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. Author manuscript; available in PMC: 2013 Sep 1.
Published in final edited form as: Diagn Microbiol Infect Dis. 2012 Jun 20;74(1):34–38. doi: 10.1016/j.diagmicrobio.2012.05.020

The role of horizontal gene transfer in the dissemination of extended-spectrum beta-lactamase-producing Escherichia coli and Klebsiella pneumoniae isolates in an endemic setting

Yohei Doi 1, Jennifer M Adams-Haduch 1, Anton Y Peleg 2,3,4, Erika MC D’Agata 2,*
PMCID: PMC3427399  NIHMSID: NIHMS379602  PMID: 22722012

Abstract

The contribution of horizontal gene transmission (HGT) in the emergence and spread of extended-spectrum beta-lactamase (ESBL)-producing gram-negative bacteria during periods of endemicity is unclear. Over a 12-month period, rectal colonization with SHV-5 and SHV-12 producing-Escherichia coli and Klebsiella pneumoniae was quantified among a cohort of residents in a long-term care facility. Demographic and clinical data were collected on colonized residents. Transferability of SHV-encoding plasmids and pulsed-field gel electrophoresis was performed to quantify the contribution of HGT and cross-transmission, respectively. A total of 25 (12%) of 214 enrolled patients were colonized with 11 SHV-5- and 17 SVH-12-producing E. coli and K. pneumoniae. Clonally-related isolates were detected among multiple residents residing on the same and different wards. Among 12 clonally-distinct isolates, HGT of SHV-5- and SHV-12-encoding plasmids was identified among 6 (50%) isolates. HGT among clonally-distinct strains contributes to the transmission dynamics of these ESBL-producing gram-negative bacteria and should be considered when evaluating the spread of these pathogens.

Keywords: SHV-5, SHV-12, nursing homes, clonal, plasmid

1. Introduction

Extended-spectrum beta-lactamase (ESBL)-producing gram-negative bacilli are rapidly spreading throughout the world (Bradford 2001). ESBL genes are typically encoded on self-transmissible plasmids and can be mobilized between the same and different species of Enterobacteriaceae. As such, dissemination of ESBL-producing gram-negative bacilli is a complex process that involves both propagation of predominant clones between patients and horizontal gene transfer (HGT) of the responsible resistance genes between different clones ([Baraniak et al. 2002], [Fernandez et al. 2007], [Harris et al. 2007a], [Li et al. 2003], [Sidjabat et al. 2009], and [Velasco et al. 2007]).

Identification of the same or similar ESBL-encoding plasmids from different clones or species has been documented ([Fernandez et al. 2007], [Harris et al. 2007a], [Li et al. 2003], [Sidjabat et al. 2009], and [Velasco et al. 2007]). These studies typically investigated isolates that were collected from certain geographic areas in a cross-sectional fashion, or single patients over time. However, data regarding the actual contribution of HGT in non-outbreak, endemic settings remain rare. Our present study was thus carried out to define the relative role of plasmid transfer as opposed to clonal expansion in a longitudinal cohort of patients, using a long-term care facility (LTCF) as a model.

2. Materials and Methods

From October 2006 through September 2007, rectal swabs were collected every 1–3 months from 214 residents of a 600-bed long-term care facility in Boston, MA, residing in 4 of 12 wards. This institution-wide infection control initiative focused on the clinical and molecular epidemiology of multidrug-resistant gram-negative bacteria, the results of which have been reported elsewhere ([O’Fallon et al. 2009] and [O’Fallon et al. 2010]). This study specifically focuses on the molecular characterization of the subset of ESBL-producing E. coli and K. pneumoniae isolates. The Institutional Review Board at Hebrew Senior Life and the Clinical Committee of Investigation at Beth Israel Deaconess Medical Center approved the conduct of this study.

2.1. Resident data collection

All demographic and clinical data were collected from electronic medical and pharmacology records, and the Minimum Data Set (MDS). The MDS is a federally mandated, standardized assessment instrument required to be completed for all residents of nursing homes in the United States (Hawes et al. 1995). Clinical data included age, gender, co-morbidities and fecal and urinary continence. The Activities of Daily Living (ADL) score, obtained from the MDS, was used to characterize the ability of residents to conduct tasks independently (Katz et al. 1993). This metric ranges from 0, those residents completely dependent on healthcare workers for their ADLs, to 6, those fully independent. A composite score for co-morbidities was calculated using the Charlson score (Charlson et al. 1983). Antibiotic exposure and hospitalizations during the 12 months prior to enrollment and during the time period between the baseline culture obtained at enrollment and the follow-up culture was also collected.

