While sometimes difficult to recognize, plasmids have previously been linked to outbreaks of drug-resistant Gram-negative bacteria. In 1985, O’Brien and colleagues reported an outbreak of plasmid-mediated gentamicin resistance, which moved to six species of the Enterobacteriaceae
over 8 years in geographically diverse areas of the United States and Venezuela (16
). Outbreaks of enterobacteriaceae resistant to sulfonamides, β-lactams hydrolyzed by ESBL, aminoglycosides, and fluoroquinolones have also been reported (17
). Most recently, a multispecies outbreak of plasmid-mediated VIM-1 metallo-carbapenemase has been reported in Spain (22
), and A/C-type plasmid carriage of the New Delhi metallo-β-lactamase gene blaNDM-1
has emerged in Enterobacteriaceae
in India and the United Kingdom (22
). While movement of blaKPC
-containing plasmids between Enterobacteriaceae
has been described (7
), the degree of horizontal transmission between strains and species observed in this outbreak is unprecedented.
Why did the outbreak of CRE occur in such a wide assortment of strains in our institution? One possibility is that the plasmid-mediated nature of the outbreak may indicate that pUVA01 is more mobile than other blaKPC plasmids. This is supported by the observation that pUVA01 moved between a wide array of strains over a short period of time, often in the absence of carbapenem pressure. In contrast, horizontal transmission of pUVA02 or pUVA03 was not observed during the study period. Clearly, facile transfer of blaKPC plasmids adds an additional layer of complexity to the epidemiology of this rapidly spreading resistance mechanism. A second possibility is that by screening all possible ESBL-producing Enterobacteriaceae, we may have captured non- or weakly lactose-fermenting KPC-producing organisms that may have been overlooked in a targeted surveillance and evaluation program, as well as organisms with low carbapenem MICs that would otherwise have been considered carbapenem susceptible. Regardless of the etiology, we predict that the diverse nature of this outbreak will render detection and control of CRE spread even more problematic if it becomes increasingly widespread.
One important aspect of this investigation was the use of ARB-PCR to rapidly identify causative KPC resistance plasmids during the investigation. This technology allowed us to quickly recognize the high degree of genetic relatedness among the outbreak isolates at the level of plasmid carriage—relationships missed by traditional species- and strain-based analysis. Investigations of so-called “plasmid outbreaks” are difficult for all but sophisticated molecular epidemiology labs. Analysis of large resistance plasmids from clinical isolates of the Enterobacteriaceae, such as the ones observed in this study, can be laborious and technically challenging. ARB-PCR allows rapid determination of DNA sequences that flank a conserved element, which can be used as the molecular signature for a transmissible plasmid. Given its simplicity, ARB-PCR should be translatable to not only transposons but also resistance genes and other conserved components of mobile DNA elements. ARB-PCR does have several limitations. A product may not able to be generated from all samples, as was seen in this study. However, we were able to obtain products in all but one of our isolates without optimization of the protocol conditions or primer sets. Another limitation is that only one product will be generated from an isolate, even if it contains more than one plasmid with blaKPC, as was found with isolate Cfr1047. Finally, generation of flanking sequence does not ensure that the product necessarily has a plasmid origin; transfer of drug resistance to a recipient bacterial strain and demonstration of a plasmid-encoded drug-resistant determinant are required to prove the presence of a resistance plasmid in the donor strain.
The ease of horizontal transmission of carbapenem resistance observed in this study has serious public health and epidemiological implications. Dissemination of blaKPC
through mobile genetic elements could allow carbapenem resistance to move to the community, as has been observed with the worldwide emergence of community-onset CTX-M-15 β-lactamase-producing E. coli
). Additionally, transmission of blaKPC
to enterobacteriaceae of reduced or no virulence could lead to the establishment of a reservoir of carbapenem resistance genes in patients and/or the environment. Understanding the dynamics of blaKPC
horizontal gene transfer will be crucial to limiting the spread of CRE, since epidemiological and containment efforts must focus not only on the spread of multidrug-resistant strains but also on the associated resistance plasmids and transposons.