was not statistically different by hospital size but showed major differences between geographic regions. Resistance rates to third-generation cephalosporins and fluoroquinolones increased, and imipenem resistance emerged in several hospitals.

CONCLUSIONS

This report provides evidence of an increase in E. aerogenes infections in Belgian hospitals and documents an increase in antimicrobial resistance of E. aerogenes strains. These figures provide a baseline for further surveillance data.

Most Enterobacter aerogenes strains in France belong to a prevalent clone

The aim of this study was to determine the distribution in France of the Enterobacter aerogenes prevalent clone isolated in the hospitals of the Marseille area (A. Davin-Regli, D. Monnet, P. Saux, C. Bosi, R. Charrel, A. Barthelemy, and C. Bollet, J. Clin. Microbiol. 34:1474-1480, 1996). A total of 123 E. aerogenes isolates were collected from 23 hospital laboratories and analyzed by random amplification of polymorphic DNA and enterobacterial repetitive intergenic consensus-PCR to determine their epidemiological relatedness. Molecular typing revealed that 21 of the 23 laboratories had isolated this prevalent clone harboring the plasmid encoding for extended-spectrum beta-lactamase of the TEM-24 type. Most isolates were susceptible only to imipenem and gentamicin. Their dissemination seems to be clonal and was probably the result of the general use of broad-spectrum cephalosporins and quinolones. Four isolates showed an alteration of their outer membrane proteins, causing decrease of susceptibility to third-generation cephalosporins and imipenem and leading to the critical situation of having no alternative therapeutic. The large dissemination of the E. aerogenes prevalent clone probably results from its good adaptation to the antibiotics administered in France and the hospital environment, particularly in intensive care units.