Abstract

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.

Resistance to broad-spectrum antimicrobials, such as the extended-spectrum cephalosporins, is a well-recognized problem among Enterobacteriaceae [1]. Carbapenems have served as an important antimicrobial class for the treatment of these organisms and, until recently, resistance to carbapenems has been uncommon among Enterobacteriaceae in the United States. However, the emergence of novel β-lactamases with direct carbapenem-hydrolyzing activity has contributed to an increased prevalence of carbapenem-resistant Enterobacteriaceae (CRE). CRE are particularly problematic given the frequency with which Enterobacteriaceae cause infections [2], the high mortality associated with infections caused by CRE [3–5], and the potential for widespread transmission of carbapenem resistance via mobile genetic elements [6, 7].

Although CRE have primarily been recognized in health care settings [3, 8], Enterobacteriaceae are common causes of both health care and community infections, raising the possibility of spread of CRE into the community. These issues, combined with the limited therapeutic options available to treat patients infected with these organisms, have made CRE of epidemiologic importance nationally. In this brief review, we will describe the epidemiology of CRE in the United States, with an emphasis on carbapenemase-producing strains, and discuss strategies for prevention.

EPIDEMIOLOGY

CRE appear to have been uncommon in the United States before 1992. Using data from the National Nosocomial Infection Surveillance (NNIS) system from 1986 to 1990, Gaynes et al found that only 2.3% of 1825 Enterobacter isolates tested nonsusceptible to imipenem [9]. However, over the last decade CRE have been reported more commonly. In the Meropenem Yearly Susceptibility Test Information Collection Program, meropenem resistance among clinical isolates of Klebsiella pneumoniae increased significantly from 0.6% in 2004 to 5.6% in 2008 [10]. Among isolates reported to the National Healthcare Safety Network (NHSN) in 2006–2007, carbapenem resistance was reported in up to 4.0% of Escherichia coli and 10.8% of K. pneumoniae isolates that were associated with certain device-related infections [2].

The Emergence of Klebsiella pneumoniae Carbapenemases

Although initial reports described that carbapenem resistance among Enterobacteriaceae was due to overproduction of AmpC-mediated β-lactamases or extended-spectrum β-lactamases (ESBLs) in organisms with porin mutations [11–13], carbapenemases have now become another mechanism for carbapenem resistance among CRE in the United States. The most common carbapenemase in the United States is Klebsiella pneumoniae carbapenemase (KPC), an Ambler molecular class A enzyme that utilizes serine at the active site to facilitate hydrolysis of a broad variety of β-lactams [14].

KPC-producing Enterobacteriaceae were first reported in a clinical specimen from a patient in North Carolina in 2001 [7]. Subsequently, outbreaks and transmission of KPC-producing organisms were reported, predominantly from the northeastern United States [3, 8]. In a 2002–2003 surveillance study in New York City, 9 of 602 K. pneumoniae isolates were found to contain the blaKPC gene. In the following year, 20 additional KPC-producing isolates were identified from 2 hospital outbreaks in the city [3]. Since that time, KPC-producing isolates have become more widespread nationally. Although the Centers for Disease Control and Prevention (CDC) does not yet perform systematic surveillance for these organisms, as of December 2010, KPC-producing isolates have been received or identified from 36 states, Washington, DC, and Puerto Rico (unpublished CDC data).

In addition, reports of KPC-producing Enterobacteriaceae have emerged from other parts of the world—some associated with receipt of medical care in the United States—suggesting intercontinental spread of these organisms [15]. In Israel, a number of facilities reported increases in KPC-producing Enterobacteriaceae beginning in 2006 [16, 17]. Pulsed-field gel electrophoresis (PFGE) analysis of KPC-producing K. pneumoniae from 8 hospitals and 5 chronic care centers demonstrated a clonal relationship between many of these isolates, some of which appeared to be genetically related to strains reported from outbreaks in the United States [17]. These organisms have now spread widely; countries from which KPCs have been reported since 2001 are shown in Figure 1.