2.2. Microbiological methods

Specimens were plated onto McConkey agar plates supplemented with ceftazidime (2mg/L) to minimize the recovery of gram-negative bacteria that do not produce ESBLs. Species and susceptibility testing using standard disk diffusion method was performed as per the Clinical and Laboratory Standards Institute (CLSI) methodology (CLSI, M02-A10 and M100-S20 (Clinical and Laboratory Standards Institute 2006). ESBL production among E. coli and K. pneumoniae isolates that were resistant to ceftazidime was confirmed using the double disk method defined by the CLSI (M100-S20).

2.3. Detection of ESBL genes

The three common ESBL genes (TEM, SHV and CTX-M types) were detected by PCR experiments as described previously (Fernandez et al. 2007). In addition, blaCTX-M-15-specific primers were used for the blaCTX-M–positive E. coli isolate and all PCR products were sequenced to determine the exact ESBL gene (Fernandez et al. 2007).

2.4. Pulsed-field gel electrophoresis (PFGE)

All isolates producing SHV-5 or -12 ESBL were evaluated for genomic clonality by pulsed-field gel electrophoresis (PFGE). Fingerprints were generated by restriction endonuclease XbaI (New England Biolabs, Ipswich, MA) and subjected to electrophoresis using a CHEF-DR III system (Bio-Rad, Hercules, CA) as described previously (Fernandez et al. 2007). The relatedness of PFGE patterns was determined by the unweighted-pair group method using average linkages and cluster analysis with the Dice setting on Bionumerics software, version 6.01 (Applied Maths, Sint-Martens-Latem, Belgium).

2.5. Plasmid profiling

Plasmids from isolates producing SHV-5 or -12 ESBL genes were analyzed. The transferability of SHV-encoding plasmids was assayed by both transformation and conjugation as described previously (Fernandez et al. 2007). In brief, E. coli DH10B transformants or E. coli J53AziR transconjugants were selected by using 50 mg/L of ampicillin as the selective agent. The presence of the SHV gene was confirmed with PCR and supported by the ESBL phenotype in the transformants and transconjugants. The plasmids were then extracted using the standard alkaline lysis method (Sidjabat et al. 2009.) The fingerprints of the SHV-encoding plasmids were generated by digesting plasmids with restriction enzyme HpaI or PstI (New England Biolabs). After an overnight electrophoresis, plasmid fingerprints were visualized by ethidium bromide staining and analyzed using the Bionumerics software as for PFGE.

3. Results

A total of 39 (18%) of 214 patients were colonized with ceftazidime-resistant isolates, of which 27 were K. pneumoniae and 12 were E. coli. Among these isolates, ESBL production was phenotypically confirmed among 23 K. pneumoniae and 7 E. coli isolates, recovered from 27 patients (3 patients were co-colonized with K. pneumoniae and E. coli). Antimicrobial susceptibility profiles are shown in Table 1.

Table 1.

Antimicrobial susceptibility profile of Klebsiella pneumoniae and Escherichia coli isolates recovered from 28 patients, using standard disk diffusion method

K. pneumoniae (n = 23) E. coli (n = 7)
Susceptible Intermediate Resistant Susceptible Intermediate Resistant
Ceftazidime 0 0 23 (100%) 0 0 7 (100%)
Cefotaxime 0 0 23 (100%) 0 1 (14%) 6 (86%)
Cefoxitin 19 (83%) 4 (17%) 0 6 (86%) 0 1 (14%)
Cefepime 21 (91%) 2 (9%) 0 6 (86%) 0 1 (14%)
Aztreonam 0 0 23 (100%) 0 0 7 (100%)
Ertapenem 23 (100%) 0 0 7 (100%) 0 0
Imipenem 23 (100%) 0 0 7 (100%) 0 0
Gentamicin 22 (96%) 0 1 (4%) 7 (100%) 0 0
Amikacin 4 (17%) 13 (57%) 6 (26%) 6 (86%) 1 (14%) 0
Ciprofloxacin 3 (13%) 0 20 (87%) 6 (86%) 0 1 (14%)
Cotrimoxazole 10 (43%) 7 (30%) 6 (26%) 6 (86%) 0 1 (14%)
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3.1. ESBL genes

All 23 K. pneumoniae isolates had SHV-type ESBL genes. Of those, 5 were SHV-5, 17 were SHV-12 and 1 was SHV-31. Of the 7 E. coli isolates, 6 had SHV-5 and 1 had CTX-M-15. All E. coli and all but one K. pneumoniae isolates also had TEM-1, which is a non-ESBL that confers resistance to penicillins.