Figure 1.

International dissemination of Klebsiella pneumoniae carbapenemase (KPC)–producing Enterobacteriaceae. This map indicates countries where KPC-producing Enterobacteriaceae have been described in published reports available as of 11 February, 2011. Because of lack of systematic surveillance for these organisms, countries not highlighted in this figure might also have unreported KPC-producing Enterobacteriaceae.

Figure 1.

International dissemination of Klebsiella pneumoniae carbapenemase (KPC)–producing Enterobacteriaceae. This map indicates countries where KPC-producing Enterobacteriaceae have been described in published reports available as of 11 February, 2011. Because of lack of systematic surveillance for these organisms, countries not highlighted in this figure might also have unreported KPC-producing Enterobacteriaceae.

In the United States, much of the dissemination of KPC-producing CRE isolates also appears to be clonal [18]. A sample of KPC-producing K. pneumoniae isolates sent to the CDC for reference testing from 1996 to 2008 was characterized using PFGE and multilocus sequence typing (MLST). This analysis revealed that a dominant strain, ST258, accounted for approximately 70% of all KPC-producing K. pneumoniae isolates sent to the CDC during that time period [18].

In addition to β-lactams, KPC-producing isolates demonstrate resistance to many agents commonly used to treat gram-negative bacteria, including quinolones and aminoglycosides [19, 20]. Among 344 isolates of KPC-producing Enterobacteriaceae sent to CDC for evaluation from January 2007 through October 2009, 312 (91%) had a colistin minimum inhibitory concentration (MIC) ≤ 2μg/mL, and 304 (88%) had a tigecycline MIC ≤ 2μg/mL. Only 2 isolates were nonsusceptible to both colistin and tigecycline (unpublished CDC data). “Pan-resistance” to antimicrobials agents has also been reported [21].

Novel Phenotypes: The Metallo-β-Lactamases

The Ambler class B metallo-β-lactamases (MBLs) differ from other carbapenemases by the utilization of zinc at the active site to facilitate hydrolysis [14]. Although MBLs have been described in Pseudomonas species [22], they have only rarely been reported among Enterobacteriaceae in the United States. In other parts of the world, however, MBL-producing Enterobacteriaceae are more common. Until recently, the most common MBLs found worldwide in Enterobacteriaceae were VIMs (Verona integron-encoded MBLs) and IMPs (active on imipenem).

In 2009, a novel MBL, the New Delhi MBL (NDM), was described [23, 24]. NDM was first recognized in a K. pneumoniae isolate from a Swedish patient who had received medical care in India [24] and was soon recognized as an emerging mechanism of resistance in multiple species of Enterobacteriaceae in the United Kingdom [23]. Many of the early cases in the United Kingdom were associated with receipt of medical care in India or Pakistan [23, 25].

NDM has also been recognized among Enterobacteriaceae in India. In 1 study of Enterobacteriaceae from a tertiary care center in Mumbai, 22 of 24 consecutively collected CRE isolates contained blaNDM, the gene encoding NDM [26]. Kumarasamy et al found that among a convenience sample of Enterobacteriaceae obtained from patients in India, between 31% and 55% of CRE isolates were NDM-producers [25]. Many of the NDM-producing isolates from India were from patients with community-onset infections. Countries that have reported NDM-producing Enterobacteriaceae since 2009 are shown in Figure 2.

Figure 2.

International dissemination of New Delhi metallo-β-lactamase (NDM)–producing Enterobacteriaceae. This map indicates countries where NDM-producing Enterobacteriaceae have been described in published reports available as of 11 February, 2011. Because of lack of systematic surveillance for these organisms, countries not highlighted in this figure might also have unreported NDM-producing Enterobacteriaceae.

Figure 2.

International dissemination of New Delhi metallo-β-lactamase (NDM)–producing Enterobacteriaceae. This map indicates countries where NDM-producing Enterobacteriaceae have been described in published reports available as of 11 February, 2011. Because of lack of systematic surveillance for these organisms, countries not highlighted in this figure might also have unreported NDM-producing Enterobacteriaceae.

In the United States, between January 2009 and February 2011, 7 NDM-producing Enterobacteriaceae have been identified among clinical isolates sent to CDC (Table 1). In addition, 6 Enterobacteriaceae containing VIMs or IMPs have also been identified between November 2009 and November 2010 (Table 1). Of these 13 MBL-containing Enterobacteriaceae, 8 were in patients whose primary risk was exposure to health care in countries where these organisms are more common.

Table 1.

Cases of Metallo-β-Lactamase–Producing Enterobacteriaceae in the United States Reported to CDC, 2009–2010

Case MBL type Culture date Organism Culture site Received medical care outside United States Additional patient information 
NDM Apr 2009 Enterobacter cloacae Urine Yes Hospitalization in India 
NDM Dec 2009 Klebsiella pneumoniae Urine Yes Hospitalization in India 
NDM May 2010 Escherichia coli Urine No Travel in India, history of multiple comorbidities, indwelling medical device 
NDM Sep 2010 K. pneumoniae Respiratory Yes Hospitalization in Pakistan 
NDM Sep 2010 E. coli Respiratory Yes Received medical care in India, no hospitalizations 
NDM Dec 2010 K. pneumoniae Urine Yes Hospitalization in India 
NDM Feb 2011 K. pneumoniae Respiratory Yes Hospitalization in India 
IMP Nov 2009 K. pneumoniae Urine No No known travel outside United States 
IMP May 2010 K. pneumoniae Urine No No known travel outside United States 
10 IMP Jun 2010 K. pneumoniae Urine No No known travel outside United States 
11 VIM Jul 2010 K. pneumoniae Blood Yes Hospitalization in Greece 
12 VIM Sep 2010 K. pneumoniae Urine Yes Hospitalization in Italy 
13 VIM Oct 2010 K. pneumoniae JP drain No Overlapping ICU stay with case-patient 11 during United States hospitalization 
Case MBL type Culture date Organism Culture site Received medical care outside United States Additional patient information 
NDM Apr 2009 Enterobacter cloacae Urine Yes Hospitalization in India 
NDM Dec 2009 Klebsiella pneumoniae Urine Yes Hospitalization in India 
NDM May 2010 Escherichia coli Urine No Travel in India, history of multiple comorbidities, indwelling medical device 
NDM Sep 2010 K. pneumoniae Respiratory Yes Hospitalization in Pakistan 
NDM Sep 2010 E. coli Respiratory Yes Received medical care in India, no hospitalizations 
NDM Dec 2010 K. pneumoniae Urine Yes Hospitalization in India 
NDM Feb 2011 K. pneumoniae Respiratory Yes Hospitalization in India 
IMP Nov 2009 K. pneumoniae Urine No No known travel outside United States 
IMP May 2010 K. pneumoniae Urine No No known travel outside United States 
10 IMP Jun 2010 K. pneumoniae Urine No No known travel outside United States 
11 VIM Jul 2010 K. pneumoniae Blood Yes Hospitalization in Greece 
12 VIM Sep 2010 K. pneumoniae Urine Yes Hospitalization in Italy 
13 VIM Oct 2010 K. pneumoniae JP drain No Overlapping ICU stay with case-patient 11 during United States hospitalization 

NOTE. MBL, metallo-β-lactamase; NDM, New Delhi metallo-β-lactamase; IMP, “active on imipenem”; VIM, Verona integron-encoded metallo-β-lactamase; ICU, intensive care unit.