3.2. Clinical data among patients colonized with SHV-5 and SHV-12-producing isolates

A total of 25 (12%) patients were colonized with 28 isolates producing SHV-5 and SHV-12. The average age of residents was 94 years (range 81–96 years). Twenty-one residents (84%) were female and 22 (88%) were Caucasian. The average Charlson score was 2.6 (range 0–6) with the majority having a diagnosis of dementia (22 [88%]). The average ADL was 0 (range 0–2). Urinary or fecal incontinence was documented among 24 (96%) of residents. A total of four (16%) and 6 (24%) residents had been hospitalized or received at least one course of antimicrobials during the year prior to enrollment, respectively. The average length of stay in the LTCF prior to the first culture was 8 years (range 5 months-14 years).

3.3. Clonality of SHV-5 and SHV-12-producing E. coli and K. pneumoniae isolates

The 28 isolates producing SHV-5 or SHV-12 were subjected to PFGE. Of the 6 E. coli isolates, all of which had SHV-5-encoding plasmids, 4 were closely related with >90% identity, whereas the other 2 isolates were unrelated (Fig. 1). The 5 SHV-5-producing K. pneumoniae isolates were grouped into 4 groups using a cutoff of 80% (Fig. 2). Thus, there were 7 groups of isolates producing SHV-5 for E. coli and K. pneumoniae combined. The 17 SHV-12-producing K. pneumoniae isolates were grouped into 5 groups using a cutoff of 80%, with the largest group containing 9 isolates (Fig. 2).

Fig. 1.

Fig. 1

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PFGE dendrogram of SHV-5-producing E. coli.

Fig. 2.

Fig. 2

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PFGE dendrogram of SHV-5/12-producing K. pneumoniae.

Ward location of residents harboring these isolates is shown in Figures 1 and 2. Clonally-related isolates were identified among residents residing on the same and different wards.

3.4. SHV-5 and SHV-12 plasmid analysis

Isolates representing each group as defined by a cutoff of 80% on PFGE were included for plasmid profiling. This included 7 SHV-5-producing isolates (3 E. coli and 4 K. pneumoniae) and 5 SHV-12-producing isolates (all K. pneumoniae). E. coli DH10B transformants were used for all isolates except one (K. pneumoniae R63RC), for which only E. coli J53 transconjugant was available. Transconjugants could be obtained for 10 isolates except K. pneumoniae R36RA and R215NA. As can be seen in the Fig. 3 and 4, 4 of the 7 SHV-5-encoding plasmids (2 from E. coli, 2 from K. pneumoniae) shared nearly identical restriction patterns (87 to 93% identity). Of the other 3 SHV-5-encoding plasmids, 2 of them (1 from E. coli and 1 from K. pneumoniae) shared 67% identity with each other using PstI but not with HpaI. The sizes of the SHV-5-encoding plasmids ranged between 50 and 80kb.

Fig. 3.

Fig. 3

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Restriction profiles of SHV-12-encoding plasmids obtained from transformants, with the exception of K. pneumoniae R63, which was obtained from its transconjugant. Lanes M, l DNA-HindIII digest marker; 1 and 7, K. pneumoniae R8; 2 and 8, K. pneumoniae R18; 3 and 9, K. pneumoniae R36; 4 and 10, K. pneumoniae R152; 5 and 11, K. pneumoniae R178; 6 and 12, K. pneumoniae R63. Lanes 1 through 6 were digested with HpaI; lanes 7 through 12 were digested with PstI.

Fig. 4.

Fig. 4

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Restriction profiles of SHV-5-encoding plasmids obtained from transformants. Lanes M, l DNA-HindIII digest marker; 1 and 7, E. coli R27; 2 and 8, E. coli R31; 3 and 9, E. coli R173; 4 and 10, K. pneumoniae R173; 5 and 11, K. pneumoniae R1RC; 6 and 12, K. pneumoniae R215. Lanes 1 through 6 were digested with HpaI; lanes 7 through 12 were digested with PstI.

Of the 5 SHV-12-encoding plasmids, 2 were nearly identical with each other using both enzymes (84.0 to 93.7% identity), with another pair sharing the majority of restriction bands with this pair.

Therefore, in our assessment of 12 clonally-distinct isolates producing SHV-5 or SHV-12, HGT of ESBL-encoding plasmids was identified among 6 (50%) isolates.

3.5. Spatial and temporal analysis among patients colonized with identical or nearly identical plasmids

A total of 6 residents were colonized with clonally-distinct K. pneumoniae and E. coli carrying similar plasmids. The average age of these residents was 91 years old (range 88–96 years) and all were female and Caucasian. The average Charlson score was 2 (range 1–6) and all six had an ADL score of 0. Five had a diagnosis of dementia. Only one (17%) and 2 (33%) residents were previously hospitalized or received at least one course of antimicrobials, respectively, in the 12 months prior to study enrollment.

A total of 4 residents were colonized with K. pneumoniae and E. coli carrying similar SHV-5 encoding plasmids. All four residents resided on the same ward (Ward A) an average of 6.4 years (range 5 months to 13.5 years) prior to enrollment. The ESBL-producing isolates were identified from the baseline rectal culture obtained at study enrollment among 3 of these residents. None of these residents received antimicrobials during the 12-months prior to the baseline culture. For the fourth resident, ESBL-producing K. pneumoniae was first identified on the third follow-up culture obtained 162 days after the baseline culture. The only antimicrobial exposure in this resident was a 7-day course of oral trimethoprim-sulfamethoxazole received during the interval between the baseline and third follow-up culture.

The two residents colonized with K. pneumoniae carrying the similar SHV-12 encoding plasmid resided on different wards (Ward A and B). Their average length of stay in these wards prior to obtaining the baseline culture was 8 years (2 and 14 years). Neither resident received antimicrobials or was hospitalized in the 12 months prior to enrollment. Colonization was identified from the baseline culture among both residents.

4. DISCUSSION

This study characterized the spread of SHV-5 and SVH-12-producing E. coli and K. pneumoniae isolates during a 12-month period of endemicity. At least 12% of residents in this LTCF were colonized with these ESBL-producing gram-negative bacteria. Although cross-transmission of clonally-related strains between patients was documented, transfer of ESBL-carrying plasmids also contributed to the spread of these multidrug-resistant organisms. In this study, 50% of clonally-unrelated strains harbored identical or nearly identical SHV-5 and SVH-12-carrying plasmids. This HGT occurred between the same Enterobacteriaceae species as well as between E. coli and K. pneumoniae.

In the outbreak setting, clonal dissemination is the predominant mechanisms of acquiring ESBL-producing gram-negative bacteria (Filozov et al. 2009). However, during periods of endemicity, several studies have documented that cross-transmission of these resistant bacteria only explains 4–50% of acquisition events ([Harris et al. 2007a], [Harris et al. 2007b], and [Kola et al. 2007]). One study showed that endogenous acquisition of multidrug-resistant gram-negative bacteria, defined as recovery of a non-MDR-gram-negative bacteria at baseline with subsequent recovery of an MDR-gram-negative bacteria of the same species and PFGE pattern during follow-up cultures, occurred among 69% of patients in an out-patient chronic hemodialysis unit over a 4-month period (Pop-Vicas et al. 2008). The findings of our study corroborate these data and imply that during periods of endemicity, both exogenous and endogenous acquisition of these ESBL-producing gram-negative bacteria contribute to their dissemination.

HGT occurs predominantly in niches with high bacterial densities, where the likelihood of cell-to-cell contact, and therefore plasmid conjugation is increased (Sorenson et al. 2005). After gene transfer, the newly formed resistant strain requires amplification via antimicrobial pressure ([Faure et al. 2010] and [Tenover et al. 2006]. In this study, only one of six residents in whom HGT was documented received antimicrobials, a 7-day course of oral trimethoprim-sulfamethoxazole. The minimal antimicrobial exposure suggests that there may be other factors involved, although a small sample size could also explain these findings.

The following limitations need to be considered. First, only isolates that were fully resistant to ceftazidime were analyzed for ESBL-production and therefore the prevalence of these pathogens may have been underestimated. Second, the study was conducted in a LTCF, a unique setting which allows evaluation of the transmission dynamics over prolonged periods of time, but differs from other healthcare environments. In this study, the average length of stay in the LTCF prior to enrollment was eight years. This prolonged exposure to other residents would provide substantially more opportunities for cross-transmission than in the hospital setting, and therefore the results of this study may not be generalizable. Third, due to logistical reasons, only residents from 4 of 12 wards were enrolled thereby providing a limited sample size. Lastly, the ESBL screening method using 2μg/L of ceftazidime in McConkey agar was designed to identify SHV- and TEM-type ESBLs, which were prevalent at the time of the study. This approach may have missed some CTX-M-type ESBL producing organisms, which are not resistant to ceftazidime, especially those producing CTX-M-2 or CTX-M-9-group ESBLs, but would have, and did, identify those producing CTX-M-15, which are the most prevalent CTX-M-type ESBL in the U.S. (Johnson et al. 2012).