CRE Risk Factors and Associated Mortality

In studies evaluating risk factors for CRE acquisition or infection, exposure to health care and antimicrobials are among the most prominent risks [4, 5, 20, 27]. Patel et al found that invasive infections with carbapenem-resistant K. pneumoniae (CRKP)—likely primarily KPC-producers—were independently associated with recent organ or stem-cell transplantation, receipt of mechanical ventilation, exposure to antimicrobials, and longer length of stay when compared with patients with carbapenem-susceptible K. pneumoniae (CSKP) [4]. Other risk factors associated with the acquisition of CRKP include poor functional status and intensive care unit (ICU) stay [5]. Of note, use of several classes of antimicrobials has been associated with CRKP carriage or infection, including carbapenems [4, 20], cephalosoprins [4], fluoroquinolones [5, 20], and vancomycin [27].

When outcomes for patients with CRKP are compared with those for patients with CSKP, carbapenem resistance has been independently associated with an increase in mortality [4, 5, 28]. Age, mechanical ventilation, malignancy, heart disease, and ICU stay have been associated with increased mortality among those with CRKP infections [4, 5, 28], whereas removal of the focus of infection (eg, catheter removal, debridement, or drainage) was independently associated with survival [4].

Long-term Care and CRE

The presence of CRE carriage has been described in a number of investigations involving patients from postacute care facilities [29–31], particularly long-term acute care hospitals (LTACHs) [29, 30]. Perez et al found that greater than 50% of patients with carbapenem-resistant gram-negative organisms were admitted from postacute care facilities, suggesting that these settings may be important reservoirs for the transmission and dissemination of these organisms [30]. In addition, small numbers of CRE clinical cases may be associated with larger reservoirs of colonized patients in these settings. In an investigation of 3 patients with KPC-producing CRE infection transferred to a hospital from a LTACH, active surveillance cultures from residents in the same LTACH unit as the case-patients identified CRE colonization among 49% of residents (unpublished CDC data).

PREVENTION

Although antimicrobial development efforts remain a cornerstone of CRE response efforts [32], interventions aimed at preventing the transmission of, and infections with, these organisms are also important. Delaying the emergence of carbapenem resistance, particularly in areas where this resistance is still uncommon, can decrease the impact of these organisms as we await additional treatment options. More research is needed to determine the best ways to prevent CRE transmission, but single-center studies and 1 national effort [33] have suggested that bundled prevention strategies can be successful in outbreak [34–36] and endemic [37] settings. The next section highlights important prevention activities and describes the CDC's current recommendations for preventing CRE transmission in acute care facilities [38].

Laboratory Detection

Accurately identifying CRE in the clinical laboratory is an important first step in prevention. Early studies have demonstrated that some KPC-producing isolates have carbapenem MICs that remain in the susceptible range [39]. As a result, failure to detect these organisms may have underestimated CRE prevalence in early reports. To improve the detection of carbapenemase-producing Enterobacteriaceae, in 2008 the Clinical Laboratory and Standards Institute (CLSI) recommended that Enterobacteriaceae with elevated MICs to carbapenems (2–4 μg/mL) or reduced disk diffusion zones be tested for production of a carbapenemase using the modified Hodge test (MHT) [40]. If test results were positive, it was recommended that the presence of a carbapenemase be noted in the medical record. CLSI reevaluated the carbapenem breakpoints for Enterobacteriaceae and in 2010 recommended lowering the carbapenem breakpoints for ertapenem, imipenem, and meropenem and established new breakpoints for doripenem [41] (Table 2). These new breakpoints were established to more accurately predict carbapenem treatment outcomes without the need for a special test to detect carbapenemase production.

Table 2.