Current efforts to curtail the spread of ESBL-producing gram-negative bacteria focus on preventing cross-transmission and decreasing antimicrobial exposure, both during outbreak and endemic periods. The outcome of interest, in evaluating the efficacy of these interventions, is predominantly the reduction in spread of clonally-related strains. The results of this study suggest that quantification of HGT is also necessary during periods of endemicity.

Acknowledgments

Funding: This work was supported in part by a National Institute of Allergy and Infectious Diseases grant (K22AI080584) and the Pennsylvania Department of Health (#4100047864) (DY).

We would like to thank Erin O’Fallon and Lata Venkataraman for data collection and specimen processing.

Footnotes

Transparency Declaration: Y.D. has received funding from Merck and served on an advisory board for Pfizer, J.M.A.:none, A.Y.P has been to an advisory meeting for Abbott Molecular and Ortho-McNeil-Janssen, E.M.C.D.: none,

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References

  1. Baraniak A, Fiett J, Sulikowska A, Hryniewicz W, Gniadkowski M. Countrywide spread of CTX-M-3 extended-spectrum beta-lactamase-producing microorganisms of the family Enterobacteriaceae in Poland. Antimicrob Agents Chemother. 2002;46:151–9. doi: 10.1128/AAC.46.1.151-159.2002. [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. Bradford PA. Extended-spectrum beta-lactamases in the 21st century: characterization, epidemiology, and detection of this important resistance threat. Clin Microbiol Rev. 2001;14:933–51. doi: 10.1128/CMR.14.4.933-951.2001. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Charlson ME, Pompei P, Ales KL, MacKenzie CR. A new method of classifying prognostic comorbidity in longitudinal studies: development and validation. J Chronic Dis. 1987;40:373–83. doi: 10.1016/0021-9681(87)90171-8. [DOI] [PubMed] [Google Scholar]
  4. Clinical and Laboratory Standards Institute. Approved standard M2-A9. Wayne, PA: Clinical and Laboratory Standards Institute; 2006. Performance standards for antimicrobial disk susceptibility tests. [Google Scholar]
  5. Faure S, Perrin-Guyomard A, Delmas JM, Chatre P, Laurentie M. Transfer of plasmid-mediated CTX-M-9 from Salmonella enterica serotype Virchow to Enterobacteriaceae in human flora-associated rats treated with cefixime. Antimicrob Agent Chemother. 2010;54:164–9. doi: 10.1128/AAC.00310-09. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Fernández A, Gil E, Cartelle M, et al. Interspecies spread of CTX-M-32 extended-spectrum beta-lactamase and the role of the insertion sequence IS1 in down-regulating bla CTX-M gene expression. J Antimicrob Chemother. 2007;59:841–7. doi: 10.1093/jac/dkm030. [DOI] [PubMed] [Google Scholar]
  7. Filozov A, Visintainer P, Carbonaro C, Aguero-Rosenfeld M, Wormser GP, Montecalvo MA. Epidemiology of an outbreak of antibiotic-resistant Klebsiella pneumoniae at a tertiary care medical center. Am J Infect Control. 2009;37:723–8. doi: 10.1016/j.ajic.2009.02.006. [DOI] [PubMed] [Google Scholar]
  8. Gardam MA, Burrows LL, Kus JV, et al. Is surveillance for multidrug-resistant Enterobacteriaceae an effective infection control strategy in the absence of an outbreak? J Infect Dis. 2002;186:1754–60. doi: 10.1086/345921. [DOI] [PubMed] [Google Scholar]
  9. Harris AD, Perencevich EN, Johnson JK, et al. Patient-to-patient transmission is important in extended-spectrum beta-lactamase-producing Klebsiella pneumoniae acquisition. Clin Infect Dis. 2007a;45:1347–50. doi: 10.1086/522657. [DOI] [PubMed] [Google Scholar]