Clinical and Laboratory Standards Institute Interpretive Criteria for Carbapenems and Enterobacteriaceae [41]

 Previous breakpoints (M100-S19)MIC (μg/mL)
 
Revised breakpoints (M100-S20)MIC (μg/mL)
 
Agent Susceptible Intermediate Resistant Susceptible Intermediate Resistant 
Doripenem … … … ≤1 ≥4 
Ertapenem ≤2 ≥8 ≤0.25 0.5 ≥1 
Imipenem ≤4 ≥16 ≤1 ≥4 
Meropenem ≤4 ≥16 ≤1 ≥4 
 Previous breakpoints (M100-S19)MIC (μg/mL)
 
Revised breakpoints (M100-S20)MIC (μg/mL)
 
Agent Susceptible Intermediate Resistant Susceptible Intermediate Resistant 
Doripenem … … … ≤1 ≥4 
Ertapenem ≤2 ≥8 ≤0.25 0.5 ≥1 
Imipenem ≤4 ≥16 ≤1 ≥4 
Meropenem ≤4 ≥16 ≤1 ≥4 

NOTE.MIC, minimum inhibitory concentration.

Although the CLSI breakpoint changes were recommended in 2010, the Food and Drug Administration (FDA)–approved breakpoints have not been changed, so the manufacturers of automated testing devices have not been able to provide clinical laboratories with tests whose performance has been validated against the new CLSI breakpoints. Thus, it appears that many clinical laboratories continue to rely on the older, higher breakpoints combined with phenotypic tests for determining carbapenem nonsusceptibility among Enterobacteriaceae.

In addition to the issues described previously, the identification of CRE is complicated by the fact that different definitions exist. Current definitions may include different bacterial species, different carbapenem susceptibility results, or results of additional testing (eg, carbapenemase testing). A conservative definition used at the CDC is nonsusceptibility to imipenem, meropenem, or doripenem using the revised 2010 CLSI breakpoints. Although this definition can be used for all Enterobacteriaceae, including the most common carbapenemase-producing strains (eg, Klebsiella species and E. coli), it might not apply equally to genera with higher baseline MICs to imipenem (eg, Providencia species, Proteus species, and Morganella morganii).

Recognizing CRE Cases

It is important for health care facilities to understand how common CRE are in their institutions. In investigations conducted by the CDC, failure to recognize CRE infections when they first occur in a facility has resulted in a missed opportunity to intervene before these organisms are transmitted more widely. This omission is often related to 2 issues: first, a failure to recognize CRE as an epidemiologically important organism that requires specific attention, and second, the lack of an established communication mechanism between infection-prevention personnel and the clinical laboratory. Based on current recommendations for the control of multidrug-resistant organisms (MDROs), the CDC recommends that, in areas where CRE are not endemic, acute care facilities review microbiology records for the preceding 6–12 months to determine whether CRE have been isolated at the facility [38]. If previously unrecognized cases are identified, a round of surveillance cultures (ie, a point-prevalence survey) in high-risk areas (eg, ICUs or wards where previous cases have been detected) should be considered to identify unrecognized cases. In addition, facilities should ensure a system is in place to promptly notify infection-prevention personnel when CRE are identified in the laboratory. All identified CRE case-patients should be placed on contact precautions, and some experts have also recommended patient cohorting and use of dedicated staff for these patients [42].

Surveillance Cultures

If previously unrecognized CRE cases or hospital-onset CRE infections are identified via either clinical cultures or point-prevalence surveys, facilities should consider surveillance cultures from patients with epidemiologic links to CRE case-patients. The goal of these cultures is to identify additional unrecognized CRE-colonized patients who are a potential source for transmission. If additional CRE-colonized patients are recognized, appropriate isolation precautions should be implemented.

Data from several studies have suggested that clinical cultures identify only a portion of patients colonized with CRE. In a point-prevalence study reporting a 5.4% carriage rate of CRKP among inpatients at a hospital in Israel [27], fewer than one-third of these patients had positive clinical cultures for CRKP. In another study, surveillance cultures were responsible for identifying more than one-third of the patients infected or colonized with CRKP, resulting in an estimated 1400 days saved from unprotected exposure through early detection and implementation of contact precautions [43].