  10. Harris AD, Kotetishvili M, Shurland S, et al. How important is patient-to-patient transmission in extended-spectrum beta-lactamase Escherichia coli acquisition. Am J Infect Control. 2007b;35:97–101. doi: 10.1016/j.ajic.2006.09.011. [DOI] [PubMed] [Google Scholar]
  11. Hawes C, Morris JN, Phillips CD, Mor V, Fries BE, Nonemaker S. Reliability estimates for the Minimum Data Set for nursing home resident assessment and care screening (MDS) Gerontologist. 1995;35:172–8. doi: 10.1093/geront/35.2.172. [DOI] [PubMed] [Google Scholar]
  12. Johnson JR, Urban C, Weissman SJ, et al. Molecular Epidemiological Analysis of Escherichia coli Sequence Type ST131 (O25:H4) and blaCTX-M-15 among Extended-Spectrum β-Lactamase-Producing E. coli from the United States (2000–2009) Antimicrob Agents Chemother. 2012 doi: 10.1128/AAC.05824-11. [Epub ahead of print] [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Katz S. Assessing self-maintenance: activities of daily living, mobility, and instrumental activities of daily living. J Am Geriatr Soc. 1983;31:721–7. doi: 10.1111/j.1532-5415.1983.tb03391.x. [DOI] [PubMed] [Google Scholar]
  14. Kola A, Holst M, Chaberny IF, Ziesing S, Suerbaum S, Gastmeier P. Surveillance of extended-spectrum beta-lactamase-producing bacteria and routine use of contact isolation: experience from a three-year period. J Hosp Infect. 2007;66:46–51. doi: 10.1016/j.jhin.2007.01.006. [DOI] [PubMed] [Google Scholar]
  15. Li CR, Li Y, Zhang PA. Dissemination and spread of CTX-M extended-spectrum beta-lactamases among clinical isolates of Klebsiella pneumoniae in central China. Int J Antimicrob Agents. 2003;22:521–5. doi: 10.1016/s0924-8579(03)00157-2. [DOI] [PubMed] [Google Scholar]
  16. O’Fallon E, Schreiber R, Kandel R, D’Agata EMC. Multidrug-Resistant Gram-Negative Bacteria within a Long-Term Care Facility: assessment of residents, health care workers and inanimate surfaces. Infect Control Hosp Epidemiol. 2009;30:1172–9. doi: 10.1086/648453. [DOI] [PMC free article] [PubMed] [Google Scholar]
  17. O’Fallon E, Kandell R, Schreiber R, D’Agata EMC. Acquisition of Multidrug-Resistant Gram-Negative Bacteria: incidence and risk factors within a long-term care population. Infect Control Hosp Epidemiol. 2010;31:1148–53. doi: 10.1086/656590. [DOI] [PubMed] [Google Scholar]
  18. Pop-Vicas A, Strom J, Stanely K, D’Agata EMC. Multidrug-resistant gram-negative bacteria among patients who require chronic hemodialysis. Clin J Am Soc Nephrol. 2008;3:52–8. doi: 10.2215/CJN.04651107. [DOI] [PMC free article] [PubMed] [Google Scholar]
  19. Sidjabat HE, Silveira FP, Potoski BA, et al. Interspecies spread of Klebsiella pneumoniae carbapenemase gene in a single patient. Clin Infect Dis. 2009;49:1736–8. doi: 10.1086/648077. [DOI] [PubMed] [Google Scholar]
  20. Sidjabat HE, Paterson DL, Adams-Haduch JM, et al. Molecular epidemiology of CTX-M-producing Escherichia coli isolates at a tertiary medical center in western Pennsylvania. Antimicrob Agents Chemother. 2009;53:4733–9. doi: 10.1128/AAC.00533-09. [DOI] [PMC free article] [PubMed] [Google Scholar]
  21. Sorenson SJ, Bailey M, Hansen LH, Kroer N, Wuertz S. Studying plasmid horizontal transfer in situ: a critical review. Nat Rev. 2005;3:700–10. doi: 10.1038/nrmicro1232. [DOI] [PubMed] [Google Scholar]
  22. Tenover FC. Mechanisms of antimicrobial resistance in bacteria. Am J Med. 2006;119:S3–S10. doi: 10.1016/j.amjmed.2006.03.011. [DOI] [PubMed] [Google Scholar]
  23. Velasco C, Rodríguez-Baño J, García L, et al. Eradication of an extensive outbreak in a neonatal unit caused by two sequential Klebsiella pneumoniae clones harbouring related plasmids encoding an extended-spectrum beta-lactamase. J Hosp Infect. 2009;73:157–63. doi: 10.1016/j.jhin.2009.06.013. [DOI] [PubMed] [Google Scholar]

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