The ideal anatomic site to screen for resistant Enterobacteriaceae with surveillance cultures has been investigated in a number of studies. Among these, perianal and rectal cultures are generally the most reliable [27, 44]. The CDC has primarily obtained surveillance cultures for Enterobacteriaceae from the perirectal area and wounds during outbreak investigations. A laboratory protocol for processing of these swabs is available on the CDC's Web site (http://www.cdc.gov/ncidod/dhqp/pdf/ar/Klebsiella_or_Ecoli.pdf).

Active surveillance cultures have been used as part of a comprehensive strategy to interrupt transmission of KPC-producing K. pneumoniae in several investigations [34–36]. Evaluation of the impact of this intervention has generally taken on a quasi-experimental design and has often involved multiple interventions that make it difficult to understand the impact of surveillance cultures alone. During a CRKP outbreak, Ben-David and colleagues obtained active surveillance cultures from ICU patients on admission and weekly thereafter, and from non-ICU patients with epidemiologic links to CRKP case-patients, as part of their intervention. They reported a 4.7-fold reduction in the incidence of CRKP infections following implementation of their prevention effort [34]. Similarly, Kochar et al found a decrease in the incidence of CRKP in an ICU with endemic CRE using a multifaceted intervention that included rectal surveillance cultures obtained at admission and weekly [37]. Although the exact role for active surveillance cultures is not known, screening patients coming from highly endemic settings at admission for these organisms might be a consideration for some facilities.

In addition to active surveillance, Zuckerman et al assessed the eradication of CRE carriage using oral gentamicin. They achieved a 66% CRE eradication rate; however, more research is needed before this can be recommended more widely [45].

Antimicrobial Stewardship and Minimizing Devices

Antimicrobial stewardship has been suggested as an important part of efforts to control MDROs [46]. However, multiple antimicrobial classes have been identified as possible risk factors for infection or colonization with CRE [4, 5, 20, 27]. Therefore, antimicrobial stewardship might be most effective if efforts are directed toward an overall decrease in antimicrobial use rather than targeting a specific antimicrobial class. Carbapenem restriction has been associated with lower rates of carbapenem-resistant Pseudomonas aeruginosa [47]; however, more research is needed to clarify the effect on CRE.

Limiting use of invasive devices is another potentially important intervention for CRE prevention. CRE have been identified from device-associated infections, particularly catheter-associated urinary tract infections. Therefore, strategies to prevent device-related infections, as described in previously published guidelines [48], should be implemented. For urinary catheters, prevention efforts include inserting catheters only in those patients with appropriate indications and removing them as soon as possible, using aseptic technique and sterile equipment for insertion, and maintaining a sterile closed drainage system [48].

Prevention Beyond Acute Care and Role of Public Health

Although much of the effort surrounding CRE control has focused on acute care facilities, nonacute care settings also provide care for patients colonized or infected with these organisms [29–31]. Limiting prevention efforts to acute care settings fails to take into account the presence of MDROs across different health care settings. Broadening the approach to prevention requires employing setting-specific infection prevention strategies in all health care arenas but also requires a method for enhanced communication to ensure that proper infection-control practices [46] are continued when patients are transferred between levels of care.

CRE can become an issue not only in individual institutions but also across an entire community, thus highlighting a role for public health in CRE-prevention efforts. Public health has the ability to reach across the spectrum of health care to improve community situational awareness with respect to CRE and to assist with coordinating prevention efforts. Toward this end, a number of states have added, or are considering adding, CRE to the state's reportable conditions list. In support of this approach, a review of the experience in Israel suggests that centrally coordinated efforts to prevent these organisms have been associated with large decreases in the incidence of CRKP [33].

We thank Carolyn V. Gould, MD, MSCR, and Arjun Srinivasan, MD, for their thoughtful review of the manuscript.

The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention.

Potential conflicts of interest. A. J. K. reports that his spouse has served as a consultant for Kimberly-Clark. All other authors: no conflicts.

All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed in the Acknowledgments section.

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